[Chhanda Das]

DIY 12 volt Portable Lithium Power Station

Warning : The long post/s ahead could test your patience.

We started this project on Valentine's day this year and dedicate it to our dear India on this momentous occasion of the 75th Independence Day on the 15th of August 2022.

Disclaimer : All voltages mentioned are for direct current or DC. I do not have an academic background in science and I am not very knowledgeable about electronics/electricals. My son has a background in Mathematics but he too is not very knowledgeable about this stuff. So we had to do quite a lot of research. The planning was done jointly by myself and my son while the execution of this project was largely done by my son. If you want to replicate this DIY project yourself then please do so at your own risk. And kindly bear with me since the following is a long read.

Hello everyone, this is about how we designed and made a lithium-based 12 volt power bank with a host of safety features along with the ability to deliver and accept an electrical charge to and from a wide variety of sources respectively while maintaining a scope for substantial capacity upgrades in future. We can also daisy-chain this to several other 12 volt power banks or lead-acid batteries if necessary. Basically, we wanted to make a power station along the lines of something like the ones made by brands like Jackery, Bluetti, Goal Zero, Rockpals, etc without paying exorbitant sums of money for the same.

For those of you who live in your car and/or go car-camping, a portable power station is an absolute necessity. However, some of us may not be able to access one for various reasons. For example, compact portable power stations like those made by Jackery, Bluetti, etc are usually not available here in India. The few importers who do sell them do so without any warranty support from the manufacturer. Additionally, the price at which they are listed here is usually above 200% of the price at which the manufacturer sells them due to 100% in import taxes alone. This makes such products very cost-prohibitive for us.

As some of you may know, I often go car camping in my 2017 Creta petrol 1.6 AT and I had installed a DIY solar battery charging system for the same. Although it was relatively successful, there were still a few issues that needed to be sorted.

Firstly, the charge controller that I was using was a 10 ampere rated one for lead-acid batteries which would accept very little solar power if the solar panel's voltage was lower than 13 volts. Although the 50 watt monocrystalline panel was rated for a maximum of 18 volts and was specifically designed for low light situations, the actual voltage output would often be below 13 volts in low light situations. This was because I had placed the panel on the rear parcel tray of my car and did not mount it on the roof to avoid possible legal issues or bureaucratic red tape as well as water leakage and/or rusting.

Secondly, there was little to no scope for daisy-chaining multiple batteries since I did not want to keep a lead-acid battery in the cabin of my car as those release fumes and my car's cabin also serves as my living/sleeping space. The ones that do not release fumes are quite expensive and nearly impossible to get in a desired shape/size.

Thirdly, lead acid batteries are expensive to the tune of more than INR 100 per Ampere Hour. So I had practically no choice other than to go for lithium batteries since those are way cheaper (approximately INR 18-20 per AH at 12 volts). More on this pricing aspect later. The life of lithium batteries is also higher than those of lead-acid batteries. For example, lead acid batteries like the ones used in cars have an average life of 5 years. While lithium batteries only lose around 20% of their rated capacities after 8-10 years. We still have a 5000 mAh lithium power bank from Sony that we purchased in 2012 which works fine to this day.

Fourthly, lithium batteries have very high depth of discharge (DoD) ratings compared to the 50% DoD ratings for lead acid batteries. This means that lead acid batteries can only be drained upto 50% of their rated capacities without causing permanent damage to them but lithium batteries can often be drained nearly completely and they would still run fine.

Planning and design

Lithium batteries have a host of issues mainly related to safety. As we all know, lithium batteries are especially sensitive to heat (thermal runaway) and all of us have seen/heard of explosions in phones, electric cars, electric bikes, electric scooters, tablet PCs, etc. Naturally, we wanted to avoid such situations with proper planning and extensive/meticulous testing.

Choosing the type of lithium battery :

There are three main types of lithium batteries/cells available in India suitable for our particular use case - Lithium-ion (Li-ion), Lithium Iron Phosphate (LiFePO4) and Lithium Titanate (LTO) with varying ranges of voltages, charging/discharging rates and capacities. Amongst these, the LTO ones are the most expensive, longest lasting, most resistant to heat and most difficult to find while the Li-ion ones are at the total opposite ends of the spectrum. We had initially decided to go for the LiFePO4 ones since they seemed to be a good balance between price and performance but we changed our mind soon after we came to know about a dirty secret of the lithium battery business in India. Almost all of the individual lithium cells or lithium battery packs sold in India are reclaimed, refurbished and imported mainly from China even if our local sellers claim them to be brand new and this is true for Li-ion, LiFeP04 as well as LTO batteries/cells. Please note that this is not true for big brands who make electronic gadgets like laptops, phones, etc or EVs or battery packs for power stations, communication stations, etc.

We started searching for sellers across India including Kolkata where I stay but we could not find a single seller with ready shipping at reasonable rates across India. The prices they quoted for refurbished cells matched the bulk prices of new cells in China. We were naturally very dejected but still didn't lose hope. We started looking into Li-ion cells but we were appalled to see that the same situation as with LiFePO4 cells existed here as well. However, the sellers of Li-ion cells were far more in number which indicated the easy availability of these cells.

Just out of curiosity, we started looking into what refurbishing the lithium cells entails or how one refurbishes/reclaims lithium cells. Refurbishing the cells is nothing but discharging and recharging the individual cells a few times at the rated current and voltage to see if the cell holds the charge or not after a few days after the cell is disconnected from all loads and charging. If the cell retains the voltage after a few days then it is considered reusable. It is as simple as that. Li-ion cells can be easily reclaimed from dead/defective laptop battery packs and power banks. These are sold by aftermarket laptop repair shops at prices approximately between INR 60-120 with each laptop battery pack containing around 6 cells rated at approximately around 2000 mAH or 2 AH each on an average. The cells would invariably be of the 18650 size. We got a bunch of these battery packs for around INR 60 each. Please note that LiFePO4 cells are usually larger both in capacities and dimensions.

Fortunately, we had a Li-ion 4 cell 18650 charger lying around which we had procured from Aliexpress before it was banned here in India. We had procured this USB based charger primarily to recharge the Nickel Metal Hydride (NiMH) AAA batteries for an earlier project involving DIY puddle lamps in our car but it can also recharge Li-ion 18650 batteries, LiFePO4 32650/32700 batteries and NiMH AA/AAA batteries as well. So the battery cell testing situation was sorted.

The 18650 lithium cell charger

Choosing the layout of the cells

We could not use a lead-acid battery charge controller for charging lithium cells. This is because the float voltage for lead-acid batteries is approximately around 14.2 volts while the float voltage for the 18650 model lithium battery cells (4.2 volts each) in 3S (3 cells in series) configuration is around 12.6 volts and the same for a 4S (4 cells in series) configuration is 16.8 volts. Float voltage is the maximum safe voltage at which a minimum current can be applied to compensate for the self-discharge rate of the battery after it is fully charged. So a lead-acid charge controller can easily damage the lithium cells by overcharging the cells in 3S and undercharging the cells in a 4S configuration. Moreover, a 4S configuration meant that we would have to find a solar charge controller supporting that configuration but we could not find such a charge controller at cheap rates even after looking for 6 months. A 4S configuration also meant that we would have to use a buck converter to reduce the battery voltage for running 12v gadgets/appliances and that would push up the cost while reducing efficiency. So it had to be the 3S configuration.

Choosing the BMS

Next, we had to choose a battery management system (BMS) circuit board for the 3S configuration which should be able to handle a lot of current while having the highest safety within reasonable limits. BMS boards are necessary because lithium cells tend to get disproportionately charged/discharged over a period of time compared to similar cells in the same series. This happens with lead acid batteries (not individual cells) too especially when they are connected in series. BMS boards also have a host of safety features like overcharge protection, over-discharge protection, short circuit protection, reverse polarity protection, etc. There are two kinds of BMS boards available - active and passive. Active boards are way better in the sense that they actively monitor the voltages of each row of cells in parallel with connectivity features like App control via Bluetooth, WiFi, etc with some of them even featuring a RS232 port for LAN connections. However, these are very expensive and nearly impossible to find in India at reasonable rates, especially with the ongoing import restrictions. The only major downside of passive boards is that we have to individually balance the voltages of each lithium cell as close to each other as possible before performing the initial connections but this is easily manageable.

Tools required/used :

1) A small manual screwdriver kit for making the wire connections and adjusting the potentiometers on the circuit boards.

2) A wire cutter and stripper

3) A portable 12 volt DC spot welder with an adjustable 90-130 ampere welding current.

4) A small 25 watt soldering iron

5) A cheap multimeter to measure voltages upto 23 volts DC.

6) A cheap ammeter to measure direct current upto around 35 amperes with a suitably rated shunt.

7) A heat gun or a powerful hair dryer

8) A 2 ampere USB based 4 bay lithium cell charger

9) A pair of strong scissors for cutting nickel strips.

Assembling/Preparing the spot welder

Please note that before using the aforementioned spot welder for the first time you will need to assemble it.

Firstly, the connectors need to be crimped onto the wires. So you will need pliers for that.

Secondly, the capacitor and the buzzer would need to be soldered onto the printed circuit board (PCB) at their respective spots while matching their respective polarities to the PCB. So you will need the soldering iron and some lead-free soldering wire for that. Here it is very important to note that if soldering the capacitor is not done properly with copious amounts of solder then this spot welder will not work as intended and will only deliver a limited amount of current.

Thirdly, you will need a battery that can deliver very high amounts of current within a short time for this spot welder. So a vehicle starting battery is perfect for this. Considering the fact that an overwhelming majority of us are vehicle owners here, arranging for a vehicle starting battery is not a problem at all.

Fourthly, you will need to install heat shrink tubes on the soldering leads for electrical insulation. Any source of flame like a candle, cigarette lighter, etc can be used for this.

Finally, you will need to screw the soldering lead wires and the wires for drawing power on the PCB (according to the colour codes and polarities) with the help of nuts. These nuts need to be tightened substantially for maintaining a proper flow of current.

The spot welder

As you can see above, the small button on the spot welder PCB beside the buzzer needs to be pressed and held multiple times to adjust the number of times (1-5) the current will be delivered upon a single contact on the welding surface. This adjustment is also indicated by aural feedback from the buzzer by the number of beeping sounds in quick succession.

Now that we have the tools sorted, let us first get into making the 3S battery pack. Here is a list of the components used :-

1) Square-shaped interlocking cell spacers for the 18650 model cells. These are required for passive heat dissipation during usage and holding the cells in place for spot welding. These are also available in a honeycomb shape but those are very rare. These need to be arranged in parallel in multiples of 3 for the 3S configuration. We assembled two sets containing 33 spacers in each set. These spacers cost us INR 1 each. For a larger battery pack, obviously you will need more spacers in multiples of 6.

2) High-strength plastic adhesive to help the individual cell spacers stick to each other after getting interlocked.

3) Thirty three numbers of 18650 model cells salvaged and refurbished by ourselves. Cost us approximately INR 12 for each 2 AH cell of 3.7 - 3.8 volts after accounting for and discarding all the defective cells. For a larger battery pack, obviously you will need more cells in multiples of 3. Here are the specifications that we found online for most of the cells. We used 6 cells of each of the 18650 cell types whose images are provided below. The relevant images B1, B2, B3 and B4 are attached. We could not find any details online for the last 9 cells which were salvaged from a Lapcare branded battery pack and seemed to have a generic origin. These 9 cells seem to be rated at 3.7 volts each with an energy rating of 7.4 Wh (watt hour) each.

Here are the specifications that we found online for most of the cells. We used 6 cells of each of the 18650 cell types whose images are provided below.

We could not find any details online for the last 9 cells which were salvaged from a Lapcare branded battery pack and seemed to have a generic origin. These 9 cells seem to be rated at 3.7 volts each with an energy rating of 7.4 Wh (watt hour) each.

4) A 3S lithium BMS (battery management system) circuit board that can handle your desired amount of current without getting hot. We chose a 40 ampere BMS and it can handle upto 20 amperes of current (charging/discharging) without getting hot and upto 40 amperes of discharging current with additional cooling. We did not need more than 15 amperes of current for our setup and hence no additional cooling was necessary. This cost us approximately INR 120.

5) Thirty three numbers of single-hole electrically insulating adhesive mat for 18650 model battery cell terminal insulation. This cost us INR 1 each. For a larger battery pack, obviously you will need more mats in multiples of 3.

6) Pure nickel strip H type or 2P type with 0.15 mm thickness. You can also go for the slightly cheaper single-line strips of the same thickness as above but please avoid the nickel-plated strips. Pure nickel double line (H type or 2P type) strips cost us approximately INR 260 for 50 grams. You will need more for a larger battery pack. Also, please note that using the single line strips makes the job of spot welding them more arduous.

7) Adhesive barley paper or fish paper for covering the nickel strips and terminals after making the battery pack. As an alternative, Kapton tape can also be used. Barley paper will cost you approximately INR 12 per metre and Kapton tape will cost approximately INR 20 per metre for the same width as the 3S battery pack. Another good alternative would be epoxy-fibreglass sheets if you can find them.

8) PVC heat shrink sleeve of width 150 mm cost us INR 15 for one metre.

9) One feet each of colour coded 2.5 square milimetre AC wires to handle upto 20 amperes of direct current (DC). Please note that you can use the more recommended DC wires but those are way thicker and will not fit the AC connectors properly.

10) Lead-free solder wire

11) One female 6 ampere rated 2 pin AC connector. We chose to use regular 2 pin AC connectors as our basic connectors for this entire setup instead of the more recommended DC connectors like XT30, XT60, XT90, etc. This is because the XT connectors are too tight to connect and disconnect on a regular basis and they are not widely available as well. The 2 pin 6 ampere rated AC connectors can handle quite a high amount of direct current (way more than 6 amperes) without getting hot. However, for a larger battery pack, you may have no choice other than using the XT connectors as per the load requirements. We used some red nail polish to mark the polarities on the AC connector.

12) Some sand paper or emery paper

13) Double-sided adhesive tape

Procedure for making the battery pack :-

Step 1 : First you need to rub both the positive and the negative terminals of each 18650 cell with sand paper or emery paper to prepare them for the spot welding later. You need to be very careful not to allow dust particles to clog the vents around the positive terminals of each cell. Also, do not apply too much pressure on the cells (especially the positive terminal) or else you may damage the cells. Also, while sanding the terminals, try not to damage the PVC coverings or else you will have to rewrap the cell with the PVC again.

Step 2 : You need to stick the single-hole electrically insulating adhesive mats (1 each per cell) around the positive terminal of each cell as evenly as possible without covering the actual positive terminal itself in any way.

Step 3 : Then you need to arrange the cells within the spacers in such a way that the cells in the same series have the exact same rated specifications (actual voltage readings should match closely too) and the highest capacity series should be closest to the BMS. Accordingly, the battery capacities in series should be in decreasing order the farther they are from the BMS. The cell manufacturer's product datasheet should give you an idea of the cell's specifications. The cells will also need to be arranged in such a way with respect to their polarities that there is minimum usage of nickel strips. This means that in a series of 3 cells the positive terminal of the 1st cell has to be close to the negative terminal of the 2nd cell and the positive terminal of the 2nd cell has to be close to the negative terminal of the 3rd cell.

Step 4 : Now place the H type or 2P type (double line) nickel strip on the battery pack in such a way as to cover the positive terminals of the row of the first cells in every series and the negative terminals of the row of the second cells in every series. It is important to note here that if the sizes of the nickel strips and the cell spacers is mismatched then you might have to cut the nickel strips and then rejoin them. Additionally, please cut the nickel strips in such a way that it sits accurately on one side of the battery pack while extending for half an inch outside the side of the battery pack on the other side lengthwise. Please note that these extensions always have to protrude on the same side of the battery pack. This is necessary so that you can connect the nickel strips to the BMS later. Also please note that if your electrical load requirement is high then you might have to use thicker nickel strips or add additional layers to the existing ones.

Step 5 : Now it is time to use the cheap spot welder. After connecting it to a fully charged vehicle battery, you need to set it to 5 or 6 beeps for it to work correctly. Please wear adequate eye protection before proceeding further. You need to spot weld the nickel strips at a minimum of 4 spots on each terminal of each cell. All you need to do is place one of the spot welding rods' tips to one of the 4 spots with a tiny amount of pressure and then do the same with the other tip of the spot welder's rod to any one of the remaining 3 spots on the same terminal of the same cell. Repeat this procedure till all 4 spots on the terminal are connected to the nickel strip. Then do the same for the remaining cells under the same nickel strip. Please do not be afraid of a few sparks from the welding spots. At the same time, you should keep a sharp eye on the temperature of the spot welder's PCB and its wires because those will become very hot and hence you will need to allow them to cool down for at least 2 minutes after spot welding at each and every terminal of each cell. You might need to do this earlier too before all four spots of a single terminal are completely welded to prevent the spot welder from getting damaged. Please note that during this time the spot welder may shut down to conserve energy and you will then have to restart it. However, if you use expensive/commercial spot welders then you may not have to wait at all since those usually have active cooling systems. You need to be extra careful while spot welding the positive terminals of each cell since the working area in the middle is going to be very small and you should absolutely not try spot welding anywhere outside the positive terminal for each cell. And do not allow the welding rods to come in direct contact of each other. And one more thing, do not apply too much pressure on the 18650 cells (especially on the positive terminals) with the spot welding rods or else you may damage the cells. And too little pressure will not help make a proper weld. The pressure has to be just right.

Step 6 : Now turn the battery pack around and place the H type or 2P type nickel strip on the battery pack in such a way as to cover the positive terminals of the row of the second cells in every series and the negative terminals of the row of the third cells in every series. Everything else will be the same as Step 4 above.

Step 7 : Now repeat Step 5 for the nickel strips placed during Step 6.

Spot welding in progress

Step 8 : If you did not buy single-line nickel strips earlier then you will need to cut the H type or 2P type (double line) nickel strip appropriately to convert them into the single line nickel strip for this step. Turn the battery pack around and place the single-line nickel strip on the battery pack in such a way as to cover the negative terminals of the row of the first cells in every series. Everything else will be the same as Step 4 above.

Step 9 : Now repeat Step 5 for the nickel strips placed during Step 8.
Step 10 : Again turn the battery pack around and place the single line nickel strip on the battery pack in such a way as to cover the positive terminals of the row of the third cells in every series. Everything else will be the same as Step 4 above.

Step 11 : Now repeat Step 5 for the nickel strips placed during Step 10. After connecting the cells in a 3S configuration, they should look like what is shown below.

First voltage check after connecting the cells in 3S

Step 12 : To prepare the BMS for the next steps, you need to apply liberal amounts of lead-free solder with a soldering iron (not spot welder) to all 6 terminals marked +, -, 0v, 4.2v, 8.4v and 12.6v on the BMS as evenly as possible. Please take special care of the terminals marked + and - while ensuring that you apply a tiny bit of extra lead-free solder. Do not short the terminals.

Step 13 : Strip the insulation from the 2.5 millimetre colour-coded AC wires for approximately 1 centimetre from each side. Now connect one end of each of these colour-coded AC wires to the 2 pin AC connector's positive and negative female pins accordingly and solder the other ends of the wires to the terminals marked + and - on the BMS as per the respective polarities of the wires. Before soldering the wires to the BMS, please ensure that you have applied a generous amount of solder to the wire ends.

Step 14 : Cut off the two unnecessary protrusions of the nickel strips connecting the row of the second or middle cell in every series. You should be left with 4 protrusions of the nickel strips (one for each corner) on the same side of the battery pack. Now stick some double-sided adhesive tape to the side of the battery pack that has the protruding nickel strips.

Step 15 : Now stick the back of the BMS board to these adhesive tapes in such a way that the single-line nickel strip connecting the negative terminals of the first cell in every series is close to the terminal marked 0v on the BMS. As a result, the single-line nickel strip connecting the positive terminals of the third cell in every series will be close to the terminal marked 12.6v on the BMS.

Step 16 : Now smoothly bend the protruding edges of the nickel strips towards their respective terminals on the BMS. Please avoid sharp bends here or else the nickel strips may break. If the nickel strip protruding edge is too big then you might have to cut it a little.

Step 17 : Now you need to spot weld the nickel strips to the respective terminals in the order mentioned as follows :-

Firstly, you need to spot weld the single line nickel strip connecting the negative terminals of the first cell in every series to the terminal marked 0v on the BMS.

Secondly, you need to spot weld the single protruding line of the H type or 2P type nickel strip connecting the positive terminal of the 1st cell in every series and the negative terminal of the 2nd cell in every series to the terminal marked 4.2v on the BMS.

Thirdly, you need to spot weld the single protruding line of the H type or 2P type nickel strip connecting the positive terminal of the 2nd cell in every series and the negative terminal of the 3rd cell in every series to the terminal marked 8.4v on the BMS.

Fourthly and finally, you need to spot weld the single line nickel strip connecting the positive terminals of the 3rd cell in every series to the terminal marked 12.6v on the BMS. Now your battery pack is electrically ready for use.

The BMS

First voltage check after connecting the BMS

Side view of the battery pack

Bottom view of the battery pack

Top view of the battery pack

Step 18 : Now you need to stick the adhesive barley paper or fish paper on the nickel strips and the spacers on the battery pack. Then you can cover the BMS with some Kapton tape.

Step 19 : Then insert the battery pack into the PVC heat-shrink sleeve completely. Just the wires and the 2 pin AC female connector should be sticking out. Now it is time to use a heat gun or a powerful hair dryer on the PVC heat-shrink sleeve. The heat should be spread evenly and should not be concentrated in a few spots or else you will damage the cells in those spots. Also, please be careful not to shrink the PVC sleeve too much or else it will start to tear.

Step 20 : The battery pack is now completely ready for use and should show a voltage above 11.1 volts and below 12.6 volts. I would suggest charging the battery pack with a small direct current of around 1 ampere till the battery pack's voltage reaches 12.6 volts before using the battery pack for the first time.

[Chhanda Das]

Charging the battery pack

The next important step is to incorporate the ability to recharge the battery pack from a variety of sources like solar, home electricity and the car. Here are the relevant steps as follows :-

a) Solar : For recharging the battery pack via solar, we will need a solar charge controller with the ability to recharge lithium (ion/polymer) batteries in a 3S configuration. This means that the charge controller should cut off the power supply from the battery pack at 9 volts while completely restricting the battery pack from recharging once it reaches 12.6 volts. Even though the BMS would have restricted the charging voltage to between 9 and 12.6 volts had we used a solar charge controller for lead acid batteries, we wanted to have an additional layer of safety in case of a BMS failure. All we had to do was to get a cheap PWM (pulse width modulation) based solar charge controller for 3S lithium batteries and connect its battery terminals to a male 2 pin AC plug. Then we need to plug it into the female AC socket of the lithium battery pack while matching the polarities accurately. We did not choose an MPPT or maximum power point tracking solar charge controller because those are very expensive. Next, we need to use 2.5 square millimetre AC wires to connect the solar input of the charge controller to a male 2 pin AC plug. For safety reasons, you can keep this plug's metal prongs covered using a female 2 pin AC socket. Please note that all charging will have to pass through the solar input terminals of this PWM charge controller irrespective of whether the lithium battery pack is being charged by solar or home electricity or the car's electrical system.

Solar charge controller for a 3S lithium cell configuration

b) Home electricity : We used a 220/110v AC to 14v DC 3 ampere adapter for charging the battery pack via the solar input of the charge controller. Please note that a 220/110v AC to 12v DC adapter may be enough for recharging the 3S lithium battery pack through the solar charge controller for lithium batteries since good quality 12v DC adapters usually put out approximately 13.7-13.8 volts. However, if you decide to switch to lead acid batteries, then the 12v DC adapter won't be enough since lead acid batteries require at least above 14.2 volts to be fully charged. The 14v DC adapter that we chose puts out approximately 15 volts. Also, please note that you will have to cut off the connector on the DC end of the adapter and replace it with a 2 pin AC female socket.

220/110v AC to 14v 3A DC charger

c) Car electricity : This is the most complicated part of the charging system. The DC electricity in a car with an internal combustion engine (ICE) can be usually accessed within a voltage range of around 12-14.4 volts all things considered. But there is a problem. A car's battery can deliver a huge amount of current (approximately around 400 amperes at the very least) which is guaranteed to damage the 30 ampere rated charge controller and the 20 ampere rated BMS (without additional cooling). So we cannot connect the solar charge controller's solar input or the lithium battery pack directly to the car's battery. We tried looking for a way to restrict the current with our limited knowledge but sadly, we could not find any way.

This is when we stumbled upon an advanced microcontroller based 1.2 kilowatt DC to DC boost converter module that can accept upto 20 amperes of current with an active cooling system involving a heatsink and fan. This module is apparently very commonly used in electric bicycles and electric rickshaws to boost the battery voltage for the motor. This can deliver a user-adjustable constant current between 0.5 and 12 amperes.

1.2 kilowatt DC-DC boost converter

As you can see in the image above, this module has 2 positive and negative terminals each (total 4), connected in parallel respectively on the input side while it has just 2 terminals (positive and negative) on the output side. It also has 3 potentiometers (small blue boxes with a screw each on top) for adjusting various parameters.

First, we connected two 2.5 square millimetre wires to an AC 2 pin male connector and marked the positive and negative poles on this connector with some nail polish. Then we divided the copper strands on each wire equally into two. The positive wire strands were connected to the two positive terminals while the negative wire strands were connected to the two negative terminals on the input side, both on the input side of the 1.2 kilowatt DC to DC boost converter module. Next, we connected two 2.5 square millimetre wires to an AC 2 pin female connector and marked the positive and negative poles on this connector with some nail polish. The other ends of the wires were connected to the single positive and negative terminals on the output side of the 1.2 kilowatt DC to DC boost converter module.

Calibrating the 1.2 kilowatt DC to DC boost converter module

We need to calibrate/adjust the 3 different parameters on the the 1.2 kilowatt DC to DC boost converter module to suit our particular charging requirements. Ideally, we need a bench power supply for this but those are quite expensive. So we needed an alternative arrangement. While salvaging the lithium cells from the dead/defective laptop battery packs, we had noticed that not all of them were completely dead. Some of them were slightly defective while the others could not be matched in multiples of 3. Take 4 or more of these cells and connect them in series to achieve a target voltage of around 12.6 volts. Now you have a battery for voltage calibration/adjustment for the 1.2 kilowatt DC to DC boost converter module. Connect this battery of 4 or more cells to the 2 pin AC male connector on the power input end of the 1.2 kilowatt DC to DC boost converter module.
One thing to note here is that turning the screws on the potentiometers clockwise will increase the value of the parameters that they are supposed to help control and vice versa and small clicking sounds will let you know that you have reached the maximum or minimum value. However, do not turn the screws on the potentiometers too fast or you may damage them. Let us begin the calibration process for charging our power station :-

Step 1 : The potentiometer on the left near the 4 terminals (refer to image above) is for shutting off the voltage boosting mechanism at the voltage of our choice to prevent the source power supply (car battery in our case) from draining too low since that would prevent the engine from starting. We did not have much use for this feature since we did not intend to charge our lithium battery pack once the engine was shut down. This was because once the low voltage was triggered on the 1.2 kilowatt DC to DC boost converter module, it would still transmit power at the same voltage as the source. Still, for the sake of safety, we chose to keep it at 12.6 volts by turning the screw on the potentiometer with a small flathead screwdriver. Once you reach the desired voltage here, the small LED beside this potentiometer will turn on and stay on even if you go lower than the voltage of the source (12.6 volts in our case). The glowing of this LED means that the module is not boosting the voltage of the source.

Step 2 : The potentiometer on the right near the two output terminals is for adjusting the boost voltage. Using a cheap voltmeter, we chose to keep it at 14.4 volts by turning the screw on the potentiometer with a small flathead screwdriver. We chose 14.4 volts because that is the maximum charging voltage for a lead acid car battery and we didn't want to have to adjust this 1.2 kilowatt DC to DC boost converter module again if we decide to go back to lead acid batteries in future. Please note that the LED beside this potentiometer will turn on the moment you connect the power source to this module.

Step 3 : The potentiometer near the middle of the module is for adjusting the amount of current. We set it to approximately the minimum that the 1.2 kilowatt DC to DC boost converter module allowed us to set it at the 14.4 volt mark which we later found out to be around 3-3.1 amperes. This was not a problem at all since lithium batteries can last for a long time with a slow charging speed. Please note that the lithium battery pack that we made can accept a charging rate of up to around 22.5 amperes without even becoming warm. The best part about the 1.2 kilowatt DC to DC boost converter module is that when the battery voltage is close to the 14.4 volt mark (when the engine is on), it does not boost the voltage at all and keeps it the same as the source but it still restricts the current to 3-3.1 amperes. Obviously, you can set it to higher current values upto 12 amperes for charging the DIY lithium power station even faster. Please note that the DC to DC boost converter module is also available in variants with 1.5 kilowatt, 1.8 kilowatt, 2.2 kilowatt, 2.5 kilowatt and even higher capacities. So you can get those for fast charging larger lithium battery packs as well.

Now, we can charge our DIY lithium power station from the car. For that, we need a male cigarette lighter car plug and need to connect it with 2.5 square millimetre wires to a female 2 pin AC socket with a fuse socket on the positive wire. Mark the positive and negative terminals on the AC socket. We used some nail polish for this. You can use a 7.5 ampere rated fuse for the fuse socket but you can go higher if you have set a higher charging speed and your car's cigarette lighter socket allows it.

Male cigarette lighter plug to 2 pin AC female socket

For charging our DIY lithium power station in the car, all we have to do is plug the male cigarette lighter plug into the car's cigarette lighter socket and connect the other end to the 1.2 kilowatt DC to DC boost converter module's 4 terminal input. Then we can connect the solar charge controller's solar input to the 1.2 kilowatt DC to DC boost converter module's output and voila our DIY portable lithium power station will start getting recharged. Once charging is complete, you can disconnect everything.
Hence, charging our DIY portable lithium power station with solar, home's AC and car's DC is sorted. Here is an image taken during the final successful charging test of the DIY portable lithium power station where the source of power was a regular 12v lead acid car battery. Here is an image taken during the final successful charging test :-

Final successful charging test

Using the DIY portable lithium power station to run electrical loads

Although, we can easily run basic DC loads like lights, fans, motors, etc through the solar charge controller's output terminals. Car gadgets like vacuum cleaners, tyre inflators, power inverters, tea/coffee makers, electric cookers, etc cannot work directly from the solar charge controller's output or the lithium battery or even a regular lead acid battery of a car (unless the car's engine is on). This is because the voltage range (9-11.4 volts) of a 3S lithium battery pack is too low for the aforementioned gadgets since they usually like a voltage between 11.8-14.2 volts. Hence we need another 1.2 kilowatt DC to DC boost converter module to boost the voltage of our DIY lithium battery pack with the ability to deliver a high amount of current. You will also need a cooling fan to cool it down. That is why there is a small fan header on the back of the PCB (printed circuit board) of the 1.2 kilowatt DC to DC boost converter module. We found that a cheap 12 volt DC 0.15 ampere rated 6010 model fan took care of the cooling job perfectly. We can use some riser mounts or some zip-ties to mount the fan to the heatsink.

Rear/bottom view of the 1.2 kilowatt DC-DC Boost converter with an aluminium heatsink and a cooling fan

However, the calibration of this 1.2 kilowatt DC to DC boost converter module will be different from the earlier one that was used for charging the lithium battery pack with the car's electricity. Let us begin the calibration process for running different loads from the 1.2 kilowatt DC to DC boost converter module :-

Step 1 : You can take 3 or more defective or partially drained lithium cells in series from earlier to make the 9 volt power source for the 1.2 kilowatt DC to DC boost converter module. The potentiometer on the left for the low voltage boost disconnect near the 4 terminals would have to be set at 9 volts. This is because we want to use all the power of the DIY 3S lithium battery pack upto as low as 9 volts.

Step 2 : Since the potentiometer on the right near the two output terminals is for adjusting the boost voltage, we chose to keep it at 14.2 volts by turning the screw on the potentiometer with a small flathead screwdriver while continually monitoring the output voltage with a cheap voltmeter. We chose 14.2 volts because that is the exact output of the stock 12v cigarette lighter sockets in our car and hence that is the ideal voltage required by our car gadgets.

Step 3 : Since the potentiometer near the middle of the 1.2 kilowatt DC to DC boost converter module is for adjusting the amperes that can be drawn, we set it to the maximum of 12 amperes even though our load requirements would not exceed 110 watts (car 12v cooker) under any circumstances and this would translate to a 125 watt power draw by the 1.2 kilowatt DC to DC boost converter module from the DIY lithium power bank.
Now, we can run all our DC loads from the DIY lithium power station. For that, we need a female cigarette lighter socket and we need to connect it with 2.5 square millimetre wires to a male 2 pin AC plug with a fuse socket on the positive wire. Mark the positive and negative terminals on the AC plug. We used some nail polish for this. We used a 15 ampere rated fuse for the fuse socket since the maximum load would not exceed 125 watts at any given point of time. Technically, we could have used a 10 ampere fuse but it tends to get quite hot under sustained 125 watt loads at 14.2 volts and that is why we went with the 15 ampere fuse.

Now we need to make a wired plug/socket converter with a female cigarette lighter car socket on one end and a male 2 pin AC plug with a fuse socket on the positive wire on the other end (image below). Mark the positive and negative terminals on the AC socket. We used some nail polish for this. We used a 15 ampere 12 volt rated fuse here.

2 pin male AC plug to female cigarette lighter socket

Hence, now all we need to do is plug in the 2 pin AC male plug of the input of the 1.2 kilowatt DC to DC boost converter module to the 2 pin AC female socket connected to the load terminals of the solar charge controller. Then we plug in the 2 pin AC male plug (with the cigarette lighter socket at the other end) into the 2 pin AC female socket connected to the output terminals of the 1.2 kilowatt DC to DC boost converter module. Then press the load on/off button on the solar charge controller to turn on the load. The fan on the boost converter should turn on immediately. Now we can run car gadgets like vacuum cleaners, tyre inflators, tea/coffee makers, electric cookers, etc by simply plugging them into the cigarette lighter socket connected to the 1.2 kilowatt DC-DC boost converter.

For a 220/110v AC output, we simply plugged our car power inverter into the cigarette lighter socket connected to the 1.2 kilowatt DC to DC boost converter module's output. Our DIY portable lithium power station is now complete. Maybe someday we will make a proper housing assembly for it if we need it.

Finally, we need to make two small accessories for our DIY portable lithium power station as follows :-

1) A DC 2.1 mm x 5.5 mm male plug to a 2 pin AC female socket converter. We can use this for running a small 2 ampere load from the solar charge controller.

DC male to 2 pin AC female socket

2) A 2 pin male to 2 pin male connector for bypassing the solar charge controller in case the battery voltage drops too low resulting in the BMS or battery management system circuit going to sleep mode.

2 pin male AC plug to 2 pin male AC plug

In conclusion, here are some of the salient features of our DIY portable lithium power station :-

1) It has somewhat similar features to a Jackery 240.

2) It has slightly more capacity (25 Ah x 11.1 v = 277.5 Wh) than a Jackery 240 (16.8 Ah x 14.4 v = 241.92 Wh). This is approximately the same as a 45 AH lead acid battery when it comes to the total amount of usable power without damaging the battery.

3) Unlike any Jackery, it has a user-adjustable charging speed ranging from twice that of a Jackery 240 all the way down to a cell phone charger putting out just more than 1.5 amperes of current. That means it can be charged with almost any consumer charger or power supply. This adjustable charging speed is very important if you want to increase the longevity of lithium batteries.

4) It can be charged at home with 220/110v AC to 12v DC adapter. It can be charged by solar panels. It can be charged in the car as well.

5) Unlike any Jackery, battery capacity upgrades can be easily done for approximately INR 21-25 or approximately US$ 0.31 per AH (ampere hour) at 12 volts.

6) Its highly modular design allows for adding features and modules to it too very easily if necessary.

7) And the best part is that it cost us less than INR 4500 or US$ 57 to make which will include its housing assembly as well and this is way lesser than a Jackery 240. This obviously does not include the price of the tools. If you want to consider the price of tools then you can add approximately INR 1000 or US$ 13 to the cost mentioned above.

We chose to compare it to the Jackery 240 because Jackery is by far the most well-known brand in the entire world when it comes to making portable power stations and their 240 model is the closest to our DIY portable lithium power station both in terms of capacity and features.

This was the most complex, dangerous, time-consuming and mentally exhausting DIY project that we had undertaken to date. So please judge us kindly. Also, please feel free to mention any criticisms that you may have along with any clarifications that you may require. Thank you in advance and we wish you all a very Happy 75th Independence Day 🙂

[anantsood]

Nicely detailed post. Thanks for sharing - learnt quite a bit. Do share your experience in usage and how the battery pack holds up.

[Chhanda Das]

Originally Posted by anantsood

Nicely detailed post. Thanks for sharing - learnt quite a bit. Do share your experience in usage and how the battery pack holds up.

Thank you very much for your kind words. We have been using this mostly at home since April this year while charging it with a small 50 watt monocrystalline solar panel almost every single day.

My son sometimes runs a DIY 12v 0.5 ampere rated table fan with this portable power station and it can run continuously for more than 44 hours before the lithium solar charge controller shuts off the load.

He also runs a modified 45 watt laptop computer with this portable power station. He modified the laptop by replacing its hard disk drive (HDD) with a solid state drive (SSD) to increase the power efficiency as well as increase data safety for usage in a moving car. He also replaced the compact disk (CD) drive with a hard drive caddy for increasing the power efficiency even more. The laptop now runs for approximately 4 hours and 17 minutes while drawing boosted power from this DIY portable power station before the 1.2 kilowatt boost converter stops boosting the voltage. Once the boost converter stops boosting the voltage, the 12v to 220v pure sinewave car inverter shuts down and the laptop switches to its own battery.

Our car refrigerator runs for slightly longer with the portable power station than the laptop above because the compressor of the fridge shuts down from time to time once the desired cooling is reached.

With this portable power station, we can cook 3 meals back-to-back using our 100 watt rated 12v car cooker.

And we can also make tea in a 12v car tea maker for approximately 9 times with this portable power station.

Thankfully, we have not noticed any degradation of the battery pack as of now

[rajeshrajan]

Thank you for this post. Fantastic DIY project. Kudos on the details of the specs. Well laid out post for anyone who wants to get down and make it on their own. A perfect power source for a camper. Once again thank you.

[mathur2012]

Hi, Kudos to you for the awesome project. I also do car camping always. I have installed a 100w Mono Solar Panel on my car's roof top for my power needs.

The parts you used in the project, were they bought offline or online? Going by the prices I assume it's offline.

Also you mentioned a 12v cooker and a kettle. Can you please share the link to those? I use a 12v kettle which lasts only for a trip and goes kaput after that. Have to buy a new one on almost every trip.

[Chhanda Das]

Originally Posted by rajeshrajan

A perfect power source for a camper

Thank you very much for the kind words of appreciation. Our primary intention was to make a reasonably cheap power source for camping or other uses. The prices of branded portable power stations are absolutely exorbitant. For example, I just checked the prices of the Jackery 240 in India and it is a jaw-dropping INR 46k

Originally Posted by mathur2012

The parts you used in the project, were they bought offline or online? Going by the prices I assume it's offline.
Also you mentioned a 12v cooker and a kettle. Can you please share the link to those? I use a 12v kettle which lasts only for a trip and goes kaput after that. Have to buy a new one on almost every trip.

Thank you very much for the kind words of appreciation. We procured the components used in this project from a wide variety of sources including both online stores as well as regular brick-and-mortar stores. For example, we bought the connectors/plugs/sockets, the wires, the nickel strip, the BMS, the 14v adapter and the defective/dead laptop battery packs from regular brick-and-mortar shops near our location in Kolkata. We got the plastic 18650 cell spacers and the lithium solar charge controller online from Amazon India on a lightning deal. We were able to procure the PVC sleeve and the fish/barley paper online from the Quartz Components website at cheap rates. We got the spot welder and the single-hole adhesive mats from the Robu website. They usually provide substantial discounts during their month-end sales and on special occasions like Republic Day, Independence Day, etc.

I definitely mentioned a 12v cooker but I never mentioned anything about a kettle anywhere above. The 12v tea/coffee maker that I use is usually sold as a car kettle but it is essentially a large mug or cup. It is an unbranded one that we had got from Amazon India in 2017. Although it is mentioned as a 12v gadget, it works properly only between 13.8v and 14.3v. I got the 12v 100 watt cooker from ebay India in 2018. Sadly, that website does not exist anymore. There were similar products on Aliexpress too but sadly, that website got banned only in India. I would have shared the links with you but the problem is that they do not work anymore. The link from Aliexpress does not work due to obvious reasons but the link to the 12v car tea maker on Amazon does not work because the seller does not exist anymore. However, there are many others on Amazon India who sell the same 12v car tea maker.

Still, here is an image showing the 12v tea/coffee maker and the 12v 100 watt cooker that we use. The steel one with a black cap is the tea/coffee maker and the green coloured one is the cooker. I hope this helps

[mathur2012]

Originally Posted by Chhanda Das

The 12v tea/coffee maker that I use is usually sold as a car kettle but it is essentially a large mug or cup. It is an unbranded one that we had got from Amazon India in 2017.

I also have the exact same tea/coffee maker, I use that just to boil water( not exactly boil) but good enough to make tea/coffee while the engine is on. I am quite surprised yours is working since 2017. Mine doesn't last more than a trip!

[Chhanda Das]

Originally Posted by mathur2012

I also have the exact same tea/coffee maker, I use that just to boil water( not exactly boil) but good enough to make tea/coffee while the engine is on. I am quite surprised yours is working since 2017. Mine doesn't last more than a trip!

It is quite surprising that the 12v tea/coffee maker does not last long. Maybe it is getting damaged due to water ingress from the top-sides or the bottom. Please do not wash this cup under running water. Instead, please wipe it with a wet cloth and dishwashing liquid.

Another reason could be that the wires may be getting damaged due to extreme folding. You may also want to check the fuse inside the 12v cigarette lighter plug of the 12v tea/coffee maker.

Also, please try to avoid too many/hard vibrations or physical shocks to the 12v tea/coffee maker or else the heating coil inside it may get broken. What we do is that we place a small square towel handkerchief inside the cupholder space in our car before placing the 12v tea/coffee maker in it. That way there is no chance of the plastic cupholder space getting melted with the heat and the lower portion (lower sides and bottom) of the tea/coffee maker remains insulated from vibrations to a large extent. Additionally, the tea/coffee maker heats up faster due to less wastage of heat as a result of the insulation provided by the towel handkerchief. Please try this and let me know if it works.

[adrena]

Thank you for such detailed posts. I will need more than a couple of reads to just unpack what you've achieved. As you know, I've been waiting for such a DIY for a long time. Back I started, i wasn't getting options for good battery cells. You've got me excited enough to go at it once more.

[Chhanda Das]

Originally Posted by adrena

Thank you for such detailed posts. I will need more than a couple of reads to just unpack what you've achieved. As you know, I've been waiting for such a DIY for a long time. Back I started, i wasn't getting options for good battery cells. You've got me excited enough to go at it once more.

Thank you for your kind words of appreciation.

Since your intended usage for the battery is going to be as a starting battery for a two-wheeler, I would suggest a 4S configuration for the LiFePO4 cells that you intend to use. Additionally, it would be more suitable to go for the 32650/32700 cells with a high discharge rating instead of the 18650 LiFePO4 cells since the 32650/32700 cells can hold more energy per unit volume considering the fact that you have space constraints. Also, you can go for the more recommended prismatic LiFePO4 cells instead of the cylindrical 32650/32700 cells but those prismatic cells are slightly more expensive. However, we do not need a spot welder for connecting those prismatic cells since they can be connected with wires and ring connectors just like lead acid batteries.

For the BMS, you will need a 4S rated one for LiFePO4 battery packs with a discharge rating way above what is recommended for your motorcycle to account for future electrical load additions. The cold cranking amperes or CCA rating and the reserve capacity or RC rating of the stock battery of your motorcycle should provide you with the recommended minimum discharge specifications for the BMS.

I hope this helps and I wish you the best of luck for your upcoming DIY project

[sharan_si]

Nice, detailed technical stuff, appreciate the effort put in writing up this. What about the safety for lithium battery, is there a way to pack it in package with insulation.

[Pulse500]

Originally Posted by Chhanda Das

Choosing the BMS

Next, we had to choose a battery management system (BMS) circuit board for the 3S configuration which should be able to handle a lot of current while having the highest safety within reasonable limits.

Electronics has been my hobby since middle school days, and looking at such DIY projects by others excite me, help me improve my own built gadgets, or gives me some new ideas/sources of materials or spares. Coincidently I also retrieved Lithium cells from old laptop battery packs and built battery packs with 3S, 4S, 5S and 6S configurations for different purposes.

I use the 3S to provide uninterrupted power to my 12V broadband routers and as generic supply for LED worklight. 4S was built mainly as power supply for my portable RS-775 motor based DIY portable mini drill machine and car headlamp/glass polisher. 5S and 6S as general purpose power banks with in-built DC-DC buck converters for laptop, max 2 Amp.

All these lithium power banks had become a necessity during the COVID lockdowns in 2020 where we could not even buy spare laptop batteries, and that there were frequent power outages during monsoons. These gadgets helped sail through tough times, enabled kids attend online classes/exams and work from home routines.

A few small suggestions. Instead of using the female AC socket for DC where there will be a risk of inserting the plug with wrong polarity, and to enable higher DC current handling etc, you may want to use these XT connectors.





I am mainly using https://robu.in/product/t-style-conn...-caps-1-pairs/. You can check out alternatives too, or if you need connectors supporting higher current. https://robu.in/?s=xt+connector&prod...t_type=product

Disclaimer: I have no vested interests with Robu.in, I am just an ordinary customer of Robu.in and numerous such online sites, as now AliExpress is not accessible.

[Chhanda Das]

Originally Posted by sharan_si

Nice, detailed technical stuff, appreciate the effort put in writing up this. What about the safety for lithium battery, is there a way to pack it in package with insulation.

Thank you for the kind words of appreciation. Yes, of course there are various ways to pack it with proper insulation. As you can see in the quote below from near the end of my very first post above, the adhesive barley/fish paper, the Kapton tape and the PVC heat-shrink sleeve can handle the insulation aspect very well. However, if you want the lithium battery pack to be able to handle very rough use then you can cover it on all sides with some acrylic sheets of adequate size. Please note that the acrylic sheets have to be used after Step 18 but before Step 19. Another alternative to the acrylic sheets could be the case/container of suitably sized dead lead-acid batteries/cells. If you choose this case/container option then you need to do this only after Step 19. I hope that this helps

Originally Posted by Chhanda Das

Step 18 : Now you need to stick the adhesive barley paper or fish paper on the nickel strips and the spacers on the battery pack. Then you can cover the BMS with some Kapton tape.

Step 19 : Then insert the battery pack into the PVC heat-shrink sleeve completely. Just the wires and the 2 pin AC female connector should be sticking out. Now it is time to use a heat gun or a powerful hair dryer on the PVC heat-shrink sleeve. The heat should be spread evenly and should not be concentrated in a few spots or else you will damage the cells in those spots. Also, please be careful not to shrink the PVC sleeve too much or else it will start to tear.

Originally Posted by Pulse500

I use the 3S to provide uninterrupted power to my 12V broadband routers and as generic supply for LED worklight. 4S was built mainly as power supply for my portable RS-775 motor based DIY portable mini drill machine and car headlamp/glass polisher.

Very nice and cool
From what I remember from researching an earlier DIY project involving DIY fans (link below), isn't the RS-775 motor rated at 24 volts (maximum) ? If that is true then I am very curious as to why you chose the 4S configuration to power it. I am curious because the maximum voltage of a 4S lithium-ion battery would not exceed 16.8 volts. So wouldn't the 5S configuration have been more suitable with a maximum voltage rating of 21 volts ?

https://www.team-bhp.com/forum/diy-d...r-camping.html (DIY: Fan setup for car camping)

Originally Posted by Pulse500

A few small suggestions. Instead of using the female AC socket for DC where there will be a risk of inserting the plug with wrong polarity, and to enable higher DC current handling etc, you may want to use these XT connectors.

You are absolutely correct here and what you have suggested here is indeed the proper way. However, as I mentioned in my very first post above in the list of components used, we chose to use regular 2 pin AC connectors as our basic connectors for this entire setup instead of the more recommended DC connectors like XT30, XT60, XT90, etc. This is because the XT connectors are too tight to connect and disconnect on a regular basis and they are not widely available as well. The 2 pin 6 ampere rated AC connectors can handle quite a high amount of direct current (way more than 6 amperes) without getting hot. Additionally, all the critical modules like the BMS, the solar charge controller, the AC to DC power adapter, etc have inbuilt reverse polarity protection. Also, the prices of the XT connectors are at least more than double of what the 2 pin AC connectors' pairs cost us. Hence, all things considered, we chose to go with the 2 pin AC connectors instead of the superior XT connectors. I hope that I was able to assuage your concerns.

[Pulse500]

Originally Posted by Chhanda Das

From what I remember from researching an earlier DIY project involving DIY fans (link below), isn't the RS-775 motor rated at 24 volts (maximum) ?

I am pretty happy with 4S to power the RS775 clone for drilling/cutting in hobby projects which primarily use softer materials (Aluminum profiles, wood, plastics etc) or with for small scrubbing/polishing pad.

Use of XT connectors was only a suggestion as I saw in the picture that you are using 11 cells in parallel and the BMS supports high current. BTW, I also use jugaad parts/connectors when specific / optimal parts are not available readily or too expensive.

I personally use identical cells in a single battery (Same battery chemistry, protection, capacity, and where possible, same manufacturer/batch. Best to avoid mix/match to prevent asymmetric charging/discharging and for safety reasons.