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-   -   HV Critical Alert in Tata EVs (https://www.team-bhp.com/forum/electric-cars/273808-hv-critical-alert-tata-evs.html)

We all probably know by now that HV critical alerts are a potential occurrence for TATA EVs , I am not sure if this error has ever been reported by other EV manufacturers. To date all that TATA does is replace the pack under warranty as they( local dealership) do not have the expertise to fix the issue. I am at 12k kms on my Tiago EV and almost worried each day when I may face this dreaded issue.

Many have suggested that its a local cell issue. But why does a single cell going bad have to strand a EV and make it unusable for weeks ( battery packs are not off the shelf items at the TASS ). TATA so far has not provided a proper explanation for why this occurs, how it can be avoided or why the BMS cannot just bypass the one cell and still keep the vehicle drivable.

Opening this thread so all HV alerts can be consolidated here and here is to hoping some one from TATA reading this and takes this matter seriously. I feel we are all sitting on a ticking clock and never know when it may bite us.


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My new Nexon EV LR has done 4000km and I have not faced this HV Critical error. Or any other battery or drivetrain or charging related bug. I’ve also done 6 back to back DC fast charging sessions on an out station trip. Car continues to perform as expected.

My observation is that these bugs have been limited to the Tiago and the old Nexon EV (Prime). The smaller battery applications. The old Nexon EV Max was niggle free, as is the new long range model.

That's an interesting observation, my Tiago is 12K done in 6 months, I have only seen the alert once during a DCFC session, but it never hampered my trip, also I have done countless DCFC sessions since then and many road trips, its just the fear of "when".
Where as my friend who owns a Tiago EV bought 3 months ago already had the batter replaced due to the HV alert and car is just 3000km old

Yes, there are HV error issues in Tata cars - Tiago specifically. Based on my experiences in the EV group, Tata HV error is a generic error for any kind of High voltage architecture related anomaly detected by the car. It can be hardware or software. It is more like a windows blue screen.

The reasons for HV errors that I have observed so far are

1. Improper charging practices - Majority of the issues are arising out of this. SC is now educating owners on charging best practices.
2. DC charger related issues - Some chargers specifically BPCL, HPCL, IOCL ones don't slow down at higher percentages causing software to throw an HV error.
3. Battery issues or other component issues as well.

I don't know why people are worried about point 3, the battery or electric motor failures. The warranty is 8 years and 1.5 lakh km which is more than enough according to me. And Tata is very receptive is replacing these components. Yes, the only thing that can be a concern is turnaround time.

P.S. - A birdie told me there is a new BMS being tested which is going to make it easier to solve/analyze some of the HV errors and even preempt HV errors caused due to improper charging or cell balancing issues.

I have not got any HV errors so far. Close to 1 year ownership (+20K Kms) of Nexon Max and mostly done only slow charging. Have done fast charging 4 times and the longest trip was Palakkad-TVM-Kanyakumari-TVM-Palakkad. I have always charged till 100% SoC on slow charging. Car has been niggle free so far.

However a distant relative of mine has a Tiago which has stopped fast charging (charge only goes up by 2%). TASS in mostly clueless is what I understand

Too soon to say for me (800km on odo). But I can believe improper charging practices is a contributor.

Though the Tata manual clearly says slow-charge to 100% whenever possible, GTO's parked first post on this subforum still says "For city commuting, I doubt anyone needs a level over 80%." He has removed an earlier statement that "Fact = many informed EV owners intentionally don’t go over 80% charge levels to preserve battery health" but retained a quote from another poster saying one "should charge till about 80% for battery health."

I wonder how widespread such misinformation is. An LFP battery's voltage-SoC curve is very flat between 10% and 80%, meaning if you maintain the charge in that range throughout, it is very hard for the BMS to accurately estimate the SoC. For the BMS to have a good estimate of SoC you MUST charge to 100% (slow charge) whenever you can. The manual also says, after 4 fast DC charges do a slow AC charge to 100%. I wonder how many of the HV critical alert cases are due to this not being followed.

I think it came from the early tesla cars which were predominantly NMC where it is fine to charge till 80%. In think tesla manual also said that. Also there is unwanted fear on the life of the battery getting affected, which although true is irrelevant as the battery pack will most likely survive beyond the car. Better SoC prediction is a more relevant thing.

To be doubly sure I checked the BYD Atto 3's manual (another LFP based system) which also suggests charging till 100%. But interestingly the Mahindra XUV 400 manual also suggests charging till 100% even though it is an NMC system (there is more nuance but essentially if you use regularly keep slow charging till 100%)

https://bydautomotive.com.au/brochur...dbook-2022.pdf

https://storagewyh.blob.core.windows...e-139-5B0FFD3F

As an aside I liked the fact that XUV400 manual seems to be available in html and you can link directly to the section.

I hope the BMS would be a SW update and not limited to new cars only

Yes it will be a SW update and it should be for all vehicles sold already. That is what i heard from a credible source, rest we should wait and watch.

I have a Nexon EV (Prime) 2021, driven for 24k km. Haven't faced this issue so far. I think it depends upon the charging pattern of a person, i charge my car to 100% almost every time. Hardly 1-2 out of 10 charges are not upto 100%. Once a month I try the under 20%-100% charge.
I'll be giving my car for the 2 year service in 2-3 weeks, will explicitly ask the service centre to give me the battery state of health (SOH). Will update here once I get it.

It will be interesting to know the SoH. Also I hope there will be some ObD tools that can help us get the information ourselves. Lots of western EVs support OBD2 and provide SoH data

I faced HV Critical Alert probably 20 times on my Tiago EV (started at 7000 kms and got resolved by10,000 kms). But it used to disappear on its own in 10-15 minutes and the car would start. But the alert came while driving on the road and the car would just stop in the middle of the road so it was a dangerous issue. At 2 occasions, the alert did not disappear and the car had to be towed.

Tata ASC was quite helpful and they changed the MCU first which did not resolve the issue and then they changed some wiring harness after which the car has done 10,000 kms without any issue. The only problem was that the car was at the service center for 2-3 weeks. I was reimbursed cab fare for my office commute as they did not have a loaner car to give.

Probably mine is the only case where the problem was resolved without changing the battery? The link to the detailed issue of my car is already in the first post.

My car has done 20,000 kms now.

Well, no one likes to be stranded in unusual hours or in urgent cases, battery alone costs 50% of car cost and it's motor costs another 25-30%. The worry is natural when technology is comparatively newer than ICE tech.

I agree it's all covered in warranty but still not enough

I had an HV Critical Alert which was apparently unrelated to any charging method or issue. Per the Service Advisor, it was a fault with the AC compressor, which then got replaced in warranty. Post that, it's been almost 1 month and 500km - haven't had a recurrence of the HV Critical Alert again.

I do feel the range has dropped a bit though - still testing out whether it's traffic/driving characteristics which are causing the drop in range and whether there is an actual drop in the real range.

Does anyone know are the critical HV alerts issue only with Tata EV vehicles or is it with others as well? Same issue but the naming can be different.