Battery drain issues on my Mahindra e-Verito: Here's how I solved it

Its been 4 months now and all the issues and niggles have been solved.

BHPian WD-42 recently shared this with other enthusiasts.

Hey, I have an e-Verito. I've faced a very similar problem as you with my car - Tata Tigor EV breaks down twice in 3 days.

The most likely cause is that your Aux battery has failed and is now Shorting the entire circuit, thus causing a quick discharge of your HV battery.

HV high discharge protection system is kicking in and trying to kick your car into safe mode, however, the terrible firmware is probably not catching the errors correctly and tripping everything.

My suggestion

Replace your Aux battery with a new battery and then charge the car overnight to 100%.

My experience

I had been facing an idle discharge/vampire battery drain of 12-15%. The dashboard would sometimes light up like a Christmas tree and the car would go into either eco or safe mode.

Took the car to Sireesh Auto Singasandra, Bangalore. They charged me ~2500 and said nothing wrong, we had to update the software which we did, and you will not face the problem anymore. Came home, with no change and still losing 15% every day. Called up Mahindra Sireesh Auto Service Center to ask for an explanation, but didn't get one.

Left home one day with a 60% charge, and went to a supermarket 4 km away. When I returned car went into safe mode. Hard reset it and the car went back into normal mode.

Left home with an 80% charge, and drove to a supermarket 1 km away, when I came back car went into safe mode with an HV battery and Aux battery alert. I reset the car (e-Verito has a hard-reset button), and the warning lamps were extinguished but the SoC showed 0% while I had left home with 80%. Called Mahindra Service Center (Sireesh Auto, Singasandra), but they were beyond useless, the S.A. told me it is Saturday evening at 5:40 pm, we are leaving the service centre and going home and offered no solutions or help, and just told me to tow my vehicle home, and to tow it to the service centre on Monday morning.

Towed the car home using a local tow service because RSA was again useless. I had purchased the car directly from the Mahindra factory (the reason for Sireesh Auto's recalcitrant attitude in helping me). Called the factory sales agent, chap was helpful, and explained that he'd seen many cars have this problem, especially ones that had sat idle at the factory or dealer yard for months before dispatch/sale. Please replace the AUX battery and charge it to 100%, he said. I was a little afraid. My line of thinking is that the car "thinks" it has 0% while in reality, the HV battery is at 80%, what if the car tries to charge to 100% and overcharges? The agent chap assured me that he's done this multiple times and to try it and that the car once it reaches the real 100% will recognise and reset SoC to 100%.

A good friend of mine works in the EV industry as a battery engineer, so I had a quick chat with him, and his diagnosis was:

• The car either has a short/leak in the wiring harness. My friend's opinion was this is the worst-case scenario since it is very hard to diagnose and nearly impossible to repair unless you rip the whole wiring out of the car and replace it.
• HV battery has a dent or physical damage, thus losing capacity, can be determined by taking the battery pack out and examining it, but smaller dents while hard to find can cause such issues.
• HV battery is failing, best case scenario funnily enough, because it is easy to identify and replace.

Our joint conclusion was to tow the car to Mahindra since we didn't know what type of overcharge protection or issue the car had. Called Mahindra's Sireesh Auto again, and they said even if I towed the car there, they would just try charging it to 100% as a first try.

Called up my Mahindra agent again. He again reiterated replacing the aux battery first and said don't bother charging it if you aren't going to install a new aux battery.

So I decided to actually figure out what was going wrong. A 13% drop in a day is equal to a loss of ~1.8kWh of power. Broke out my ammeter and noticed the current draw fluctuating between 1 and 8 Amps across the 12v battery.

The big problem is the Aux battery showed 13.6-13.8V when I measured it with a Voltmeter, leading me (and a battery shop guy) to conclude that it was working properly. Under constant load, it showed ~12.5V. When I left the voltmeter connected and tried various loads it would momentarily for less than a split-second drop to ~8-10V, barely noticeable, but all the systems reliant on that would trip, shut down and reset completely.

When I had given the car to Mahindra-Sireesh for service, they too just checked the voltage and said the AUX battery looks good.

Replaced the battery, and charged the car overnight, the car stopped after charging 20%, and SoC reset from 20% to 100% (the original SoC was 80% before it shut down).

Result

It's been 4 months since I put in a new Aux battery. Having 0 issues. All of my safe mode issues, dashboard Christmas trees, and breakdowns are resolved.

Learnings

Things I learnt while trying to figure out my issue:

1. Most EVs use VRLA-AGM deep discharge batteries for the aux battery, which are normally used for either UPS/Lift UPS/Telecom UPS/Solar-P.V. applications. These aren't your typical SMF-SLA 12V car batteries.
2. EVs (from Teslas to Volvos to Mercs to Mahindras and Tatas) use the Aux battery as a UPS (Uninterrupted Power Supply) to keep the circuits on even when the car is off. See James May's struggles with his Tesla Model S' Aux battery here.
3. If the car hasn't been used for a while, or if the Aux battery is weak, the car will top off/recharge the Aux using the HV battery, thus draining the HV battery.
4. As the Aux battery degrades, either the charging current from the HV system just "passes through". The Aux battery effectively is just a big conductor at this point. As to why the chemistry degrades this way I'm yet to learn, or the charging system constantly tries to charge the AUX from the LV and drains the HV.
5. When the car requires a higher current from the AUX which the aux is unable to provide, the car systems that rely on the Aux lose power. If one of those systems is a memory system that keeps the current state (drive mode, SoC) etc. that memory is wiped and reset. Thus, the car can go suddenly from 90 or 80 or whatever % SoC to 0.
6. A new Aux battery will solve all these issues; however, the new battery will degrade in much the same way as the old one so this is going to be a cycle. I've instead just connected the 12v now to a battery tender to keep it topped up, so it doesn't drain the HV.
7. The main reason for this aux dying seems to be low running or a long break that would've drained the UPS-style battery. Apparently, on most EVs, the aux is recharged by the re-gen. And since it functions as a UPS battery it is continually discharged.
8. However, most batteries including UPS batteries are not designed for continuous discharge, with sparse charging cycles, but deep discharge followed up by immediate charging. Hence my choice to use the battery tender on the 12V aux.

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