Strange battery problem in my Tata Nexon EV

I had parked the car at 45% and couple of hours later when I switched it on, it was at 20%!

BHPian josephraj recently shared this with other enthusiasts.

Strange issue today. I had parked the car at 45% and couple of hours later when I switched it on, it was at 20%! Home was 24km away. Reached with 12% remaining but 25% drop for no reason is scary!

A week ago, I had stopped the car at 52%. After a few hours, when I started the car it showed 6%, connect charger! I restarted the car and percentage was back to 52%!

Not sure, if I have a battery issue. Car is less than a month and 2000 kms old.

Here's what BHPian Shresth_EV had to say on the matter:

Just searched a little on Google, you’re not alone. Few other owners are also facing this SoC drop issue.

PlugInIndia Nexon Thread

Looks like some of it gets mended itself during charging process in next cycle (one gentleman says that it jumped 8% during charging) after the SoC drop occurs and you put vehicle to charge. Could be the cell balancing like others note.

Some part of the said SoC drop is undoubtedly from vampire drain of electronics, just like all batteries lose some charge even when idle — so that’s that. If it’s double digit, it’s probably the cell balancing.

It’s also been noted that software had to be downgraded for owners. I suggest you check the link to that thread out. NexonEV Owners club also responded same to some users who took to Twitter for the issue.

Seems like a cell balancing/OS issue.

BHPian josephraj provided an update:

Well, it looks like some issue with the BMS. In the next charge cycle it jumped from 67% to 95% in about 30mins (slow charging at home)!

The BMS is fairly complex, if one of the cell drops voltage then it will show that percentage.

So, it is either of two cases:

• One or more cells becoming weak (highly unlikely since it is a new car)
• The BMS software has a bug (this is more likely now that I know others are facing the issue after the update)

The warranty on battery is 8 years. That is a bit of a relief but it looks like a lot of owners facing this issue and Tata is still firefighting.

Here's what BHPian mayd85 had to say on the matter:

0% indicated charge (on dashboard, connected car app or anywhere) does not mean completely discharged battery. Similarly 100% indicated charge does not mean 100% charged battery. Indicated 0 to 100% is mapped to something like 20 to 80% or 15 to 85% of actual SOC on the battery. These buffers are kept for protecting the battery pack from deep discharge as well as over charging condition. Imagine a near full battery pack and regenerative braking trying to pump in more charge in the battery pack and the pack is overcharged, thus damaging the cells. There are complicated algorithms and strategies which decide this mapping and control the operation of the battery pack in order to:

• Always operate the battery pack in a safe operating region (of its operation)
• Maximize SOC utilization
• Maximize battery pack life
• Protect from extreme temperatures etc.

A lot of experimentation goes in defining these control strategies based on battery chemistry, pack size, etc.

Check out BHPian comments for more insights and information.