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Battery balancing Technic

Derek Kessler

Active Member
Apr 15, 2016
1,193
1,876
Cincinnati
1) The battery is always balancing itself. But.. the calibration of the range meter may be off after 30k miles. You can get it to recalibrate by draining the battery down close to zero.

2) Supercharging is more damaging to the battery than even high-amperage home charging. It takes a lot of Supercharging to hurt the battery, but if that's the only way you're charging it would be a problem for the battery pack's health. If you can charge at home, that's recommended for battery health, your own convenience, and not taking up a Supercharger stall that somebody driving a long distance could use. Outside of California that last point isn't a big problem, but as Teslas become more common it will be.

3) Not mentioned in your post, but constantly charging to 100% will also damage the battery. Tesla recommends 90% for day-to-day driving, though I set my S at 60-70% when I know I'm not going to need all that range.
 
1) The battery is always balancing itself. But.. the calibration of the range meter may be off after 30k miles. You can get it to recalibrate by draining the battery down close to zero.

2) Supercharging is more damaging to the battery than even high-amperage home charging. It takes a lot of Supercharging to hurt the battery, but if that's the only way you're charging it would be a problem for the battery pack's health. If you can charge at home, that's recommended for battery health, your own convenience, and not taking up a Supercharger stall that somebody driving a long distance could use. Outside of California that last point isn't a big problem, but as Teslas become more common it will be.

3) Not mentioned in your post, but constantly charging to 100% will also damage the battery. Tesla recommends 90% for day-to-day driving, though I set my S at 60-70% when I know I'm not going to need all that range.
Thanks for the info.
 
1) The battery is always balancing itself. But.. the calibration of the range meter may be off after 30k miles. You can get it to recalibrate by draining the battery down close to zero.
...

I understood that battery balancing happens with the SOC (state of charge) above 93%. I don't think that's changed? I generally charge to 100% once a year to make sure the battery is fully balanced, as I know others here do.

It would be a nice improvement to have balancing start at any SOC, after charging has completed.
 
That was my understanding as well based on what the CAN bus data shows - balancing occurs >93% state of charge. The service center has also mentioned the battery cells can become "unbalanced" if you're only using and charging in a small range day after day, such as 50-70%.

That being said, what is the negative impact of having the battery pack out of balance? The only thing I can think of is obviously not having a consistent voltage across all of the batteries such that some batteries end up charging to a higher voltage. While very minimal and likely not a factor for 99% of users, I suppose that could cause slightly faster degradation on some batteries (i.e. not even "wear").
 

Rocky_H

Well-Known Member
Feb 19, 2015
8,357
10,754
Boise, ID
That being said, what is the negative impact of having the battery pack out of balance? The only thing I can think of is obviously not having a consistent voltage across all of the batteries such that some batteries end up charging to a higher voltage.
The main negative thing that can happen with this is the artificial shutdown behavior. The car will trigger a shutdown based on a cell voltage dropping below a certain point. But displayed charge level, in either % or rated miles, is based on the overall average readings of all of the cells. So there have been cases where people get pretty far out of balance and didn't know it, and then they get down to 3 or 4 rated miles left, and the car says, "I'm done--shutting down now", because one of the cells dropped too low while most still had a little energy left. So that's why it's a little risky for people to assume it's always going to be perfectly accurate down to that last 1 or 2 left on the display.
 
Recently I got to that point "shutting down now" - and the car was indicating 6%!!! (I was also 500ft away from home... so close!)
2014 MS P85D, 80k miles. I think I've only ever twice charged over 90% - I'll do 95% occasionally now that I know that balancing is triggered there.
I have 2 bricks that are currently consistently showing significantly lower voltage (20mv to 50mv) than the others in the pack (which are all within 3mv of each other).
 
Battery balancing is going on all the time. You have bricks made up of hundreds of cells connected in parallel through small resistors. How could you prevent balancing under that configuration? That's part of the story. The rest is that you have a bunch (around 90) of these bricks connected in series. Charge and discharge current flows in series through this string. If one of the bricks charges or discharges faster than another then the cells in that brick can be subject, at either end of the SoC range, to damage which damage, in Li ion cells can lead to disaster (fire) so you can be pretty sure that the BMS monitors the voltage on each brick. I have no ideas of what the actual BMS mechanism might be in these cars but what it attempts to do is transfer excess charge from the higher charged bricks to the lower charged ones. There are various efficient mechanisms for doing this of which the "flying capacitor" scheme at least has the coolest sounding name.
 
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