90% the new recommended SOC?

[JRP3]

Essentially, the BMS top balances the pack by calculating how much power needs to be drained from each group of cells, and goes until it has drained the requisite amount from each. It doesn't matter if charging stops during the process, or if you drive 100 miles while the balancing is happening.

That doesn't seem possible, presumably the BMS is reading cell voltage to engage resistors to bleed off excess charge. Drawing power while driving would cause voltage sag and throw off the voltage reading.

 

[tga]

That doesn't seem possible, presumably the BMS is reading cell voltage to engage resistors to bleed off excess charge. Drawing power while driving would cause voltage sag and throw off the voltage reading.

I just assumed that the BMS calculates the number of Wh's that need to be bled off, and constantly monitors current flow through, and voltage across, the bleeders to keep a running total. The math isn't that hard. But @wk057 could give a definitive answer.

 

[JRP3]

I just assumed that the BMS calculates the number of Wh's that need to be bled off, and constantly monitors current flow through, and voltage across, the bleeders to keep a running total. The math isn't that hard. But @wk057 could give a definitive answer.

True if it's a Wh calculation and not voltage that would work.

 

[Paul73]

I'd prefer to charge to a selectable cell voltage (e.g. 3.9 V, approx 60%?) instead of percentage, and show a graph of both the lowest brick voltage and the highest brick voltage vs time.
If only needing a small daily topup for short trips, 3.9 V is probably optimum for the battery life (sudden collapse after some number of cycles from oxidisation, regardless of gradual degradation - many many more cycles before collapse by not charging >60% when not needed)
Hopefully the pack is always being balanced even if you never charge to a high soc?
A voltage graph will look noisy in use with charge and momentary peak discharge currents, but that's ok. I'd like two selectable guidelines to indicate the voltage window you want to usually stay within.
Displaying cell temperatures would also be good (Jeff Dahn emphasized how important temperature is --

Lucky in the UK, the average temperature is about 10 Celcius over the year.

 

[wk057]

The bleeder resistors are toggled off during voltage reads. It takes a negligible amount of time to disable the bleeder, read voltage, and re-enable the bleeder.

 

[JRP3]

The bleeder resistors are toggled off during voltage reads. It takes a negligible amount of time to disable the bleeder, read voltage, and re-enable the bleeder.

So then I'd assume there is no balancing going on when driving, correct?

 

[wk057]

So then I'd assume there is no balancing going on when driving, correct?

AFAICT, only gross imbalances cause the bleeders to remain enabled while driving. Otherwise, only when "off" or charging.

 

[calleserra]

Shallow charging doesn't hurt the battery. It may confuse the system's idea of what the capacity of the battery is, but it doesn't hurt the battery.

You are spot on. When shallow charging (Tesla's recommendation per the "always plugged-in" concept) confuses the car's estimation of the range which in most cases tend to show a lower number, most people get confused by it into thinking that the battery's actual range had gone down.

 

[calleserra]

Second this. Tesla batteries are not magical, nor do they defy physics. They have significantly reduced the negative impacts of temperature and high voltage/charge to the point where it's pretty inconsequential where you set your charge setpoint. This is why Tesla is now recommending 90%. It's because you get the most utility out of the range, and I guess in some cases, it allows the car to give you a better SoC reading.

The reality is that the lower the voltage, the longer the battery will last, but there's a tradeoff between utility and longevity. I have a laptop battery that is now 12 years old and still in near perfect condition because I kept it between 20% and 35% SoC. This worked out fine because the laptop was never mobile and was plugged in 100% of the time.

I get you and I'm with you. It's just that there are folks who either work from home, or retired, or have five cars that tend to drive the Tesla just maybe twice a week to go to the corner store. Charging and keeping it at 90% might be less beneficial overall than, say 70%.