How often should we charge to 100%?

[Peter Lucas]

I've never needed to charge to 100% yet. I exclusively Supercharge to 90% as it's nearby and my rental townhouse doesn't have L2 charging.

Does only charging to 90% mean the same 10% of cells never get charged? How often should you go to 100% even if you don't need to?

How often should you charge to 100%?
Rarely. Only when absolutely necessary. And when you do charge to 100%, minimize the time that the battery sits there with 100% charge.
I've lost the reference, but a careful study of lithium batteries maximum number of duty cycles at various limits of recharge revealed that there is a sweet spot around 2/3 to 3/4 charged. This maximizes battery lifespan. Battery lifespan dropped off dramatically as you approach 100% recharging.

 

[sorka]

The two biggest factors that degrade batteries charged to the top are 1) How long you left it there. Charging to 100% and leaving it over night is WAY worse than starting a drive as soon as it hits 100%. 2) The hotter the battery is, the worse it is to charge it to the top. Charging to 100% when it's cold outside is far better than when it's 100F+. The warmer the battery, the more it expands as it fills up. The exansion causes anode cracking which causes degradation.

The amount of expansion on the rate of anode cracking is non linear.
This is also why charging from 0% to 100% X times is way worse than charging 25% to 75% 2 * X times and why EV batteries last way longer in cold environments.

There's a quote around from Elon saying that Tesla batteries in Alaska should last millions of miles.


....which I can't seem to find any more.

 

[David99]

Very low discharges are probably worse than 100% charges. That's why Tesla doesn't allow the battery to be discharged to true 0% but they do allow it to be charged to 100%. I think it's just as important to avoid very low discharges and avoid high power draw when the battery is low.

 

[sorka]

Very low discharges are probably worse than 100% charges. That's why Tesla doesn't allow the battery to be discharged to true 0% but they do allow it to be charged to 100%. I think it's just as important to avoid very low discharges and avoid high power draw when the battery is low.

The danger with low discharges is if a single cell in a string is exposed to reverse polarity. I don't see how in the worse possible case where you drive the car until it dies that this could ever happen since the pack will have at least a few kWH left. You'd have to get to near zero volts on a string for individual cells to have the possibility of going negative.