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Charging to 80%

Posted: Fri Jul 02, 2021 5:39 pm
by james_scott22
Interesting having plugged the car in that it recommends only charging to 80% and will offer to cap it there. Wonder if this is for charging efficiency, because that last 20% takes a lot longer, or to be kinder to the battery....

Re: Charging to 80%

Posted: Fri Jul 02, 2021 7:52 pm
by H_uue
It's mainly battery life. Avoid going to the bottom 10% or above the top 10% is good advice. Save the 100% charges for longer trips not day to day.

Re: Charging to 80%

Posted: Fri Jul 02, 2021 8:53 pm
by james_scott22
Didn't know about the fully draining not being good either, thanks for that. I've fully charged it this evening, mainly to see what I get as first return off a full battery

Re: Charging to 80%

Posted: Sat Jul 03, 2021 9:22 am
by RichR
I posted a longer explanation a week or two ago, but this is normal for all lithium batteries (such as phones and laptops too). The battery management unit needs to balance the charge in all of the cells, which takes a little bit of time to allow them chemical reactions to settle. If you charge slowly then this isn't an issue - so when you charge on AC at home at up to 7.2kW, you can safely charge to 100%. But if you charge at 125kW, the battery will get hotter and the cells have less chance to balance. So it's always recommended to stop at about 80% to prevent degrading the battery's overall capacity.

There's a buffer at the bottom too so that when the gauge shows 0%, there's still enough power left for the car to drive a bit further but more importantly keep the various coolant pumps running to prevent battery damage. Obviously it's very much not recommended to drive an EV until it stops, but even if you do, it won't really do any damage if it's then recharged nice and slowly - the BMU will limit it anyway as its purpose it to maintain battery health.

You'll notice that you can charge at a higher rate if the battery is warm and low, but if it's cold and more than half full you won't get the full rate the charger can supply, because the BMU will be limiting it to help the cells balance properly. Again - phones and laptops do exactly the same, but it's not always obviously that it's doing it.