As battery capacity decreases, does charging energy also decrease? - Page 1
Forum section: electronics / general

author: johankj
author_rank: Contributor
timestamp: March 05, 2020, 08:49:51 am
content: More specifically, I drive a Nissan Leaf EV, and over the years the battery capacity has decreased (say about 25%). The pack is a 24kWh lithium ion type, and I guess most EVs use lithium ion.So my question is then, as the battery capacity decreases, is there a corresponding decrease in the energy required to recharge it?

author: sleemanj
author_rank: Super Contributor
timestamp: March 05, 2020, 08:51:38 am
content: Yes, (but not necessarily 1:1)

author: Rerouter
author_rank: Super Contributor
timestamp: March 05, 2020, 10:04:44 am
content: At the beginning yes, and then usually no assuming the same rate of charge, then finally back to yes.As the battery charges and discharges, you both loose some capacity in non-ideal reactions that are not easily reversible, and increase the ESR of that battery.As the ESR increases you start loosing more of the charging energy as heat, this is the dominant factor after the capacity starts to decrease,So right at the start, you have lost lets say 10mAh / 3600, and your ESR has only increased by maybe 0.1%, you will use less energy to charge it as the heat loss increase is lower. as it continues to degrade you hit a turning point where the ESR becomes dominant, and the heat produced during charging exceeds the loss in capacity of the cell. then as it continues to degrade the capacity towards 0 due to the heat. your controller will just drop it, so less energy.So possibly less energy overall than new, but depending on how much heat is being produced, it could be more energy used over the entire charging cycle.

author: johankj
author_rank: Contributor
timestamp: March 05, 2020, 11:55:01 am
content: Quote from: Rerouter on March 05, 2020, 10:04:44 amQuote from: sleemanj on March 05, 2020, 08:51:38 amThat was also my suspicion after mulling over it. And though it probably varies, ESR will increase with degradation in all types of batteries, irrespective of chemistry.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 05, 2020, 02:54:48 pm
content: Energy efficiency drops slightly because of increased DC resistance of the aged cells, but the difference is small - especially at low discharge and charge rates.Power loss is I^2*R, so by charging at half speed (half the current), the power loss drops to 1/4th, for double the time, so the energy loss drops to half. Same is true for discharge.Overnight home charging is slow anyway, so little energy loss happens here. If the battery is very aged and you are concerned about efficiency, it's better to drive less aggressively to discharge more slowly.But, if the car appears to perform normally, battery efficiency is likely over 95% anyway, so there is not much to gain.

author: Gyro
author_rank: Super Contributor
timestamp: March 05, 2020, 07:46:33 pm
content: Presumably, the increased ESR will also result in the cells reaching their charge termination voltage sooner for a given charge current. That probably also drops the amount of charge that gets into the battery.

author: johankj
author_rank: Contributor
timestamp: March 06, 2020, 09:09:31 am
content: Quote from: Siwastaja on March 05, 2020, 02:54:48 pmSo my takeaway is, when a rechargeable battery degrades with use:The reduction in capacity matches a reduction in energy required to charge the batteryHowever, due to increasing ESR/DC-resistance, there are slightly higher losses to heatQuote from: Gyro on March 05, 2020, 07:46:33 pmSo, would ESR make a good model for degradation?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 06, 2020, 09:52:11 am
content: Quote from: Gyro on March 05, 2020, 07:46:33 pmYes, this could happen, but it's only significant at fast charging when there is no time to top off, i.e., when you have to terminate early to get going.When charging overnight, the charger usually has time to do a full charging cycle. The charging current tapers to some minuscule value such as C/20 at the end, so even if the ESR has doubled or tripled from the initial value, the difference in stored charge from stopping earlier is something around 0.1%, likely.What you describe happens in significant scale whendischarging, though. Unlike overnight charging, you can't practically crawl at jogging speed discharging at C/20 or so. So you are forced to stop when the cells begin getting near the low-voltage cutoff limit, and that limit has to be measured at a somewhat higher current. Combine this with the DCR being significantly higher near zero state-of-charge, than with the full battery.Storing less charge comes simply from the loss of charge storage capability (Ah) itself. Storing less energy comes from the combination of having less Ah (dominant mechanism), and decreased efficiency (smaller effect, but not completely insignificant).To get this in ballpark real-world numbers, an "end-of-life" pack might have approx 70% of original capacity, 200% of original DC resistance, and hence, say, 96% average efficiency (down from the original 98%).Finally, charging energy accurately tracks the actual energy usage defined by the driving habits. Battery condition is a fairly meaningless variable here, of course, except when the condition is poor enough that you can't use the car for the intended purpose any more.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 06, 2020, 09:58:45 am
content: Quote from: johankj on March 06, 2020, 09:09:31 amYes, usually we are interested in looking at both decrease in capacity, and increase in DCR, because they don't necessarily happen at the same rate, although usually there is a strong correlation of both getting bad.Note that the DCR is highly dependent on temperature; if you drop the temperature from 20 degC to say, 0 degC, the DCR shoots up so much you could think the battery is End-Of-Life even though it isn't. This is, to some degree, self-regulating, because high DCR means higher losses, heating up the battery internally, rectifying the issue; as long as you have decent thermal insulation around the pack so that you can keep the generated heat in.So if you want to estimate state-of-health from the DCR, always do it at the same temperature, to the accuracy of a few degrees C.

author: johankj
author_rank: Contributor
timestamp: March 06, 2020, 10:50:20 am
content: Quote from: Siwastaja on March 06, 2020, 09:58:45 amAh, so that would explain why my EV range is so much lower at 0C compared to 20C.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 06, 2020, 11:27:09 am
content: Quote from: johankj on March 06, 2020, 10:50:20 amOne of the reasons. The biggest one is likely the heating, though.And while you could trade comfort for range, you can't completely stop heating, otherwise the windows will fog.You will require something around 2kW of heating, even if you turn it down to almost levels of discomfort. This 2kW drain will be more than, say, battery efficiency drop from 98% to, say, even down to 90%, which would be 800W of extra loss if driving at 10kW RMS consumption.