There are several factors that damage the lithium batteries and make the capacity drop. Its not just one factor, but several and they all add up individually. While heat is the worst, charge level is still another factor and so is deep discharge.
Temperature 40% charge 100% charge
0°C 98% (after 1 year) 94% (after 1 year)
25°C 96% (after 1 year) 80% (after 1 year)
40°C 85% (after 1 year) 65% (after 1 year)
60°C 75% (after 1 year) 60% (after 3 months)
Here is a table I cut and paste from the battery university page. As you can see here a battery with a lower SOC lasts longer than one with a higher soc. Their text says:
"Table 3: Estimated recoverable capacity when storing Li-ion for one year at various temperatures. Elevated temperature hastens permanent capacity loss. Not all Li-ion systems behave the same."
So therefore there is no reason to plug in the car all the time if you are not going to be using it right away. Of course if you need it tomorrow then charge it up, but if you don't need it don't plug it in on Friday evening and let it sit 100% until Sunday for example when you go out. Think average charge level over the years needs to be as low as possible, not as high as possible. If you plug it in all the time then its as high as possible, and that's one X bad mark for your battery.
2nd X bad mark is for it to get hot or overheated. This could happen with it parked in the hot sun God forbid fully charged or from you driving it in the summer and constantly recharging the battery for more because you don't want to burn a drop of fuel that day. That's the worst X bad mark for the battery.
Third, over time cells do wear out and some a little faster than others. Those cells will drop quicker in voltage and they will get heated more and puff if you deep discharge the battery. That's because while other cells are holding the voltage OK, one or some are lower and they will drift less voltage and go below minimul levels which will damage the cells further. Once you have damaged cells in the pack, its going to be as good as the worst cells because they are all in series together. So to prevent some of this I recommend that you do NOT use the battery completely, a 20% SOC should be the limit meaning you should leave about 1 to 2% in the HVB battery on the MFM screen display and switch into EV later before you go into hybrid mode automatically. They will prevent the real deep discharge levels that could bring the cells down to 15% SOC. I noticed the cells will start to sway away from each other once you drop below 20% SOC. (balance wise). The newer your battery the less this is, the older the worse it will be.
And finally, you should recharge the battery if its drained down to the hybrid portion or close to it, say 20% SOC, back up to at least 8% charge level on the MFM screen before letting it sit immediately when you stop driving as when the battery is hot the voltage will be higher and when it cools its going to drop. You don't want it to drop to dangerous levels (below 15% SOC). Conditions change depending on how hot the battery is as to how much it can drop when resting. Last night for example my battery was 75F when I got home and at 19.5% SOC (I went into EV later at 1% but it still uses some battery below the set point). I charged it back up to 28.2% SOC which was 9% charge level and this morning its at 6% charge level on MFM. I just looked at the battery temp is now 64F and its at 25.6% SOC. It dropped about 3% SOC which isn't that bad but it can be worse. One time I had it drop when I parked to eat without immediately charging after the hybrid battery was in use (somewhere around 18% SOC I think) and when I came back to the car the SOC was 9.5%, the engine started immediately when I powered up the car!!! BIG MISTAKE on my part, damn, I learned something from that day.
Also when you do charge it use the 120v slower charger instead of 240v. It takes longer, puts less current into the battery which does not elevate the temp as much. In fact the HVB cooling fan runs and the temp will drop while you're charging faster. Last night it dropped from 75F to 71F while I charged it on 120v for 43 minutes. If you use 240v to do this, you would only charge it for 15 minutes to get the same charge amount and the cooling fan for the HVB would not run as much (doesn't run while not plugged in and doesn't run while not actively charging meaning plugged in and waiting to charge) so 120v is much better to be able to run the cooling fan longer and drop the battery temp as again, a lower temp is better than as higher temp and you want to get there quicker to the lower temps.
What I do is peanuts here and there, but they all add up. You can just be a person without care and plug it in all the time so its always full and always ready to go, but I guarantee that you will be noticing capacity loss quicker over the years. We all are going to lose capacity, time and recharge cycles also lower capacity, but why add more loss due to heat, SOC, and depth of discharge if you don't have to? There are things you CAN do to minimize it, I do those things because I love this car and want to keep it forever if I could.
I try to make very meaningful posts, please read this, and read it once again any questions let me know.
Edited by rbort, 01 November 2018 - 09:04 AM.