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Understanding Battery Capacity Loss

How to guard against early capacity loss

Avoid deep discharges. As mentioned above lithium ion batteries do not like to be frequently fully drained. Once in a while won’t hurt, but you don’t want to be rolling into your garage every night with the state of charge under 5%.
Don’t leave a fully charged EV sitting unused for long periods of time. While charging to 100% daily isn’t really a problem, if you are not going to be using the car for a while, like days at a time then it’s best to leave it at about 80% charged. A typical example would be if you were going away on vacation for a while. In that case, don’t fully charge the car before you leave. It would be ideal to leave it between 70% and 80% charged until you get back.
Avoid excessive fast charging. The BMW i3 will have the capability of charging on a DC quick charger which will charge the battery to 80% in about a half hour. While the batteries are not damaged by quick charging process, they can be damaged by the heat created by fast charging. Unlike the Nissan LEAF, the i3 will have a complex thermal management system that is liquid based and its sole purpose is to keep the battery at safe operating temperatures to prolong the battery life and extend the cars range. This system will definitely allow you to fast charge more often without damage then if the car didn’t have it, but most industry experts still warn against consistent use of fast chargers. The science hasn’t really proven this one way or the other just yet, as DC quick charge is just beginning to be available to EV’s, but I would prefer to err on the side of caution and only use DC quick charge when I really needed to. I’m sure a few times a month won’t have any adverse effects.
Don’t leave the car parked in a hot parking lot in direct sunlight if possible. I’m not suggesting you constantly hunt for a shaded paring spot when you run to the shopping mall, but if it is an extremely hot day(90+ degrees) and you’ll be leaving the car parked for many hours, it would be wise to find a spot where the car isn’t baking in direct sunlight. One of the biggest enemies to the li-ion battery cells is heat. The ideal temperature for the battery is 68 degrees Fahrenheit and as the battery temperature rises to about 90 degrees the cells begin to degrade. Once the battery temperature exceeds 105 degrees there is definite cell damage and capacity loss. I have only witnessed such a high battery temperature twice in my ActiveE since the thermal management system is constantly working to cool off the batteries when it’s hot out. I suspect the i3’s thermal management system will work even better since it’s been engineered and refined for about four years now, and the ActiveE’s system was only designed to be used on a short-term test car. In fact, if you look at the above graph you can see a period where the capacity dropped rapidly. That period was immediately following the summer of 2012, when I experienced my highest battery temperatures. I can’t say for sure whether or not that is directly related to the rapid capacity loss, but I do suspect it played a role.
If you don’t need all the range the car can offer on a daily basis, then don’t fully charge it every night. I know above I said it’s not a problem for daily use, however if you don’t really need to then it’s better not to always fully charge to 100%. I may be nitpicking a bit here and others may say it’s not a problem, but if you know you only drive 30 or 40 miles a day commuting, then there is no need to fully charge your EV if it has an 80 -100 mile range. You can set it on a timer to stop charging before it’s fully charged or use the feature that many EV’s have which allows you to set the amount of charge the car accepts. You can charge to 80% daily and then set it to fully charge on the occasional days you need more range. I wouldn’t really worry too much about doing this, but if you are a low mileage driver, then it certainly won’t hurt.



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