We have a 2016 i3 and were thinking of topping up the battery every night - but are worried about damaging the battery. (we do not have any special power outlet installed.
I hate answering this question but I'll give you my (I hope) properly nuanced answer.
BMW says go for it. BMW says plug it in and charge it to 100%. BMW gives you the owner no tools to assist you in stopping short of 100%. And you'll find plenty of i3 owners who agree and are quick to share the "ABC: Always Be Charging" meme.
I answer with this: every other EV manufacturer and battery maker recommends 80%. Charge to 100% when you need it for a trip, but best practice is 80% – or maybe a little more, or maybe a little less. Among those? BMW, who post-i3 gives their users the ability to stop charge at 80% and even limit their charge to lower amperage then what's available.
The ABCers counter, "but the i3 has a buffer, so 100% isn't really 100%."
No, it's more like 90%, but that's not 80%, and other EVs have buffers, too.
BMW first began selling the i3 in 2014 and to me it's preposterous to think that 10 year old i3 batteries and battery management has a secret sauce that today's batteries and BMS systems don't. Even BMW seems to acknowledge that with their newest crop of cars.
That said, are you going to "damage" the battery by chaging to 100% daily? No. But I'd expect it would degrade more quickly than if you were to limit your charge – even if just a little.
Maybe it's not enough degradation to concern yourself with. All batteries degrade with use. Maybe it isn't worth the hassle. Heck, a battery degrades just by being a battery. But if you want to hedge your bet and get the longest life out of the car, it certainly doesn't hurt to limit the charge to 80% through delayed start times or variable rate charging.
