With the 1.11 implemented car battery draining (which is way too high by the way), I already ended up a couple of times in the middle of nowhere with an empty battery.
As there is no way to tell the battery charge from outside and it's a pain to always open the hood to check the battery charge, I suggest implementing a simple "warning sign" in the standard car HUD, like in real life. e.g. a yellow/red battery sign popping up.

Suggestion: at 33% yellow, at 10% red