Battery drop is determined purely by defeating enemies, which is how we tried to word it in the tutorial. By explaining it that way we hope it encourages people to rush and defeat enemies (especially when their battery is low/empty) and not worry about whatever is happening under the hood, which is why you see a lot of people refer to it as driving
aggressive play style. One thing to note is that there is no RNG in the entire game, all spawns/drops/everything is deterministic, that's the only way the game can be fair.
There are a few helpers already at play with the battery though, for starters its magnetism is drastically increased if you are about to run out of battery so that you have less frustrating moments where you die with a battery 2 meters from your character, and they also can move over small gaps to be collected in that phase. Batteries drop so frequently that I'm not too keen on adding any special gameplay mechanic to collecting them (ala slow-mo) as I think the pacing of the game is very important, you'll see some slight tweaks to the system in the next patch but the one thing I desperately want to avoid is nerfing the system to the point that it isn't giving us the benefit of pushing aggressive gameplay.
I hope this isn't taken the wrong way but players failing a level by running out of battery is intentional design, it is the only way to fail in the game (As we have no lives) and I believe the game would be poorer if you just walked through the levels by holding fire and spamming to get back up with no consequences.
Btw thanks for the detailed feedback Haz, quite a few useful bits in there
Edit -
Yep, there are at least 6 new enemy types that will need to go in to the game.