I'm working on a project to make my own super-bright bike taillight using an emergency flasher LED (something like this:)
The LED package requires a 12V input, but from what I've read from other people who've done this before, it will still work with as little as about 8V. It also uses a 12V momentary charge to switch flash patterns.
I'm planning to use 3 18650 lithium-ion batteries in series to power the light. Each battery is 3.7V, so I should get 11.1 V total, which will (supposedly) be sufficient. I'll put them in a 3-battery holder like this one:
This should be easy enough to set up, but I'm wondering how I should go about charging the batteries. I haven't been able to find 18650 chargers specifically designed for 3 batteries. I've found them for 1, 2, and 4 batteries only. My (limited) understanding of lithium batteries is that all batteries that will be used in a single device should be charged together so that the cells can be balanced, and that if this isn't done, it poses a fire risk. I'd like to avoid having a fire erupt between my legs while barreling down a hill at 30 mph.
Would it be safe to use three 1-battery 18650 chargers, if I charge them all for the same time? Do they actually need to be balanced at all, or am I misunderstanding something? If they do all need to be charged together for safety purposes, then is there any way for me wire it up to get 12V out of 2 or 4 batteries, which I would then be able to charge normally?