Convert 36 V lawn mower from Lead Acid to Li Ion batteries - charging / danger
I wouldn't charge separate packs in series.

If you have a pack with series cells (if it is a 12V one, it must have series cells), the balancing electronics is built into it if it is any good. (As well as the emergency shutdown mechanism.) If it doesn't have it built into it, the battery will blow up in your face during usage anyhow, so why worry about charging :D

If you have separate cells you can charge them with, per se, a lab power supply, or with any other power supply on which you can set a maximum current and a maximum voltage. They won't explode.
The problem arises when there are multiple cells in series and one of them charges faster then the rest of them. That is the one which will blow up.

(But to be honest, in my experience, if the cells are from the same batch, the aren't likely to fall out of step with each other.)