unless you run the batteries to almost 90 - 100 % drained, I doubt you'll see any heat from these batteries. Mine have never even gotten warm. It's amazing. The 3.2v charger takes the battery up to 3.65v and stops. I don't think it's a good idea to charge them to 4.2v. I've never had mine over about 3.76v . They say that the batteries last longer if you only charge them to 3.65v.