If he's bashing with it in the grass and such that puts more load on the motor increasing it's amp draw. Long story short, let them cool before you charge them and if they don't get hot, warm is fine, while you charge them they should be fine. Basically the motor is putting more load on the battery than they can really handle which creates heat, internal resistance, capacity, amp draw, blah blah blah. Any way if you can still hold it when he's done they should be fine and I personally wouldn't worry but that's me.