no hot like hell....but its more than warm.....
This generally means you're charging them too fast, or your charger is overcharging them.
In either case, if the battery gets hot, unplug the battery, let it cool down, maybe start charging it again.
Your charger might not be working properly also... or amperage setting might be too high. If the charge speed is adjustable, set it so that it's charging at 1C or lower. (for a 1000mAh battery, 1C is 1000mA, for a 4200mAh battery, it's 420mA... so on and so forth)