Why is putting batteries in parallel a bad thing?
Discharging NiMH or NiCD in parallel is not a problem. Charging however is. When Ni based batteries are being charged their voltage rises until they are nearly full, then the voltage drops. ‘Smart’ chargers take advantage of this to detect when the batteries are full. It’s called negative delta V detection. If two strings of cells are in parallel while being charged and one string fills first its voltage will drop and ‘hog’ all of the charge current. This will result in overcharging/overheating.
