I don't know why. I'm new to all this and don't know. I figured if I set the 24v battery pack up with a bms the way it shows to do it in parallel I'd be pushing 24v towards a 12v charger. Didn't think that could handle it. I did ruin my charger doing this. Have another here Monday. But that was on me. I know what I did wrong.Why would you need a separate 24v battery charger? Set your power supply to 3.5v per cell (28v), let the BMS do the rest.