Why did my 12v battery charger blow, when connected to a 12v battery in a 48v bank?
Charger MUST float with respect to battery overall negative terminal.

Floating charger MUST have enough isolation to withstand battery voltage.

Charger may be unable to handle situation where battery voltage being charged is &gt; charger voltage. (Some can, some can't).
Adding a diode to charger output MAY help. This will probably cause charger regulation to be poor or worse, but if you are manually balancing by eye this may be acceptable.