Great read and your knowledge seems to be tempered with a fair degree of wisdom.
Now for the unwise question. What would happen if I connected a 12 volt charger to one battery in the middle of my 48 volt battery bank? Would the input equalize over the whole bank or would I be damaging the charger and/or the batteries.
Currently the batteries are connected to an UPS.
First, it would depend if the individual batteries are 6-volt or 12-volt that make up the 48-volt battery bank.
Assuming they are 12-volt and you connected a 12-volt charger to just one battery, the current would flow just through the one battery as this is the only complete circuit. The balance of the battery bank does not complete a circuit that would “circle back” around to the charger unless you connected the positive and negative leads together and that would be a very very bad thing – melted cables and possible battery explosion!
However, I don’t know why you would want to charge just one battery. Lets assume you charged this single battery to a state of charge higher than the other batteries in the battery bank. As soon as you connected the battery bank to a load, this would immediately “draw-down” the higher charged battery because all the cells will try to reach the same voltage level.
This is what happens when you equalize a battery bank. Any already fully charged cells will “boil” off gasses until the lower charged cells reach the same level of charge. I see no reason to do this unless your 48-volt battery bank is not being charged and you only have a 12-volt battery charged and are trying to charge each battery separately.
Its not good to have any battery in a string that is charged much higher than the others.
Hope this helps,