I had a battery go bad in a fire alarm system. It has two 12 batteries in series to make 24 volts.

Because it was at night on a weekend, I took one battery out of a emergency light and installed it. The combined voltage was 24 volts but when connected and being charged the voltages were 14 volts across the charged battery and 10 volts across the discharged battery. So, I had to rob a second battery out of another emergency light.

It made me wonder:

If I install two new batteries and they do not have equal charge, like one being fully charged and the other discharged, will the charged battery prevent the discharged battery from charging?

Before I install the new batteries, would it make a difference if it let them sit in a parallel connection for a couple hours so the charges balance out?