Looking at several places, a charge voltage of about 7.5 volts is just about right for a three-cell battery (6-volts). Many older charging systems were set to 7.8 volts. Now, if I take that number for a three-cell battery and multiply times 4 for a 12-cell 24-volt system, anything under 31.2 volts would be an acceptable charge level.

Anything under 28.4-volts is getting on the low side.