Bob's Shop Notes:

How not to tap a 24-volt battery to supply 12-volts

Several times a year we receive questions about tapping the center of two 12-volt batteries in a 24-volt system to supply 12-volts to some accessories. A schematic of this scheme is illustrated below:

This seems like a perfectly reasonable thing to do. After all, when the system is operating normally, one can place a voltmeter at the jumper between the two batteries and the instrument will indicate one-half nominal system voltage. Why not just hook your 12/14-volt accessories to this tap?