Let's go over some basics here.
The battery will sit at around 12.5 volts when fully charged and no load on it. With the engine running, if you see the voltage any higher than this, the battery is doing nothing but taking power from the alternator (to keep it charged).
An alternator is regulated to provide as much current as the system demands, while keeping the voltage under about 14 Volts. If you ask for it to provide more current than it can supply, the voltage will drop.
The slower the alternator spins, the less power it can supply, hence the idle is when most problems arise.
If you're running at less than 12.5 volts with the engine running, your battery is getting discharged and will get damaged. A battery is considered fully discharged once it reaches around 10 Volts (no load). A regular car battery will lose about 20% of it's capacity even if it gets discharged once. A deep cycle one won't.
Anyway..................have your car battery checked to see if it's healthy. Not because it provides any power with the engine running in a proper situation, but because an old or damaged unit can cause extra draw on the alternator, which you could be used to power your fans instead.
