Originally Posted by Nos4r2
If your battery is FUBAR then you'll get a low voltage or problems starting. They won't give more than about 13.2v even when fully charged,
While running? My F115, at higher revs shows 14.2 to 14.4 across the battery.
Since the battery switch is only dealing with the +12V, I don't see how this could affect the supply voltage to the GPS in a way that would increase it.
The output from the motor at speed and with no load (which is not a good thing to do, by the way) is likely to be in the 16 to 18V range or higher (talking potential here, as running with no load may damage things.) With a battery as a load, that usually smooths things out to anywhere from 13.5 to 14.6 or so volts. That range should not cause an overvoltage condition on your GPS.
Two things you should do: Put a voltmeter on the alternator output at the motor and see what that reads, and compare it to the voltage across the battery at a similar rpm, which will give you your cabling loss (which I would expect to be not more than a couple/few tenths of a volt, up to maybe a volt or so.) Do the same with the GPS power input. Any large discrepancy means your cabling is dropping voltage somewhere. Might want to check teh grounds by putting one lead of the voltmeter on the neg at the engine and measuring voltage at the battery (-) and the GPS (-) to see if a ground connection is resistive.
If all the (+) voltages are roughly similar (first step above), then you've got a component causing the problem, and need to start isolating that. Swapping in a different battery temporarily is probably a good place to start.