I need to test my temp gauge out of the car.

Is it OK to use a traditional style battery charger for the 12 volt feed?

I checked it with a voltmeter and it looked like about 13.5 volt output.

Will it damage the gauge somehow?

Battery chargers can provide pretty lumpy power - and I thinkg the vwoltage can be a little higher than it ought to be.

In short: I won't recommend it, but it will probably work fine.

Uh - you should be careful of having a battery charger hooked up to a car with FI while uyou are trying to start or run it - the power drain can cause some wierd power spikes which can (did) kill the FI computer (happened to my megasquirt).

For a guage it will most likely be okay. I usually use a PC power supply for testing guages and FI stuff though.