Battery Charger

We are using a Motomaster CR 19873, 6 amp, has a 6v/12v switch on it. I'm afraid of overcharging the battery and wonder if I switch it to 6v will that just give me a trickle charge? If I leave it on for too long even on 6v will that overcharge the battery? The truck is not a daily driver, it has been sitting for the winter. When I put a volt meter on it it read 13v after sitting the whole season.
Thanks for any help anyone can offer, I am female and just learning all this so any help anyone can offer to my various questions is really appreciated!

Welcome to AF, do not use 6v on a 12v car/truck, is it a step down self regulating charger, if so it will not overcharge. 13v is good.

Thanks for the info on the battery charger (and the power steering pump). What if we've already put it on 6v to try to trickle charge it previously, will that totally screw the battery or new alternator we just put in it? It was a brand new battery (Eliminator) last summer but because the alternator belt kept loosening off it wouldn't hold a charge for very long.

6v just won't charge that battery, no issue there.