I am sure you mean 2 amp or 12 amp not volt
if you think they might be frozen or all but charged ,use the lower setting but if they are down a lot and not frozen then use the high setting, most chargers these days are smart chargers and will knock the amps down once they reach about 80% of full charge but after about 6 hours you can put it on the 2 amp setting to finish topping up the charge