A convertor is not a battery charger! A battery charger "senses" the voltage in the battery and adjusts accordingly, all to bring the battery up to its capacity without "boiling" it! A convertor is like the alternator in your vehicle, the alternator feeds 12 volt electric to the accessories in your vehicle, the battery "smooths" out the voltage so when you turn on your headlights, other things will not drop in voltage. Yes it does charge if the battery is low, but that is not the primary function. And there are higher end convertors, that will charge sense what kind of battery you have and ensure that the battery will not be overcharged.