The secret is the different spreads between each charger's voltage and the battery's voltage. The charger with the biggest spread will run the longest while the others drop out as their spreads disappear in turn as the battery voltage rises.
The battery can only accept so many amps at any voltage, so the chargers will only add amps more than one charger can do, if the battery can accept that many. Otherwise you will get the same total amps with several chargers as with that one charger. In that case each charger will do a few amps and they will add up to that total. How much each charger does of that total is in proportion to its "spread"
