Well, the charger + battery in parallel (+ to +, - to -) would be 12v with chargers amps + the battery's amps. With the charger + battery in series (charger + to starter, charger - to bat +, bat - to engine ground) would be 24 v + amps. The point in putting the charger on in parallel would be to see if it starts with the chargers amps, which would prove the battery doesnt have enough amps. A battery can have 12v with a fault and not have enough amps to start.