Don't forget the lamp in series, This can be very well used to limit the current and voltage to charge battery packs.
For example use a 12 volts 3 Watt or 5 Watt or wattever wattage you need to limit the maximum current in the circuit.
The beauty of the lamp is that you have an optical indication of the charging current.
I use this way a lot to slowly recover 12 Volts substation batteries which have been discharged for a while.
After a while the 12V 21 Watt lamp gets dimmer when the batt voltage increases, then take the lamp out and leave the 12 volts charger connected to fully charge up the batt.
This process may take a few days, bear in mind that these are deap cycle batts 12 volts 100 Ah or bigger.