You will probably be better off buying another charger (we got ours from maplins) fit it with crocodile clips and charge spare battery.
We only use backpacks, two on the van and carry two spare batteries. Not often needed but if really busy day may need to change one.
With the Gardiner Back pack you will need to fit spade clips to the wires that connect the battery as the wires are soldered to the battery, making it impossible to change unless you cut wires.
Good luck
Steve G