you could have bought one of the same little batteries for about £15 on ebay and had a spare to change quickly, also the wcw backpacks i have used used to have mazzive batteries in them lasted ages !
all backpacks are pretty much the same, i have a nice little usefull one at the moment called a water genie from the cleaning warehouse comes with a little sackbarrow that it sits on and you can take two screws out and wear it on your back, i figure after about 6 months the wheels will break but then ill wear it on my back afterwards for another 6 months, getting rid of the flow controller which will defo die and changing the small pump on the way, the pumps are 5l per min diaphram pumps on ebay for £10 , as long as you have the shell of a backpack you can always fix it