go to maplins, get a nitro 1000 charger.
Cost about £15 certainly wasnt dear.
Its an intelligent charger which will warn you if its connected the wrong way round, it also warns you if there is a fault and it monitors the condition of your battery.
You can leave it connected and it wont over charge.
You may be able to get one online if you havent a branch near you.
I thought I was going to have to buy a new backpack at one stage but this sorted it so well.
Charges my backpack after a days use in just over an hour.