I wouldn't be to sure it's the battery, mine and a few others I've fixed ended up being the PCB Switch. If you switch on and the red light starts to flash after about 5 sec and you know your battery is charged then it will point to the PCB being damaged (probably due to getting wet) You can replace the PCB but I bypass this now and have a simple on/off switch wired to the pump.
You only have the one setting doing this but only cost me £6 for the parts from maplins. If you dont wear the BP and use it on the floor this method makes life easier. Jeff1 also did a tutorial on how to do this but included the PCB switch in his extension. I'm sure he will post the link if he See's this.
Wayne