I have the two man pure freedom van system. Over the last week or so with two of us the battery has only lasted a couple of hours before showing battery low. It had been on charge for 24 hours. So I'm guessing I need to replace it? There's a Denka extreme leisure in there now. Can anybody suggest where or what to replace it with? It sits in a tray so ideally I'd just replace it with another Denka so I know it kits ok, but can seem to source one from anywhere. Any suggestions? Thanks in advance?