had mine about two years and is charged on split relay but the last couple of weeks i have had trouble when filling with the booster pump as the charge runs dead before tank is full, but when i get to work and use the varistrem it runs all day without a problem, does the varistream not drain the battery like a booster pump or is the battery fine and i have another problem.
A Booster pump runs on a higher current than a varistream, although a varistream is rated at 12v most of the internal workings like the IC's (intrigated curcuit) run on 5v so a varistream takes vertually no current, its your pump that takes most of the current.
If your booster pump has a higher curent rating than your pump connected to the varistream then your battery could be on its way out, top it up with pure water to just above the plates, give it a good charge and see how it goes from there.
Halfords also sell little tablets for batteries that you drop into the cells. (may also help)