RCS,
I suspect your battery is 'knocking on a bit'; mine need replacing every 6 to 8 months. I've also never experienced any problem with the charger that was supplied with the UK backpack (the one with the light that turns from red to green when the battery is charged.
Tony,
With regards to your filter question, the UK backpack should be supplied with a filter that fits in the opening, where you pour the water in. The US e-bay backpacks don't.
If you haven't got one, I believe you used to be able to buy the filter from the Window Cleaning Warehouse (you'll find it on the internet).