Lionheart,
I've been using the backpack for over three years now; I'm on my second one and have just started using my fourth battery.
I use it every day and it gets quite a hammering.
I think when you get a brand new battery, you need to charge it for an awfully long time; I don't know how long exactly, but it wouldn't hurt to charge it for at least 24 hours. Maybe if you don't charge it fully the first time, your battery won't work as efficiently as it should?
You should be able to get a lot more than 75 litres through it though. Unless the battery was old, and when I say 'old', I mean six or seven months worth of intensive daily work use, I've never ever been able to wear one out from a days work; no matter how much I've used it and how hard I've worked. I reckon I've put at least 200 litres through one on a single charge (this mightn't sound much when the 'big boys' use 1000 litres a day, but I use it mostly on domestic accounts, tops only; with Wor Lass following, tradding the ground floor); maybe more and I believe you can get 400 litres from one, but don't ask me to prove it without you betting money on it.
I'm also not sure how right PJ is when he says it's good to 'run the battery completely flat and then recharge it'. (Sorry PJ; don't flame me
) I know this is the right thing to do with my electric razor, but I think it's wrong to do it with this type of battery. I don't think back pack batteries are designed to be ran flat; I know my caravan leisure battery (the type of which many guys here use for WFP) wouldn't like it. I think the back pack battery is just a smaller version and I normally charge mine up at the end of each day and have had no problems doing this.
I hope I've been some help.
