At the moment I have 2 Rutlands (can't remember what models) but the one is brilliant and lasts for weeks - the other, more expensive one, uses a battery about every 7 days on half the amount of fencing.
I used the top of the range Hotline to power 400m 4 x 1cm tape. The shock wasn't particularly strong because of the length and condition of the tape used, but it was enough to keep my native ponies in.
It ran off an old car battery which was no longer good enough to start a car, which I got from my local garage. It has high and low settings. I always kept it set to high.
Biggest problem, as with all fencer units, is security and someone nicked it!
I recommend buying one that you can chain to something very heavy/immovable.
The rutland electric shepherd energisers are good, but expensive and they do drain the battery quickly especially the bigger one as you have found. We have replaced all ours with the Hotline Super Hobby, which will still do 2km of fence, the battery lasts for at least 4-6 weeks - double if you buy a solar charger panel and link it up. The Hotline cost about £90 and we got most of that back from the Rutlands on ebay.
eta Have you tried swapping the batteries between the chargers just to check that one battery is not on the way out?