Frequently the batteries in these devices are rubbish but I think that does for all of the unbranded ones whether cheap or less cheap. Often ones offered cheap have been sitting around for too long as well, which kills the batteries.
However, they are still handy and you can always come across better quality batteries (hang around a UPS when the guy comes to service it).
Kevin
I got one from maplin for £20, was the biggest pile of crap ive ever used! It started the car ONCE when the battery had run down due to me having the radio on for a few hours.
The built in air compressor makes all the right noises, but no air comes out of it!
Tried using it a few weeks later to start my Vectra, and it was dead, wouldn't even make the starter motor click.
I then tried using it to start my Honda Genny (its only a single cylinder engine like fitted to lawn mowers, and it wouldnt even crank that over!)
I know now not to buy a cheap one and expect it to work!

Kevin, do you, or anyone else for that matter work with UPS's? Im after the batteries from them, and if anyone knows where I can get some from let me know!
