I parked my big '96 Chevy conversion van earlier this summer and, not suprisingly, the battery was dead when I went to start it up recently. So I hauled out my combination power jumper/air pump and it started right up. I let the engine idle for an hour or so, then shut it off for the night. The next day, everything was completely dead again (not even a click) and on top of that, my power jumper won't give any juice at all to the motor. So I got out the cigarette lighter cable and attached the power jumper to the cigarette lighter, and the thing powered up my van just fine. Next day, no juice at all again. Two problems: Why does my van's battery loose it's charge so quickly (its almost new) and second, why does the power jumper fail to give a jump, but it will charge the battery through the cigarette lighter?
More than likely your battery terminals are not making good enough contact the the battery. remove the cables and clean the posts and try again. You may also have to buy a new battery.