Second battery causes crash

[costas]

I am having a very weird problem with my 3-yr old T42: a few days ago, after being stored for about a week in a closet (in a bag, no detectable humidity) the laptop started crashing: On boot, the power LED will turn on for 1-2 seconds, and then the machine will turn immediately off (no IBM screen).

This is reproducible whenever the machine is unplugged and the second battery (Ultra-Slim bay, FRU 08K8191) is plugged in. Without the battery, the machine boots up normally and so does if it's on A/C (with or without the battery). Strangely, if I plug in the battery while on A/C, leave it to charge, then unplug A/C, the laptop will keep running for a few minutes, then shuts off.

This doesn't look like an OS crash to me (nothing in the XP Event Viewer) but more like a short-circuit within the battery.

Has anybody seen anything like this before and managed to work around it (googling or searching this forum turns up nothing)? I am guessing I'll need a new battery, and that's fine, but I am wondering if there's any way to check that the problem isn't within the laptop's circuitry rather than the battery's (which would make a second battery worthless).

Thanks in advance.