T43 hibernates with good/bad battery

[Specter]

I am having battery problems with a T43. The battery manager says the battery is in good shape with only 31 cycles and that the capacity is about 2:30 hours. But after about half an hour it hibernates due to low power.

I have tried to reset/initialize/zero (don't know the exact term in English since I read it in Swedish) the battery from battery manager but this didn't help. Is there anything else I should try?

I have bought the machine second hand so I don't know exactly how old it is, but I would guess around three years. Model is 2668-WXZ.

Thanks!