Thinkpad T42 Freezes Up On Battery Power

[Waldo324]

I have a five year old IBM T42 that freezes up when on battery power. It's fine on AC power but when I switch to battery the OS freezes up within five minutes. I have tried three batteries with the same result. I can run diagnostics but not anything using Windows. When I try to do a disk check it freezes up before completion. Any ideas?