[Thinkpad] A21P battery not seen

[ted.frater at virgin.net]

Hi All,
 
Weve 2 identical A21P's  in our household, amongst other thinkpads. ( im 
a dealer in a very small way)
one (thats mine) with its original bios  running dual boot win 
98/Slackware 10.2
My after market battery is seen both by windows and by linux.
the other  ( thats my sons's) has the latest bios, running ubuntu  and 
works fine with a genuine ibm battery tho this has only 10 minsof life 
in it after a full charge. BUT it wont see my after market battery at 
all. It thinks its running on mains when in fact itsrunning on the 
battery, which has been fully charged in my A21P.
 So how can the latest bios, which is the only difference in the 2 
machines effect the battery in this way?
As of now its a complete mystery.
 any help much appreciated.
 Now one option would be to revert the bios to the original. Can this be 
done? and if so how? would it be safe to do so?

 Ted Frater Dorset UK.