TP R51 2887-AVG running XP Pro - how to re-enable charging

[MNDK]

Recently I purchased for my daughter a used IBM TP R51 2887-AVG running XP Professional.
Seller informed me that "the battery was seemingly dead".

However it seems to me in stead that actually the initial owner of this laptop has somehow disabled the battery charging completely.
Perhaps wanting to use it solely as a desktop computer.

My laptop is a T41 which uses the same battery pack as the R51 which I recently bought.
First thing I did was to charge the "seemingly dead" battery to full capacity and test it in my own laptop successfully.
This battery pack is still very much alive !

Is my suspicion correct that you can in fact "tamper" with the charge process of the battery pack and in effect disable the charging completely ? - If yes how is this done ?

Following is a description of the behaviour of the R51:
- When I charge the battery pack to 100% in my T41 and then change the battery pack to the T51 while using AC input the T51 will inform correctly 100% remaining capacity and AC status.
Also correctly the charge indicator will be green as capacity is at 100%.
- When I insert a battery pack fx 80% full into the T51 while using AC input the T51 will inform correctly 80% remaining capacity and AC status is "Charging" (but actually no charging is going on)
Also correctly the charge indicator will be orange as capacity is at 80%.
- Regardless of remaining battery pack capacity, when I disconnect the AC power the status of the T51 will be "AC status" and "Charging" if the battery pack capacity is fx 80%. So the R51 laptop seems to keep the settings of "AC input on" even when running only on the battery pack.
- When I insert a 100% charged battery pack into the T51 and disconnect the AC power the laptop will run on battery power while informing correct remaining capacity.
- When I connect AC power on after discharging battery pack from 100% to fx 80% the remaining battery pack capacity will stay at 80%.
- When I got the R51 the BIOS version was 1VET45WW (1.03) with Embedded Controller Program version (1.01)
- With fully charged original battery pack I updated BIOS to version "1VET71WW (1.29) Release Date: 2006/12/22" and Embedded Controller Program version to "1VHT28WW (1.04) Release Date: 2005/01/31" but there was no change at all.

Now I´m hoping that someone can guide me how to re-enable charging of the battery pack.
From my job I am fairly skilled where Li-ion battery charging is concerned - Setup and maintenance of computers is really not my field though......

BR
Morten
Denmark