Again, it sounds to me like the charge card has gone walkies. I would say that:
1. You obviously need to confirm that the spring-like contacts on the charge card are not corroded and that they are actually aligned with the battery contacts. That might be possible without disassembly (though as I do not own a Clamshell, I am no authority).
2. It might be possible to revive the charge card by replacing the few electrolytic capacitors found on the thing. My theory is that the batteries in the machines that use the Battery Reset utility just don't respond well to currents that aren't just right. In roughly the same era, Wallstreet charge cards were well-known points of failure, and it is probably the same technology that was used in these machines, given that they run the same software and come from the same engineers.
3. The fact that the charge card is physically isolated from the logic board also raises the possibility, I suppose, that the connecting wires have become disconnected or the like.
Having said that, I just took a peek at the disassembly/reassembly required
in dealing with a Clamshell (ouch!). That is at least as bad as a 2400c, and that is saying something.