HP-45 Battery/Power Problem

[Accutron]

I have nearly new condition HP-45 which is behaving strangely. It works fine with the external supply, but a new, fully charged NiMH battery pack putting out 4.25V does not work correctly. If I plug the charger in for about 10 minutes, the calculator will then run off the battery for 20-30 seconds, then shut off. Sometimes it shows the low power indication *very* briefly before it turns off. It's acting like the calculator's power fail cutoff voltage is slightly higher than 4.25V. I already confirmed it wasn't a problem with the charger socket clip by shorting the two outer pins. The battery contacts are aligned properly, and all of the gold surfaces on the battery contacts and charger socket pins and clip are clean and bright (factory new). Has anybody else observed this behavior before? I'm assuming there's no easy fix for this, but I'm very reluctant to perform a label-ectomy on a pristine HP-45.