The backup (not purely a PRAM) battery of the PBs 100 and 500, at least, is intended to have power enough (50-odd mAh) to allow graceful exit to sleep, and maintenance of the contents of RAM for up to 24hr, when the main battery runs out of puff. For this purpose they are rechargeable, and remarkably rugged. I've had a dead-flat backup in a PB 160 recover full function after an estimated 12 years off the AC adapter.
But there's a catch. (There always is.) Backup batteries do not begin to recharge before the main batteries reach nearly full terminal voltage. (Nota bene: not full capacity, so if even a battery with 30-50% of original capacity reaches about 7V (100s) or 10V (500s), the backup can charge. Then the four-fingered salute No 1 can be effective in resetting PRAM and restoring the notion of startup to the PB. How to get the backup to charge? Leave the PB on the adapter for anything up to a week, checking startup ability at intervals. This certainly works for the NiCd 100s, and can usually work for the NiMH 500s, despite the interference of their stupid intelligent batteries. And no, it doesn't work if the main battery is left out.
For the 500s remember also four-fingered salute No 3: control-option-command-power. It stops them instantly, and not gracefully as power first, count five, then return, does. In case you are wondering, four-fingered salute No 2 is shift-option-command-delete, inviting Start Manager to bypass the usual startup hard drive in favour of something else (eg a CD-ROM drive) on the (usually) SCSI bus.
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