Yes. It is a secondary battery, but as in most older PBs, it does not begin to recharge before the system (main) battery has reached near-full voltage, ie not full charge. They are rugged batteries, rarely failing, but they can do so. I had one in a PB160 that recovered after at least a dozen years of disuse. So get the system battery charged if that is possible, and allow another 24hr after that for the backup battery, which may take a while to appreciate its new life.

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