After searching for this particular battery pack I am going to guess that these have become obsolete. That being said, I can't see why it wouldn't be possible to carefully split the heat shrunk plastic from your non charging packs, de-solder the connector pads and retrofit them to replacement batteries with the label or a home made version using heavy duty tape. Sometimes it's necessary to improvise when there are no options. The equivalent rated batteries are available but are not cheap. This might be a good starting point for a suitable source - Sima Np36 Cherokee Np36 Replacement 3 6 550 Mah Nimh Battery Pack For...
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After searching for this particular battery pack I am going to guess that these have become obsolete. That being said, I can't see why it wouldn't be possible to carefully split the heat shrunk plastic from your non charging packs, de-solder the connector pads and retrofit them to replacement batteries with the label or a home made version using heavy duty tape. Sometimes it's necessary to improvise when there are no options. The equivalent rated batteries are available but are not cheap. This might be a good starting point for a suitable source -
Sima Np36 Cherokee Np36 Replacement 3 6 550 Mah Nimh Battery Pack For...