The Texas Instruments TI-55 calculator was powered with the BP-7 battery pack. It was connected with a proprietary 3 pin connector. The battery pack was charged with an AC step down transformer. If the battery pack failed, the calculator could not be powered with at AC power brick, because the battery pack contained the rectifier.
The TI-30 calculator used the nearly identical BP-8 battery pack. The only difference is that the BP-8 used a standard 9 volt battery type connector. The TI-30 could be powered with either a 9 volt battery, or the BP-8 battery pack.
In this video, we attach a 9 volt battery connector to the TI-55, so that it can be powered with a 9 volt battery.

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