I tried prying it out but gave up because I didn't want to risk messing it up. The battery connects to the harness using a Molex-style connector. I have one battery so I saw the connector in the other battery compartment when I removed the cover.

The battery has to be wedged in there tight otherwise it can vibrate.

When you took out your Pi/DAC board, was one of the boards seated an an angle? Mine is at an angle because the header connector and the stand-offs are not at the correct height.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated