@canadadave trained at your fine Hospital decades ago :-)
This actually sounds like a bad battery. Could be issues with the battery control board or the cells itself. How comfortable are you taking it apart and rebuilding it? No, it is not as dangerous as it sound. It is not like any explosive ordnance disposal but needs common sense :-)
We would need some good pictures of the actual battery to see how to approach this. Hardest part is always cracking the case :-)
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