@ KennyR
Do you know if the interface has any components in it that once the batteries have dropped below their operating lifespan the interface won't allow it to recharge again, even if the failed batteries are replaced?

I can easily get the case open and separate the components out, I just thought there'd be some electronic chip/component in the interface board that wouldn't allow replacement of batteries after it had run its expected lifespan.

I've not got a spot welder, but surely a good soldering iron could connect the batteries together permanently?