Several older synths - Korg and Peavey DPM come to mind - did use recharable Ni-Cad batteries. There are(were) fed a small current any time the synth was powered on - not just plugged in - to keep them charged. Modern Lithium cells DO NOT like to be subjected to current flowing IN to them. They WILL overheat and vomit all over everything. Not quite as corrosive as the Ni-Cad juce, but still nasty. You can check the schematic to see where the charging voltage comes in - usualy through a diode and resister - and disconnect that part of the circuit. Watch that you don't break the circuit from the battery to the memory chips.
"A thing may seem impossible...until it's done."