[Jon_G4MDC]

Is it likely that any electrolyte has been lost, through inversion/leakage?
If not then a simple water top up and hydrometer test might be all that is needed.

I suppose you could empty the lot and start again fresh. Do these cells drop crud from the plates like lead acid? Pardon my ignorance.

I had a small 2Ah one once, it could deliver one hell of a lot of current. Ejected from my bedroom window into the front garden because of smoke emissions.

A shortcoming in the insulation of my charging system had become manifest...