Do these things use Lead Acid Batteries? The ones I've seen, at least the newer ones, tend to be small and claim capacities which suggest an Li based battery. Recovering them is, at best, 'interesting'. You can swap out cells but trying to recover cells (or batteries) by 'forcing' them to accept a charge can result in dramatic results.
Even recovering sulphated LA batteries is hit and miss. I've tried in the past but never managed more than a fraction of the original capacity and, even then, the results have been short lived. Certainly, modern LA batteries seem all but impossible to recover, the limited success I've had was with older ones some years back. I was given some 'magic' pills years ago- a bit like Batt Aids on steriods- they worked but I've never seem them since.