Reviving old batteries?

Its more or less a NOS battery. Bought new 2011 and only used at a test bed to run gauges, etc... When it would fall below 9 volts, the Power Wheels charger would bring it back to 12+. Maybe 3 cycles/times during about 3 year use period. It has sat in a corner for about 2 years and now, totally dead. Jumper cables from my car wouldn't bring it up at all.
They don't reuse the electrolyte because it is contaminated w/ lead? Just curious
for the price of a lawnmower battery, i wouldnt even consider it