What about trying an initial charge with a low-amp (like 1 amp) trickle charger, perhaps starting with 12VDC output? If it's not a "maintenance" charger that senses battery voltage, just a simple trickle charger like a typical AC adaptor, it may be able to put some charge in. Low amps is safest I think, don't worry about time. Just in case though, wear safety glasses and have the battery and charger on a fireproof surface or in a metal charging box designed for that.