Take it to a proffessional battery company have them check it out, they will need to keep it for at least 3 days to peform all the tests required to see if this battery is worth shooting and keeping.
Test such as deep discharge cycling, acid balance, charging & recharging, etc. things a DIY person can't do unless they have all the right equipment & training & eye wash stations w/shower - batteries can create a lot of gassing and "blow-up". Often an old battery may have a "dead' cel or two and need to be replaced or the inner cell connectors are corroded and need replacing, etc. The experts know how to do this correctly as well.