It’s a good news/bad news thing. According to Alan Kohler in the engineering department at Yuasa Battery, leaving an old-school lead/acid battery to charge unattended for any length of time is a bad idea because the water will evaporate, which you learned the hard way. The good news is there’s a decent chance of resurrecting what you’ve got. “Fill the battery with distilled water, since the acid didn’t evaporate, and give it a good charge,” Kohler says. Here’s the bad news: “The typical life of a conventional battery is two to three years, but it’s likely that this incident has shortened the life of yours considerably.” When it dies, Kohler suggests a maintenance-free absorbed glass mat (AGM) battery like a Yuasa YTX14HL, which can be left connected to an appropriate smart charger with no adverse effects.