just saved the expense of buying a replacement:https://www.optimabatteries.com/en-us/support/charging/resuscitate-deeply-discharged-battery
Here's what you need:
Battery charger (under 15 amps)
A good battery, preferably above 12.2 volts. (It can be an AGM or flooded battery- it doesn't matter.)
The seemingly dead, deeply discharged AGM battery
A voltage meter
A watch or timer
Now, here's what you do:
Hook up the good battery and deeply discharged AGM battery in parallel – positive to positive and negative to negative. Do not have the charger connected to the battery or turned on at this stage.
Now, hook up the good battery to the charger. Turn on the charger. The charger will "see" the voltage of the good battery (hooked up in parallel), and start providing a charge.
After the batteries have been hooked up for about an hour, check to see if the AGM battery is slightly warm or hot to the touch. Batteries naturally become warm during charging, but excessive heat may be an indication that there really is something wrong with the battery. Discontinue charging immediately if the battery is hot to the touch. Also discontinue the process if you hear the battery "gassing" — a hissing sound coming from the safety valves. If it's hot or gassing, STOP CHARGING IMMEDIATELY!
With your voltage meter, check back often to see if the AGM battery has charged to 10.5 volts or above. This generally takes less than two hours with a 10-amp charger. If it has, disconnect the charger from the wall outlet and remove the good battery from the charger. Now, connect only the deeply discharged AGM battery to the charger. Turn on the charger and continue until the AGM battery reaches a full charge, or until the automatic charger completes the charge process. In most cases, the AGM battery will be recovered.