recharging a battery with the voltage too low

[Wicks]

With the batteries disconnected from all loads.

Parallel both batteries together to allow the good battery to "hopefully" dump a lot of current into the bad battery, raising its voltage enough to trick the charger.

While the voltage is raised and high enough, connect the charger to both batteries.

If the charger is happy, remove the good battery while keeping the charger connected to the bad battery.

Let it charge and monitor the battery throughout the process. A charger with a desulfation mode could help as well.

Make sure the battery appears to charge properly and that it doesn't get hot.

If it appears to recover then a load test will help you determine if the battery is still good.

Then charge it again if passing the load tests.

Remember to charge the original good battery as well.