Battery Charging Subaru:
If a boost charger can be used, a deeply discharged battery can be placed on the high setting for up to 30 minutes. This will help the recharge reaction get started. After a boost charge for 30 minutes, the battery still needs at least an 8 hour slow charge. But, the battery may only reach a 60-80 percent state of charge.
Battery Charging from 11.6 to 12.1 volts may also be somewhat difficult to charge, and may need extra time to reach full capacity. With any low voltage battery, it’s a good battery charging practice to give it a second charge after the battery has been sitting for at least a day.
It is always a good practice to check gravities to determine if all the cells are charging equally. Gravities that differ by more than 30 points may indicate some potential problem with the battery. Batteries with differences in gravities of greater than 50 points between cells should be replaced.
This is one of my favorite battery chargers to use to revive a battery:
Schumacher SSC-1000A SpeedCharge 2/6/10 Amp Battery Charger and Maintainer with 50-Amp Battery Clips
If you want a real beefy wheeled battery charger you can’t go wrong with this model:
Schumacher SE-1555A 12V Automatic Wheeled Battery Charger with Engine Start