Recharging Dead Batteries
These instructions have been edited a bit to make them more universal. They were originally written to recharge my 420 amp hour bank with a TrueCharge 20.
The time of year make a lot of difference in the urgency of the situation. Cooler winter temperatures gives you a bit more "forgiveness" than high summer temperatures. Having said that, the critical time period on battery recovery is a 45 to 60 day time period of deep discharge the lead sulfate associated with discharge begins to change into a crystallized form that doesn't easily convert back into lead oxide with normal charging methods.
Depending on the size of the bank and charger it may take most of a weekend to recharge the batteries and perform some recovery process work. You could speed thing up if you have or can borrow a second charger that you can parallel with your TrueCharge 20 during the bulk and absorption charge phases
The first thing to do is to simply get the batteries recharged as soon as possible. In their present condition that will probably take at the least 24 to 36 hours with a 20 amp charger. Leave the charger on even after it has gone to float mode, because the last part of the recharge will occur with the float charge to get the last 10 to 15 percent of recharge. You can stop the normal charging when the charge current in float mode drops below about .5 amps. After that, you should perform an equalization charge on the batteries. Your charger is under-powered for a 400+ a-hr bank, so you will likely have to run 2 equalization cycle charges. I think the TrueCharge equalization cycle terminates after 6 hours, but I'll have to verify that. If you run out of time, try to get at least 1 equalization charge completed this weekend, and then get another one done as soon after that as possible.
The only good way you will have to determine if you are doing any good is to run a load test on the boat after the batteries have rested 24 hours. I assume you have a battery monitor of some sort on the boat, and your load test will be to determine if you have useable capacity again with your batteries. Rather than give you specific parameters, the bottom line for you is if the batteries will give you the capacity to allow you to use the boat the way you want. Your past experience probably gives you a few key parameters that will let you know if you are at that point.