I'm using multiple 12V lead-acid batteries in a boat with a battery switch to go between each battery or tie them together and an isolator to keep one battery from discharging into the other (diodes on heatsink). Now that all batteries are the maintenance free type and checking the gravity with a hydrometer can no longer be done, what voltage levels should I be looking at to ascertain if the batteries are up to snuff. Getting towed in because of dead batteries and a working alternator on the main engine is un-acceptable.
The battery maker (reputable) is unwilling (or unable) to provide a set of charging curves. Its a weighty chore to remove the batteries and charge them on land. Any data will be appreciated.

Cheers, DPW [Everything has limitations...and I hate limitations.]

 

12volt batts should be charged when at 10v minimum.

 

Here's a reference on using voltage to determine the state of charge.