This site has some good general information on it Dave.
They also include this information.
Wiring & Connections
- top surfaces clean and dry
- acid (electrolyte) level up to plastic liner inside holes
- fully charged specific gravity is 1.245 to 1.300 in each cell
- In a partially discharged battery, specific gravity of each cell does not vary by more than 0.050 from the other cells
- 'at rest' battery voltage is 12.1 to 12.8 Volts
- engine cranks properly for 5 seconds with each battery alone -- battery voltage is above 9.5 Volts and steady, while cranking
- cables are in good condition
- connections clean and sealed from moisture
A-Sea Marine Electrical
- contact surfaces of mechanical connections are cleaned and coated with moisture resisting sealant before being put together
- no frayed or cracked insulation (check bilge and engine compartment)
- battery switch connections clean and sealed
- fuses have clean, tight, sealed contacts
- every electrical system is documented in diagrams or schematics and these are in one location on board Alternator, Starter & Winch Motor
- with engine running, batteries fully charged, and a 1 or 2 Amp load on system (a light turned on), voltage to batteries is 13.9 to 14.4 Volts -- voltage remains constant as more loads are turned on, up to the rated output of the alternator
- external connections clean, sealed from moisture, and positive terminals are covered
- alternator drive belt(s) tight and in good shape
- starter solenoid plunger clean and lubricated
- starter and winch motors get 9.0 Volts or more while operating
I have found in the past that, once a battery shows a significant low electrolyte reading on a cell, you are on track for replacement and might as well bit the bullet and spend the $$$$$$$$$$.
If the reading is not too varied, an equalization will generally restore capacity if the battery has not been neglected for an extended period.