Because flooded lead acid batteries are a 'consumable', they wear out. Therefore, unnecessary discharging, overcharging, and the sizing of the bank to the charging/discharging determines their life. Their state day-to-day should be in the top 20% of charge. Over the years I've found that flooded lead acid cells need to be used actively. So it's an art to get the most out of them. In a primitive installation you won't need a hydrometer, shorted load test meter, timed load test program/computer, and amp hour meter but there is where your future lies. I consider 15 to 15.5 volts (nominal 12 v lead acid) for a couple of hours a month and many times there isn't enough power available so a day at 14.8 might be the best I can get. They hiss and pop, throw off nasty smelling gases so shield electronics from the spray that comes off the bank. But the best indicator of overcharge is temperature. If it gets to 150 degrees F (warmer than comfortable to the hand) I quit charging. Outback has an optional charging sensor for temperature to avoid battery damage with their inverters.
Recently I've tried a new technique called 'acid balancing'. This becomes noticeable if you have dissimilar (old) batteries mixed in a bank (deep discharge with non-deep discharge). Other uses are sulfated batteries mixed with fresh. Usually there are differences between the acid concentration, cell to cell. They say when there is a 5 point difference in a battery discard the battery! I could never afford to do this so I swap high acid cells electrolyte with watery cells electrolyte and use my hydrometer a lot. Fresh acid is available from auto parts stores for the watery cells. It's normal that bubbling cells lose electrolyte and normal that cells lose water to evaporation so you'll always be putting in more than you swap.
