Monitoring and Maintenance

Monitoring battery state of charge is the single largest responsibility of the system owner. The battery voltage should be kept at or above a 50% state of charge for maximum battery life. See the battery voltage table.

Keep the battery's electrolyte level to the indicated level and never let the plates be exposed above the electrolyte. Use only distilled water - not tap water, when refilling the batteries. Water is the only element used by your battery. You should never have to add acid to your battery. Do not over-fill the batteries or fill when the batteries are discharged. Over-watering dilutes the acid excessively and electrolyte will be expelled when charging.


A slight acid mist is formed as the electrolyte bubbles upon charging. This mist is highly corrosive, especially to the metallic connectors on the tops of the batteries. Inspect for corrosion and clean these periodically as needed with baking soda and water. Corrosion buildup can create a good deal of electrical resistance, which can contribute to shortened battery life and the waste of power. It's always a good idea to wear goggles and protective gear as the sulfuric acid will eat holes in your clothes.


Equalization is the controlled overcharging of a fully charged battery. This over­charge mixes the electrolyte, evens the charge among varying battery cells and reduces permanent sulfation of the battery plates. It is energy invested in lengthening the life of the battery. Though the PV system battery bank receives a good deal of cycling and gassing through normal activity, we believe that equalization is a complement to this activity and as a rule of thumb should be done every 60 to 90 days.

The equalization process consumes water and produces much gassing. Make sure your batteries are well ventilated during this charging. Equalization charging voltages vary widely, as do duration times, so the batteries should be monitored closely during this process. Check specific gravities of all your cells at the start, noting any low cells. Check periodically during the process. You don't have to check every cell each time, but watch any that show a higher variation. Keep checking electrolyte densities until you receive three readings of 30 minutes apart which indicate no further increase of specific gravity values. Keep a record of individual cell voltages and specific gravities before and after equalizing. Equalization will take your voltage to 15 volts or higher (30 volts on a 24 volt system) so make sure any DC loads are disconnected before you begin.


As batteries are charged they create bubbles of gas, produced when the chemical reaction can not keep up with the energy input. Some gassing is necessary in flooded cell batteries. The amount and duration of gassing varies from one battery to another. Gassing mixes the electrolyte and compensates for the tendency of the acid to stratify with the most dense electrolyte on the bottom. Gassing is the product of splitting water molecules into hydrogen and oxygen. This consumes water and creates the need for its periodic replacement.

Battery Connections

The connections from battery to battery and on to the charging and load circuits are critical. Terminals should be greased, interconnects should be clean and fastening hardware should be tight. Torquing all bolts equally avoids variations in resistance. This is also the reason we prefer to minimize the number of parallel strings in the bank. Higher resistance values on one string of batteries results in less charge to that string and consequently shorter life. We also place the main negative and positive on opposing corners of the battery bank for this reason. The goal is to keep the variation of resistance from one cell to another to a minimum.

Next - Configuration & Installation

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