Voltage meters are used to indicate battery state of charge. They are relatively inexpensive and easy to use. The main problem with relying on voltage reading is the high degree of battery voltage variation through the working day. Battery voltage reacts highly to charging and discharging. In a PV system we are usually charging or discharging and many times are doing both at the same time. As a battery is charged the indicated voltage increases and as discharging occurs, the indicated voltage decreases.
These variations may seem hard to track, yet in reality they are not. A good accurate digital meter with a tenth of a volt calibration can be used with success. The pushing and pulling of voltage, once accounted for by experience, can also help indicate the amount of charging or discharging that is taking place.
By comparing voltage readings to hydrometer readings and shutting off various charging sources and loads and watching the resulting voltage changes, the system owner can learn to use indicated voltage readings with good results.
Due to impurities in the chemicals used for battery construction, batteries will lose power to local action, an internal reaction which occurs whether we are using the battery or not. This slow discharging is termed self-discharge. Self-discharge rates vary greatly among battery types and varies with temperature. The rate also increases with the age of a battery, so much so that an old battery may require a significant amount of charging just to stay even. Even new batteries may lose 1 to 2% of charge per day. Lead calcium grid batteries have the lowest selfdischarge rates.
Power Conversion Efficiency
Energy is never consumed or produced, it merely changes form. The efficiency of conversion is never 100% and in the case of new batteries ranges from 80 to 90%. This means that to discharge 100 watts of power from a battery it must be charged with power.
Determining Battery State of Charge
Battery state of charge is determined by reading either terminal voltage or the specific gravity of the electrolyte.
The density or specific gravity of the sulfuric acid electrolyte of a lead-acid battery varies with the state of charge. The density is lower when the battery is discharged and higher as the cells are charged. See the table to the left; this is because the electrolyte is part of the chemical reaction, it changes as the chemical reaction takes place. Specific gravity is read with a hydrometer. A hydrometer reading will tell the exact state of charge. A hydrometer cannot be used with sealed or gel-cell batteries.
Another important point is freezing. At low densities, the electrolyte contains enough water that the battery can freeze. This is not a problem with PV systems where the batteries are kept both warm and charged. Batteries can survive and operate in a cold location, but the charge level should not be so low that it could freeze.
Specific gravity values can vary + or -.015 points of the specified values. This table IS for the Trojan L-16 battery in a static condition, no charging or discharging occurring, at 77 degrees F. Discharging or charging will vary these voltages substantially.
Source - Trojan Battery Company.
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