Batteries - The Heart of an off grid System
"Few batteries die a natural death, most are murdered"
A Solar Electric system is made up of a number of components, and of these, none needs as much attention as the batteries. Though the idea and usage of a battery bank is very simple, if batteries are neglected, degradation can occur at a fast pace.
As someone in the industry once put it, "few batteries die a natural death, most are murdered". The following information is designed to tell you how to get the longest life possible from your battery bank.
This is strictly flooded cell lead-acid battery information; Alkaline, gel-cell and Lithium-Ion batteries and many of their needs and characteristics are completely different.
A typical 6-12 volt flooded lead-acid battery must be taken to approximately 14.2-14.4 VDC before it is fully charged. (For 24 or 48 volt systems double these figures.) If taken to a lesser voltage level, some of the sulfate deposits that form during discharge will remain on the plates.
Over time, these deposits will cause a 200 amp-hour battery to act more like a 100 amp-hour battery, and battery life will be considerably shortened. Once fully charged, batteries should be held at a considerably lower voltage to maintain their charge typically 13.2 to 13.4 volts. Higher voltage levels will "gas" the battery and boil off electrolyte, again shortening battery life.
Battery Charger Designs
Most battery charger designs cannot deal with the conflicting voltage requirements of the initial "bulk charge" and subsequent "float" or maintenance stage. These designs can accommodate only one charge voltage, and therefore must use a compromise setting - typically 13.8 volts. The result is a slow incomplete charge, sulfate deposit build-up, excessive gassing and reduced battery life.
The charger available in our inverters automatically cycles batteries through a proper multi-stage sequence to assure a rapid and complete charge without excessive gassing. Factory battery charger settings on our inverter-charger combinations are optimal for a lead acid (liquid electrolyte) battery bank of 250-300 amp hours in a 600 F environment. If your installation varies from these conditions, you will obtain better performance from your batteries if you adjust the control settings.
Charge Voltage Guidelines
The Maximum Charge Rate in amps should be set to 2025% of the total amp-hour rating of a liquid electrolyte battery bank. For example, a 400 amp-hour bank should be charged at no more than a 80-100 amp rate. Excessive charge rates can damage batteries and create a safety hazard.
The Bulk Charge Voltage of typical liquid electrolyte batteries should be about 14.4 VDC; gel cells like the Deka about 14.1 VDC There is no one correct voltage for all types of batteries. Incorrect voltages will limit battery performance and useful life. Check your battery maker's recommendations.
The Float Voltage setting should hold the batteries at a level high enough to maintain a full charge, but not so high as to cause excessive "gassing" which will "boil off" electrolyte. For a 12 volt liquid electrolyte battery at rest, a float voltage of 13.2-13.4 is normally appropriate; gel cells are typically maintained between 13.5 and 13.8. If the batteries are being used while in the float stage, slightly higher settings may be required.
Charge voltage guidelines used here are based on ambient temperatures of 600 F. If your batteries are not in a 600 F environment, the guidelines are not valid. Temperature Compensation allows easy single dial re-scale of the voltage settings to compensate for the differences between ambient temperature and the 600 F baseline.
Temperature compensation is important for all battery types, but particularly gel cell, valve-regulated types which are more sensitive to temperature.
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