Deep Cycle Batteries

One of the most expensive pieces of equipment in solar, wind power or backup power supply systems are the batteries. Batteries used in these types of systems are usually flooded lead acid deep cycle batteries.

Unfortunately the term "Deep cycle" has been used very loosely by battery manufactures and has to often been used as a sales buzz word to imply heavy duty and long life. The fact is most batteries described as being deep cycle are nothing more than large SLI batteries and almost all use pasted grid plate technology. Active materials pasted onto the plate grids presents a very large surface area to the acid and thus will provide high amperage all be it for a relatively short period of time. Positive pasted plates will fill with sulfate when discharging - this is normal and recharging the battery will return the sulfate back into the electrolyte as sulfuric acid. However if repeatedly discharged to less than 80% the plate material will be damaged and will become so full with sulfate that it will expand and begin to break away from the plate grid thus the capacity of the battery will become significantly lower.

True deep cycle batteries are designed to withstand repeated deep discharging to much less than 80% without significant shortening of battery life. True deep cycle batteries are constructed with solid pure lead plates for the positive plates and a thick grid pasted negative plate (pasted neg plante). Pasted neg plante batteries are very tough and life expectancy if correctly maintained is around the 25 year mark.

Deep cycle batteries are very expensive and you are running solar or wind power systems and have a battery bank it can cost a small fortune just for the batteries. And when one fails it usually means replacement (more $$$).

The average life of commercially available deep cycle batteries is about 18 months to 2 years.
Although these failures are mainly due to poor maintenance it does show how easily commercially available deep cycle batteries can fail.