Batteries: the fuel tank of your alternative energy system

September 20, 2012

Most alternative energy systems require batteries to store energy. Lead-acid deep cycle batteries provide the best price to power ratio on the market. There are two classifications among lead-acid batteries: sealed or flooded. 

Flooded lead-acid (“wet cell”) batteries use a fluid electrolyte, have ports to access the fluid reservoir, and require maintenance (top-ups of fluid). Relatively maintenance-free, sealed lead-acid (“dry cell”) batteries use non-fluid electrolytes contained in inaccessible cells. Sealed lead-acid batteries have two further classifications: Absorbed Glass Mat (AGM) or Gel. Gel and AGM batteries are rugged, low maintenance and do not need to be kept upright-- a good choice for applications where the batteries are handled often or need to be left unattended in a remote location. Keep in mind, sealed deep cycle batteries are more expensive than flooded batteries and have a shorter life span. For good quality batteries, you can expect to budget approximately $120 for every 100Amp hours of capacity at 12 volts for flooded batteries and $260 for sealed..

We say, if you’re not comfortable-- neither are your batteries. Batteries maintained at a relatively constant temperature will perform better and last longer. Generally, that means storing your battery in an area between 10-26 °C/50-80°F. Battery capacity ratings are usually specified at 25°C/77°F. As batteries get colder their voltage drops and performance suffers.

Batteries are rated in terms of the number of amp hours they can provide at a given rate.  This is done to recognize that the rate at which energy is extracted influences the total energy available quite significantly.  So, one might have a battery that yields 100 Ah at the 20 hour rate and 132 Ah at the 100 hour rate.  What this means is that you could take 5 Amps out for 20 hours or 1.32 Amps out for 100 hours.  We generally use the 20 hour rate in determining the size of the battery required.

When choosing your batteries, size really does matter! A reasonable rule of thumb is to provide rated capacity equal to a week's consumption. For example, if your total weekly consumption is 1200 Wh/week, then a battery of at least 100 Ah at 12 V (W = A x V / Wh = Ah x V) would be required. A battery bank of this size would provide approximately 3 to 4 days of reserve capacity. In order to keep batteries in top condition, we design systems such that the batteries will not need to be brought below 50% of their capacity.

Be careful not to use automotive starting batteries for alternative energy applications.  They are designed to spend most of their lives in a fully charged state, and to be charged and discharged quickly.  The deep cycle batteries that are best for alternative energy systems will spend most of their lives partially discharged and will charge and discharge quite a lot more slowly.  The wrong kind of battery--even if it's free-- is hardly better than no battery at all.  Solar use of a car battery will damage the car battery quite quickly and leave you with a need for a battery.  A good deep cycle flooded battery can last 25 years and can have a warranty as long as ten years.

Your batteries are the heart of your system.  Choose them carefully and take care of them and they will take care of you.


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