Battery Power Solar Systems

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					                                                                                                                                 SOLAR ELECTRIC

                                                                  A Winning Combination—Design, Efficiency, and Solar Technology

                                                               Battery Power for Your Residential
                                                               Solar Electric System
                                                                  A battery bank stores electricity produced      sometimes used in situations in which deep-
                                                               by a solar electric system. If your house is not   discharge batteries are not available, car and
                                                               connected to the utility grid, or if you antici-   marine batteries will quickly fail if used in a
                                                               pate long power outages from the grid, you         solar electric application.
                                                               will need a battery bank. This fact sheet pro-
                                                                                                                  The Battery Bank
                                                               vides an overview of battery basics, including
                                                               information to help you select and maintain           The basic building block of a lead-acid
                                                               your battery bank.                                 battery is a 2-volt cell. A battery bank is a
                                                                                                                  collection of connected 2-, 6-, or 12-volt bat-
                                                               Types of Batteries                                 teries that supply power to the household
                                                                  There are many types of batteries avail-        in case of outages or low production from
                                                               able, and each type is designed for specific       renewable energy sources. The batteries are
                                                               applications. Lead-acid batteries have been        wired together in series to produce 12-, 24-,
                                                               used for residential solar electric systems for    or 48-volt strings. These strings are then con-
                                                               many years and are still the best choice for       nected together in parallel to make up the
                                                               this application because of their low mainte-      entire battery bank. The battery bank sup-
                                                               nance requirements and cost. You may               plies DC power to an inverter, which pro-
                                                                                     remember the flooded         duces AC power that can be used to run
                                                                                     version, which used to be    appliances. The decision to select a 12-, 24-,
                                                                                     widely used in automo-       or 48-volt battery bank will be determined
                                                                                     biles. The sealed version    by the inverter’s input, the type of battery
                                                                                     is used in most types of     you select, and the amount of energy storage
                                                                                     portable equipment.          you require.
Byron Stafford, NREL/PIX11663

                                                                                     Other names for sealed
                                                                                                                  Sizes and Costs
                                                                                     batteries are absorbed
                                                                                     glass mat, valve regulat-        To determine the number of batteries you
                                                                                     ed lead acid, and gel.       need, you must first determine how much
                                                                                     Lithium and nickel metal     energy storage you need in kilowatt-hours
                                                                                     hydride (NiMH) batteries,    (kWh). If you are connected to the utility
                                                                                     which are commonly           grid, you can use your monthly utility bill to
                                This 24-volt battery bank,                                                        calculate past energy usage for your house-
                                used at a remote home          used in cell phones, laptop computers, and
                                                               camcorders because of their energy-to-             hold. (Keep in mind that implementing
                                powered by a solar-electric                                                       energy-efficiency measures in your home is a
                                system, consists of 2-volt,    weight ratios, are very expensive and may
                                                               be difficult to use in residential solar           preliminary step to installing a solar electric
                                lead-acid batteries. The                                                          system. Reducing energy consumption and
                                batteries are flooded and      applications.
                                                                                                                  installing energy-efficient appliances are far
                                have clear cases for easy         The best kinds of batteries to use in a resi-   cheaper than purchasing larger solar electric
                                maintenance and quick visual   dential power system are deep-discharge            systems.) A second way to determine your
                                inspection of the plates and   lead-acid batteries specially designed for sta-    required kWh of energy storage is to multi-
                                the electrolyte levels.        tionary solar electric systems. Some golf cart     ply the wattage of your appliances by the
                                                               batteries may be a less expensive alternative.     number of hours you use them in a day.
                                                               Car and marine batteries are not recom-            Because watts = amps x volts, if you require
                                                               mended for solar electric system use because       1,000 watt-hours (or 1 kWh) per day, and
                                                               they are designed to give a large burst of         if you have a 24-volt battery bank, then
                                                               energy when starting a vehicle and are not         you need 42 amp-hours of useful storage.
                                                               made for deep discharges. Although they are        Because you cannot fully discharge lead-acid

batteries, you would need to install a larger         Keeping the batteries cooler than 70º-75º
battery to get the needed 42 amp-hours of             will not significantly extend their life but
capacity.                                             will reduce their capacity. Discharged bat-
                                                      teries may freeze and burst, so maintain
   Over the lifetime of the solar electric
                                                      an adequate charge on the batteries in
system, batteries will be the most expensive
                                                      cold weather.
component of the renewable energy system
in an off-grid home due to maintenance and          3. Maintain the same charge in all the
replacement costs. Initial costs for residential       batteries. Although the entire series of
batteries range from $80 to $200 per kWh.              batteries may have an overall charge of
                                                       24 volts, some cells may have more or less
What should you look for when
                                                       voltage than neighboring batteries.
purchasing a new battery?
1. A long cycle life, or how many deep              4. Inspect your batteries often. Some things
   discharges the batteries can provide.               to look for are leakage (buildup on the
                                                       outside of the battery), appropriate fluid
2. Thick lead plates—the thicker the plates,
                                                       levels (for flooded batteries), and equal
   the deeper the discharge and the longer
                                                       voltage. Your battery manufacturer may
   the battery life.
                                                       have additional recommendations.
3. If you have flooded batteries, look for          Battery Tips
   space at the bottom of the battery case
                                                    1. The largest cost, over the life of the
   to hold sloughed-off material, which can
                                                       system, is the batteries. The lifetime cost,
   lower the battery’s performance level, and
                                                       including maintenance, of your batteries
   adequate head space above the plates so
                                                       is dependent on your initial purchase
   you don’t have to water as often.
                                                       price, how well you adhere to a mainte-
   Flooded (unsealed, watered) batteries               nance schedule, and the replacement
may be the least expensive choice. However,            interval for the batteries you select.
flooded batteries require periodic electrolyte
                                                    2. The energy storage capacity of a battery
maintenance by adding distilled water and
                                                       is measured in watt-hours, which is the
equalizing the charge among cells. Keep in
                                                       amp-hour rating times the voltage. For
mind that sealed batteries still require main-
                                                       example, a 12-volt, 100-amp-hour battery
tenance, even though you don’t have to
                                                       has a storage capacity of 1,200 watt-hours,
check electrolyte levels. Sealed batteries are
                                                       which is the same as a 600-amp-hour, 2-
sometimes specified in difficult or remote
                                                       volt battery.
                                                    3. Follow manufacturer recommendations
Battery Maintenance
                                                       for voltage set points. Make sure that your
   All batteries will wear out in 1-15 years,
                                                       charger or charge controller will supply
even if they are rarely used, because the
                                                       the correct voltage.
acid in the battery wears down the internal
components regardless of use. However, you          4. Place batteries in a well-ventilated,
can maximize the life of your battery bank             temperature-moderated area because
by adhering to the following practices:                batteries give off gases that could
                                                       accumulate to form an explosive mixture.
1. Avoid repeated deep discharging of bat-
                                                       Batteries should be kept in an uncluttered,
   teries. The more a battery is discharged,
                                                       dry area of a shed or garage or placed in
   the shorter its lifetime. In addition, if your
                                                       a vented box with a strong lock for easy
   batteries are deeply discharged every
                                                       but safe access.
   day, you should increase the size of your                                                          Produced for the
                                                    5. Always refer to the battery manufactur-        U.S. Department of Energy
   battery bank.
                                                                                                      by the National Renewable
                                                       er’s recommendations for use and               Energy Laboratory, a DOE
2. Keep batteries at rated temperatures.
                                                       maintenance.                                   national laboratory
   Battery life is rated for 70º-75º tempera-
   tures. Keeping batteries warmer than this                                                          October 2002
   significantly reduces their life. Passive                                                          Printed with a renewable-source ink on
   solar is a great way to heat a battery stor-                                                       paper containing at least 50% wastepaper,
                                                                                                      including 20% postconsumer waste
   age unit, but it must be well insulated.

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