

               How you can store your generated electricity

                             Updated 11/15/95



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     Battery Information



     Lead-Acid Batteries



     Nickel-Cadmium Batteries



     Nickel-Iron Batteries



     Caring For Lead-Acid Batteries



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                            Battery Information



     All stand-alone and utility interface PV systems require battery

     storage. Photovoltaic modules charge the batteries during

     daylight hours and the batteries supply the power when it is

     needed, often at night and during cloudy weather. Utility

     intertie systems supply power directly to the utility grid, no

     battery storage is needed.



     The two most common types of rechargeable batteries in use are

     lead-acid and nickel-cadmium. Lead acid batteries have plates

     made of lead, mixed with other materials, submerged in a sulfuric

     acid solution. Nickel-cadmium batteries have plates made of

     nickel and cadmium submerged in a solution of potassium

     hydroxide. We now have nickel-iron batteries, which are similar

     to nickel cadmium batteries.



     The size of the battery bank required will depend on the storage

     capacity required, the maximum discharge rate, the maximum charge

     rate, and with lead-acid batteries, the minimum temperature at

     which the batteries will be stored. When designing a power

     system, all of these factors are looked at and the one requiring

     the largest capacity will dictate battery size. Temperature has a

     significant effect on lead-acid batteries. At 40 degrees F they

     will have 75% of rated capacity, and at 0 degrees F their

     capacity drops to 50%. The storage capacity of a battery, the

     amount of electrical energy it can hold, is usually expressed in

     amp hours. If one amp is used for 100 hours, then 100 amp-hours

     have been used. A battery in a PV power system should have

     sufficient amp hour capacity to supply needed power during the

     longest expected period of cloudy weather.



     A lead-battery should be sized at least 30% larger than this

     amount, but a ni-cad on nickel iron battery can be sized to

     exactly this amount. The reason for this is if you discharge your

     batteries over this amount on a regular bases then you will

     greatly reduce their life or ability to be recharged to full

     capacity. Please note the chart in this area.



                            Lead-Acid Batteries



     Lead-acid batteries are the most common in PV systems because

     their initial cost is lower and because they are readily

     available nearly everywhere in the world. There are many

     different sizes and designs of lead-acid batteries, but the most

     important designation is whether they are deep cycle batteries or

     shallow cycle batteries.



     Shallow cycle batteries, like the type used as starting batteries

     in automobiles, are designed to supply a large amount of current

     for a short time and stand mild overcharge without losing

     electrolyte. Unfortunately, they cannot tolerate being deeply

     discharged. If they are repeatedly discharged more than 20

     percent, their life will be very short. These batteries are not a

     good choice for a PV system.



     Deep cycle batteries are designed to be repeatedly discharged by

     as much as 50 percent of their capacity so they are a good choice

     for power systems. Even though they are designed to withstand

     deep cycling, these batteries will have a longer life if the

     cycles are shallower. All lead-acid batteries will fail

     prematurely if they are not recharged completely after each

     cycle. Letting a lead-acid battery stay in a discharged condition

     for many days at a time will cause sulfating of the positive

     plate and a permanent loss of capacity.



     We also sell a line of deep cycle sealed ( Deka Gelled )

     lead-acid batteries. These batteries are not as durable as

     wet-cell batteries, but they are maintenance-free; they never

     need watering or equalization charge. They cannot freeze or

     spill, so they can be mounted in any position. Sealed batteries

     require very accurate regulation to prevent overcharge and over

     discharge. We strongly recommend these for small to medium

     systems such as weekend cabins. Either of these conditions will

     drastically shorten their lives.



                                 [Image]



                         Nickel-Cadmium Batteries



     Nickel-cadmium batteries are nearly ideal for all types of power

     systems, but their initial cost is very high. Nickel-cadmium

     batteries cost much more than similar capacity lead-acid

     batteries, but they can last five times as long. A smaller

     capacity battery can often be used.



     Since ni-cad batteries can be discharged 100%, and they can stay

     in a partially discharged state for long periods of time without

     damage, a smaller size battery can be used. The fact that their

     capacity does not get lower at low temperatures, and that their

     voltage is very stable from full charge to full discharge, even

     under large loads, makes a smaller battery a workable choice.

     Depending on loads and weather conditions, a 30% to 50% smaller

     battery can be used.



     Ni-cads can operate down to -30 degrees F with only a small loss

     of capacity and they will not be damaged if they freeze. This

     allows them to be used in areas with severe temperature swings

     without having oversize capacity and without providing a heated

     space.



     Charging efficiency of nickel-cadmium batteries is about the same

     as that of new lead acid batteries, and it stays the same for the

     life of the battery. Their overall efficiency is higher, because

     they have an extremely low self-discharge rate.



     If the power system size is increased at a later time, additional

     ni-cad batteries of the same type can be added to the old bank

     without any problem, something that should never be done with

     lead-acid batteries after they have been in service for more than

     six months.



     Nickel-Cadmium batteries require slightly higher charging voltage

     than lead acid batteries to bring them up to full charge. This

     may cause some difficulty with equipment designed for lead-acid

     batteries. Most battery chargers will not go up to the 16.5 volts

     required to completely charge them. Always use solar modules with

     36 or more cells to insure high enough charging voltage. This

     higher full charge voltage will cause some invertors to turn

     themselves off, so they may not work near the end of a charging

     cycle. Even though these batteries reach this voltage near full

     charge, the voltage drops immediately upon discharge, so the

     condition is only temporary.



                           Nickel-Iron Batteries



     Nickel-iron batteries are very similar to nickel-cadmium. They

     use an alkaline electrolyte similar to nickel-cadmium batteries,

     which allows them to withstand low temperatures. Their charge and

     discharge voltage is about the same, as is their long life. Many

     old nickel-iron batteries that were made 50 years ago in the US

     are still working fine. The nickel-iron batteries we sell are

     made in Eastern Europe, so we do not yet know how long they will

     last. One major difference between nickel-iron and ni-cad

     batteries is discharge rate. Nickel-iron batteries cannot deliver

     the extremely high currents that ni-cads can, so if you plan to

     have heavy loads, you will need to have a larger capacity battery

     bank.



     Nickel-iron batteries have a larger voltage swing between

     discharged and fully charged than nickel cadmium batteries have.

     The voltage gets higher when charging and drops more rapidly when

     discharging. A 12 volt battery pack reaches 16.5 or 17 volts at

     full charge and drops to 10 volts when fully discharged.



     One of the most interesting aspects of nickel-iron batteries is

     that they are made without toxic lead or cadmium, which solves a

     future disposal problem and therefore they are environment

     friendly.



     The manufacturer says the service life can be over 30 years at

     discharge of 40% per day. 12 volt battery packs consist of ten

     1.2 volt cells. We supply nickel plated copper interconnects and

     output terminals. Charging efficiency is below 70% and overnight

     self-discharge is about 5%. The manufacturer says the service

     life can be over 30 years at discharge of 40% per day. 12 volt

     battery packs consist of ten 1.2 volt cells. They are currently

     warranted for a period of 3 years. They are being used in Eastern

     Europe being charged and fully discharged daily for a period of

     five years without ruination.



                      Caring For Lead-Acid Batteries



     Check the electrolyte level in wet-cell batteries at least four

     times a year and top each cell off with distilled water. Do not

     add water to discharged batteries. Electrolyte is absorbed when

     batteries are very discharged. If you add water at this time, and

     then recharge the battery, electrolyte will overflow and make a

     mess.



     Keep the tops of your batteries clean and check that cables are

     tight. Do not tighten or remove cables while charging or

     discharging. Any spark around batteries can cause a hydrogen

     explosion inside, and ruin one of the cells, and you.



     It is a good idea to do an equalizing charge when some cells show

     a variation of 0.05 specific gravity from each other. This is a

     long steady overcharge, bringing the battery to a gassing or

     bubbling state. Do not equalize sealed or gell type batteries.



                                 [Image]



                 These reading are correct at 75 degrees F



     With proper care, lead-acid batteries will have a long service

     life and work very well in almost any power system.

     Unfortunately, with poor treatment lead-acid battery life will be

     very short.



     The quickest way to ruin lead-acid batteries is to discharge them

     deeply and leave them stand "dead" for an extended period of

     time. When they discharge, there is a chemical change in the

     positive plates of the battery. They change from lead oxide when

     charged to lead sulfate when discharged. If they remain in the

     lead sulfate state for a few days, some part of the plate does

     not return to lead oxide when the battery is recharged. If the

     battery remains discharged longer, a greater amount of the

     positive plate will remain lead sulfate. The parts of the plates

     that become "sulfated" no longer store energy. Batteries that are

     deeply discharged, and then charged partially on a regular basis

     can fail in less than one year. Use a hydrometer to check the

     specific gravity of your lead acid batteries periodically. The

     chart below gives state of charge vs. specific gravity of the

     electrolyte. Check your batteries on a regular basis to keep

     track of how charged they are getting. If batteries are cycled

     very deeply and then recharged quickly, the specific gravity

     reading will seem lower than it should because the electrolyte at

     the top of each cell will not mix with the charged electrolyte

     until the battery begins to "gas" or bubble for a while.



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