How you can store your generated electricity Updated 11/15/95 --------------------------------- Battery Information Lead-Acid Batteries Nickel-Cadmium Batteries Nickel-Iron Batteries Caring For Lead-Acid Batteries --------------------------------- 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. ---------------------------------------------------- [Mr. Solar Home Page] [Catalogue] [Entire Systems] [New Products & Specials] ["Ask Mr. Solar"] [Articles] [Like-Minded Web Sites] [Global Network] [DO IT Homestead] ---MrSolar@netins.net---