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Frequently Asked Questions - System Batteries
Q. Do I need system batteries? A. All standalone and battery backup 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 grid-tie systems supply power directly to the utility grid; no battery storage is needed.
Q. What kind of battery do I need for my renewable energy system? A. The two most common types of rechargeable batteries today are lead-acid and alkaline. Lead acid batteries are what you need for your system. They have plates made of lead, mixed with other materials, submerged in a sulfuric acid solution. We do not sell nickel-cadmium batteries at The Sustainable Village because of their environmental problems when disposed. Nickel metal hydride and lithium ion batteries may be a choice in the future, but now their cost is too high for the size needed for all but the smallest of remote lighting systems.
A. That depends on the storage capacity required, the maximum discharge rate, the maximum charge rate, and the minimum temperature at which the batteries will be used. Look at all of these factors and go with the largest capacity dictated.
Q. What voltage do I need? A. We have worksheets that you can use to determine what size battery array is required for your system. Battery size is measured in amp-hours. Battery voltage is determined by the number of "cells" wired in series. All lead-acid battery cells have a nominal output of 2 volts. Actual cell voltage varies from about 1.7 volts at full discharge to 2.4 volts at full charge. 12-volt lead acid batteries are made of 6 separate cells in one case. 6-volt batteries are made of 3 cells in one case. Wiring battery cells in parallel increases amp-hour capacity, but does not change voltage.
Q. How much electrical storage capacity do I need? A. The storage capacity, or the amount of electrical energy it can hold, is expressed in amp hours. A battery should have enough amp-hour capacity to supply needed power during the longest expected period of cloudy weather (if your renewable energy is PV). A lead acid battery should be sized at least 20% larger than this amount.
Q. Are all lead acid batteries the same? A. No. There are shallow cycle batteries and deep cycle batteries. You will need deep cycle for your renewable energy system. They are designed to be repeatedly discharged by as much as 80% of their capacity. However, they will have a longer life if the cycles are shallower. Shallow cycle batteries are used in automobiles and they cannot tolerate being deeply discharged.
A. At 40 degrees F, they will have 75% of rated capacity and at 0 degrees F their capacity drops to 50%.
Q. What about maintenance? A. Sealed deep cycle lead-acid batteries, gel cells and absorbed glass mat, are virtually maintenance free. They do not need watering or equalization. They do require very accurate regulation to prevent over-charge and over-discharge. Either of these conditions will drastically shorten their lives. We recommend sealed batteries for remote, unattended power systems. For flooded batteries, see the question below on "Equalization."
Q. What SAFETY MEASURES are needed around the batteries?
Batteries can deliver extremely high current. Always install fuse protection on any positive wiring connected to batteries. Keep the tops of your batteries clean and check that cables are tight. Do not tighten or remove cables wile charging or discharging. Any spark around batteries can cause a hydrogen explosion inside, and ruin one of the cells, and YOU!
Q. How do I ensure a long battery life? A. Lead-acid batteries fail prematurely when they are not recharged completely after each cycle. Letting a lead-acid battery stay in a discharged condition for days at a time will cause a permanent loss of capacity. The positive plates change from lead oxide when charged to lead sulfate when discharged. If they remain in the lead sulfate state for a few days, part of the plate does not return to lead oxide when the battery is recharged. 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. Check your batteries on a regular basis to be sure they are getting charged. You can use a hydrometer to check the specific gravity of your lead-acid batteries.* If batteries are cycled very deeply and then recharged slowly, the specific gravity reading will be lower because of incomplete mixing of electrolyte. *PUTTING ANYTHING INSIDE YOUR BATTERY MAY INTRODUCE CONTAMINANTS! BE CAREFUL!
Q. What does it mean to "water" the batteries? A. Check the electrolyte level in wet-cell batteries at least four times a year and top-off each cell with distilled water. Do not add water to discharged batteries. Electrolyte is absorbed when batteries are discharged. If you add water at this time and then recharge the battery, electrolyte will overflow and make a mess.
Q. What does "equalization" mean? A. This is a long steady overcharge, bringing the battery to a gassing or bubbling state. It is a good idea to do an equalizing charge when some cells show a variation of 0.05 specific gravity from each other. Do not equalize sealed or gel-type batteries.
Q. What is a hydrometer and how does it work? A. A hydrometer is very accurate at measuring battery state-of-charge if you measure the electrolyte near the plates. Unfortunately, you can only measure the electrolyte at the top of the battery. When a battery is being charged or discharged, a chemical reaction takes place at the border between the lead plates and the electrolyte. During charging, the electrolyte changes from water to sulfuric acid. The acid becomes stronger and the specific gravity rises as the battery chargers. Near the end of the charging cycle, gas bubbles rising through the acid stirs the fluid to mix it. It takes several hours for the electrolyte to mix so that you get an accurate reading at the top of the battery. Always try to take readings after a period of no charge or discharge.
Q. How can I determine the "state-of-charge" of my batteries? A. Battery "state-of-charge" can be measured by an amp-hour meter, voltage or by specific gravity. Some care and knowledge is required to interpret state-of-charge from voltage or specific gravity readings. But using amp-hour meters is clear and easy to understand. At a glance, you can see system voltage, current, and battery condition.
Q. What if the battery overheats on discharge? A. This could be caused by over discharging the batteries or excessive loads, or it was not fully charged prior to discharging, or the electrolyte levels are low or it is operating in high ambient temperatures.
Q. What if the electrolyte level is low? A. Probably a broken or cracked jar. It could also happen if you did not "water" the batteries as needed or if a cell was missed when adding water. Could also result from too much over charging.
Q. What causes unequal specific gravity and cell voltages? A. There are many causes for this, such as:
Q. What causes excessive cell temperatures? A. It could be a defective or weak cell, insufficient air circulation around the batteries on charge, the charger is too large for battery, the electrolyte level is too low, or cell(s) are shorted.
Q. What if my batteries are not "holding up" as expected? A. This can be caused by any of the following:
Q: Why is charging voltage so critical to both gel and sealed lead acid batteries? A: Charge voltage is critical on these types of batteries because they both are recombinant batteries. This means that the oxygen that is normally produced on the positive plate in all lead acid batteries recombines with the hydrogen given off by the negative plate. The “recombination” of hydrogen and oxygen produces water, which replaces the moisture in the battery. Therefore the battery is maintenance free and never needs watering. The sealing vent is critical to the performance of a sealed battery. The cell must maintain a positive internal pressure, otherwise the recombination of the gasses will not take place and the cell will dry out and not perform. In addition, the valve must safely release any excess pressure that may be produced during overcharging, otherwise the cell would be irreparably damaged. It is because of the safety mechanism of the valve that overcharging hurts a sealed battery. The excessive pressure that the valve is releasing is both hydrogen and oxygen. The more you overcharge a battery, there is less oxygen and hydrogen left inside the battery to recombine and create water. In essence, the battery dries out. It’s important to note that a sealed battery must never be opened once it leaves the factory. If opened, the cell loses its pressure, and the outside air will “poison” the plates and cause an imbalance that destroys the recombination chemistry. Hence the name: Sealed, Valve Regulated (SVR) Battery. The correct charging voltage for a Gel battery is 13.8 to 14.1 volts maximum at 68°F. The correct charging voltage for our Absorbed lead acid battery is 14.4 to 14.6 volts maximum at 68°F.
Q: What is the difference between gel cell and starved electrolyte batteries? A: Both are recombinant batteries; both are sealed valve regulated. The major difference is that the “starved” or “absorbed electrolyte” battery contains an amount of liquid electrolyte added at the factory that soaks into the special separators. Therefore, it is non-spillable because all of the liquid electrolyte is trapped in the sponge like separator material. There is no “free” electrolyte to spill if tipped or punctured.
Q: What are the advantages and disadvantages of both types of sealed valve regulated batteries? A: Gel Battery Advantages:
Gel Battery Disadvantages:
Absorbed Electrolyte Advantage:
Absorbed Electrolyte Disadvantages:
Q: Do I have a battery problem or a charging problem? A: The Digital Voltmeter is a valuable tool in the Mobility Technician’s arsenal for sealed batteries. The starting point for checking batteries is always the charge voltage. In 24-volt systems we know the chances for two bad batteries are less than 1 in 10,000. So what we need to determine is WHICH battery is bad or if either battery is bad. This is accomplished by checking the voltage of each battery separately. As illustrated in photo 1, voltage for a pair of batteries can read in excess of 24-volts which can incorrectly be assumed to be a good set. However, as shown in photo 2 one battery has a voltage of 12.89 volts while the battery in photo 3 is reading 11.97 volts. Combined, the voltage of this set of batteries looks good, but clearly the battery in photo 3 is bad.
Two batteries in a 24-volt system charge and discharge together almost as one 24-volt battery. A wide voltage separation between two batteries indicates that you may need to replace both batteries. If both batteries read similar voltage, they should be fully charged before doing any further testing. If both batteries are below 12.0 volts, the question becomes, “WHY?” Is the battery charger working correctly? Could there be a problem with the wiring or other components of the wheelchair? You can determine the next step in the troubleshooting process once you know the voltage of each battery.
If you have questions or comments, please email technical@sustainablevillage.com.
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