Batteries: The Heart of the Stand Alone System
Batteries are the heart of the home solar power system: they are the reservoir that provides electrical energy at all times of the day, rain or shine. They provide huge bursts of energy required for starting and operating large appliances that a PV array alone could not. Your battery is the nucleus of your system, the Grand Central Station of energy distribution. The type and size of battery you choose will set the stage for your system’s performance.
Types of Batteries
There are two basic types of batteries used in renewable energy systems: lead-acid and alkaline. Lead-acid batteries are by far the most used type because of their universal availability and relative low cost. Alkaline batteries are either nickel-cadmium or nickel- iron type. Though alkaline batteries are commercially available, their expense and maintenance characteristics make them less desirable.
There are many types of lead-acid batteries: automobile, marine, golf cart, forklift, telephone etc. Some of these are shallow cycle and some are deep cycle. A shallow cycle battery is one that can only be discharged 10 to 30% of its capacity. A deep cycle battery is designed to be discharged 50 to 80% of its capacity.
Motive power batteries like forklift, golf cart and marine are designed to be used a good length of time between recharging and they are deep cycle. These types of lead-acid batteries are the most used types in solar electric systems.
Regular automobile batteries are shallow cycle and are designed to start a car and then be immediately recharged by a high output alternator. They will not hold up over slow-charging and discharging cycles characteristic of a solar electric system. A marine battery is a 12 volt battery, slightly bigger than a car battery, and designed to power a trolling motor, or start a boat engine. They can be adequate for a small 1 or 2 module system where intermittent use of a few small loads are required.
Golf cart batteries are probably the most common storage battery used for PV systems. They are a 6 volt battery, slightly larger that a marine type. It takes two connected in “series” to provide 12 volts. Adding multiples of two in “parallel” increases the capacity of storage. Many large systems use golf cart batteries because that is all available in their area.
For the larger home power systems the bigger industrial batteries are by far the preferred battery. Larger cells with thicker more massive plate construction will simply last longer and take more abuse than smaller cells. Industrial batteries like forklift and telephone cells make excellent storage batteries for larger systems.
Forklift batteries are two volt cells usually packed together in a steel tank in 24, 36 or 48 volt configurations. The cells range in capacity anywhere from 400 to 1200 amp hour. If used out of their tanks, they need bracing because their thin plastic casing lacks structural rigidity These cells tend to need regular watering intervals because of their low liquid to plate ratio.
Telephone batteries or technically lead-calcium batteries are heavy duty industrial grade two volt cells. Their size and capacity can range from 200 amp hours to well over 2000 amp hours and their weight per cell from 60 to 400 lbs. They are a shallow-cycle cell and should not be regularly discharged more than 30%. However, 30% of the larger cells is still is a lot of energy, so they can make excellent storage batteries for medium to large home power systems. These batteries come from the factory with an estimated 30-year life expectancy and often last much longer. They have clear, sturdy plastic cases for easy plate inspection. Dimensions of a 1680 amp hour cell are approximately 15″ square and 21″ high. We used to get these frequently, but they are getting much harder to find because the telephone companies are going to a much smaller, cheaper storage battery for the more efficient digital phone systems being employed.
Your batteries need to be large enough to provide energy for extended overcast periods when there is little to no charging.
To determine your needed battery capacity:
- Consider your average winter energy needs on a daily basis (in amp-hours at your system voltage). We use the winter months because they are usually the most overcast and the days are shorter. This figure should allow for future expansion of system and increased electrical demand.
- Now, multiply daily amp-hour demand by the number of consecutive days without significant sunshine. (For the Ozark region, 5 days of cloudiness is not uncommon.) So, multiply this number times your winter energy demand (amp-hours). This figure will give you the amount of usable battery storage your system will require. Ideally, this figure should be 50% or less of the total capacity of your batteries. If your batteries are true deep cycle, then your usable storage could equal 70 to 80% of your total capacity. If you plan on using ni-cad batteries, then, you can figure on 100% usable storage.
This big battery bank is efficiently tucked away in a room with a three-tiered steel rack. The room is actually the entryway to the basement that houses the rest of the system’s controls and the inverter. This battery room is vented and closed off from the rest of the basement. These are the ultimate conditions for safety and the health of the batteries.
This is an add-on type of battery shed in construction. This shed will be insulated and vented at the top. The top will be hinged and open up for battery inspection.
Battery location is not a factor to be taken lightly. They need to be protected from excessive temperatures, precipitation, dust, and direct sunshine. Batteries prefer a cool, stable environment. Lead-acid batteries have greater capacity when they are warm (90 degrees F.), but, age faster. At extreme freezing temperatures (sub-zero), they can lose up to 50% of their capacity. If discharged low enough they can freeze, and possibly bust their jar.
Housing batteries in an adjacent shed to the house or in the crawl space under the house is good, if accessible and centrally located, so that wire runs into the house and from the array are at a minimum. It is good to locate an adjacent shed for batteries on the south side so that some degree of passive solar heating will help keep the cells from excessive low temperatures.
Batteries need some ventilation in their enclosure so flammable and corrosive gases cannot build up to hazardous levels. Vents should be located at high points in the enclosure and on either side of the battery to allow for a cross-flow. It is good to insulate batteries from the ground, especially if the ground is subject to freezing temperatures and moisture. If the foundation for the batteries is on cement blocks or a concrete slab, use plywood and/or ¸” or 1″ rigid insulation between.
WE DO NOT RECOMMEND putting batteries in your living space. There is too great a potential for Murphy’s Law to manifest when batteries, fire and humans (especially kids) occupy the same space. Batteries don’t get lonely. They do well without human company so put them outside or build them their own room.
Happiness is a fully charged set of batteries in the independent home power system. Keeping batteries healthy is the highest priority in system performance. The health of your batteries dictates the performance of your whole system.
If system components are sized and installed properly, there is little battery maintenance required. Periodically checking water levels and your voltmeter is generally enough maintenance especially if you are reaching full charge voltages regularly.
A fully charged set of lead-acid batteries at rest (no charging or discharging happening) will have a voltage of around 12.7 for a 12 volt system and 25.4 for a 24 volt system. Most Telephone cells electrolyte have a weaker acid content than other types of lead-acid batteries and therefore have a lower full charge resting voltage–between 12.3 and 12.5 (24.6 and 25.0 for 24 volt systems).
Every four to six months a hydrometer reading or voltage reading of each cell is recommended. A voltage reading of each cell may be taken using a multi-tester. We recommend investing in a digital multi-tester and learning to use all of its functions. Standard digital meters will read volts, amps (up to 10) and ohms or resistance. It will solve a lot of mysteries for you and the digital read-out leaves little to the imagination. Hydrometers and digital multi-testers are available at most auto parts, hardware and electronics stores. It is good to take readings at two different times: (1) in the morning before charging begins and no major loads are on in the system, and (2) during peak charging.
Some cells will show about the same voltage during rest but when charging a weaker cell will show a considerably lower voltage–10 hundreds or more. This indicates that the weak cell or cells are not fully charged and a mild overcharge or equalizing is necessary. The resting voltages of each cell should not vary more than 3 or 4 hundredths of a volt. A fully charged cell will read 2.10 to 2.12 volts (2.06 or 2.07 for telephone cells). A ni-cad cell a full charge will read around 1.35 volts or 13.5 volts for a 10 cell/12 volt system. Ni-cad cells have the characteristic of maintaining consistent voltage between full and 10% state of charge–between 1.2 and 1.25.
For lead-acid batteries, hydrometer readings should be taken in the morning because acid levels have had the extra time to homogenize after a day of charging. If water has recently been added, then the acid level will be weak at the top and a hydrometer reading will read lower than the actual state of charge. To help prevent contamination of the electrolyte, keep the hydrometer in a clean container. Note that telephone batteries, because of a weaker electrolyte will show around 1.220 when full; deep cycle batteries will read between 1.275 and 1.300 when full. Use only distilled water when adding to cells.
The tops of the batteries should be kept clean to prevent the cells from discharging through the dirt particle build-up around the terminals. Use massive interconnects between terminals (the equivalent of #4 gauge wire or greater) and annually check the tightness of the terminal bolts. Each terminal and bolt should be liberally greased with petroleum jelly or other suitable anti-corrosive compound. This will help prevent oxidation and corrosion that can happen when battery gassing and humidity mix. Corrosion at connections to battery terminals will significantly diminish your system’s performance.