Battery Types, Applications, Configurations and Battery Bank Sizing
GENERATOR/BATTERY/INVERTER or "GENVERTER" SYSTEM
The most common home power configuration is the generator-battery-inverter also called the "genverter" system. With the addition of an inverter to a generator and a battery bank, this becomes a truley independant system, capable of providing the home owner with electricity for years. Independant of weather conditions, the amount of sunlight, wind, etc., a "genverter" system can supply power during high demand periods such as power tool use, dryer operation, etc. The cost per kilowatt hour can remain low if the generator is run at close to it's rated capacity and the batteries are charged while other high demand operations are performed (during winter, in Idaho, when sunlight and winds are low we produce power for about 3.5 cents per kw/hr). In the gen-battery configuration, the inverter provides power from the battery bank for AC loads during the time when the generator is not running. Battery systems are configured in either 12 or 24 volt (48 volt in rare extremely heavy use systems). Only deep-cycle batteries should be used with a independant energy system. This is a practical and cost effective system in any location, but especially in northern latitudes
Battery Configurations
Parallel connection - two batteries connected in parallel (positive to positive, negative to negative) will double the amp-hour rating of the battery bank.
Series connection - two batteries in series (positive to negative) will double the voltage of the battery bank. The amp-hour rate remains the same.
The size of your battery bank will depend on your required storage capacity, the maximum discharge rate and the minimum temperature to which the batteries will be exposed.
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 20% larger than this amount. If you have a standby generator, your battery bank does not have to be sized for the worst case.
| Battery
Size Just as important as the type of battery selected for use with your Statpower brand product is the subject of battery size or capacity. Unfortunately, there are a number of different standards for rating battery energy storage capacity. Automotive starting batteries are normally rated at cranking amps. This is not a relevant rating for continuous use. Deep cycle batteries are rated either by reserve capacity in minutes or by amp-hours. Battery reserve capacity is a measure of how long a battery can deliver a certain amount of current - usually 25 amps. For example: a battery with a reserve capacity of 180 minutes can deliver 25 amps for 180 minutes before it is completely discharged. Amp-hour capacity is a measure of how many amps a battery can deliver for a specified length of time - usually 20 hours. For example: a typical marine or RV battery rated for 100 amp-hours can deliver 5 amps for 20 hours. 5 amps X 20 hours = 100 amp-hours The batteries are an important part of your system, so we recommend you purchase as much battery capacity as possible. A large battery will extend running time and ensure your Statpower brand inverter product delivers its full rated surge. Even if your battery is in excellent shape and fully charged, with too small a battery you will likely experience poor surge power performance and unsatisfactory operating time with anything but a small AC load. We recommend a minimum battery size of 200 Ah for moderate loads (300 to 1000W) and greater than 400 Ah for heavy loads.
Determining Battery Size for Inverter Usage To determine how large a battery or battery bank you require for equipment running off a Statpower brand inverter product, simply add together the power requirements for all electrical devices that you will be running, multiplied by their approximate running times in hours between battery recharges. Each device will be rated in either watts, volts and amps, or VA. For this calculation, all three of these ratings are equivalent: volts X amps = watts
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Convert the watt-hours to amp-hours by dividing total watt-hours by 10: 1495 watt-hours / 10 = 149.5 amp-hours A 150 amp-hour battery is required to supply enough power for the above loads, and become completely discharged. Under normally circumsatnces, you would only want to discharge your battery to 50% capacity, so for the above loads you would require 300 amp-hours of battery capacity. When sizing your battery, be conservative. More capacity is better since you will have more reserve capacity and your battery therefore won't discharge as deeply. The deeper the discharge, the shorter the battery life. As your power requirements increase, to obtain sufficient battery capacity, you may need to use more than one battery. Two identical batteries can be connected positive to positive and negative to negative, in a parallel system that doubles the capacity and maintains the voltage of a single battery. It is not recommended to connect batteries from different manufacturers or with different amp-hour ratings in parallel. Decreased battery life may result. |
Battery Types
| Absorbed Glass Mat and Gel Cell Batteries |
| Absorbed Glass Mat or AGM batteries utilize a fiberglass mat saturated with sulfuric acid. AGM batteries are also sometimes called "starved electrolyte" or "dry", because the fiberglass mat is only 95% saturated with sulfuric acid and there is no excess liquid. An AGM battery is cleaner and can be shipped without any hazardous material requirements. They are far superior for most uses, can take a fair amount of abuse and are non-spilling even when broken. The major disadvantage is a higher cost than a flooded battery, approximately 2 to 3 times. In cases where fumes and leakage are not an issue, the more economical choice is probably a flooded industrial lead-acid |
| GEL Cell or sealed lead-acid batteries are frequently selected in applications where batteries cannot be vented or cannot be mounted in an upright position. Gel cells are cleaner in the sense that they do not vent gasses like lead acid batteries. However, gel cells are more sensitive to charge voltage (and cannot typically be charged with an automotive type battery charger) since they cannot vent except in emergencies (which may cause irreversible damage). In addition, the gel cells are much more sensitive to higher temperatures and cannot tolerate being discharged for long periods of time relative to a flooded lead acid battery. Therefore, the charge on gel cells must be regulated properly. Manufacturers’ recommended regulation set points must be followed. Gel cell batteries may require an external battery temperature compensated regulator. We do not recommend this type of battery. |
Lead-acid batteries fall into two categories. 1. Shallow cycle - these are the type used to start your car. They are designed to deliver a large amount of current over a short period of time. This type is unsuitable for a home power battery bank. They cannot withstand being deeply discharged, to do so shortens their life. 2. Deep cycle - Designed to be discharged by as much as 80% of their capacity, this is the type of choice for home power systems. The life of deep cycle batteries will be extended if the cycle is shallower than 80% and if they are fully recharged after each cycle (this avoids positive plate sulfating). 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.
Always use extreme caution when handling batteries and electrolyte. Wear gloves, goggles and old clothes. "Battery acid" will burn skin and eyes and destroy cotton and wool clothing. Check your batteries on a regular basis to be sure they are getting charged. Use a hydrometer to check the specific gravity of your lead acid batteries. If batteries are cycled very deeply and then recharged quickly, the specific gravity reading will be lower than it should because the electrolyte at the top of the battery may not have mixed with the "charged" electrolyte. 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.
Exercising proper care will lead to a long service life as with poor care battery life will be very short.
The chart below gives state of charge vs. specific gravity of the electrolyte.
SPECIFIC GRAVITY of ELECTROLYTE
| 100% charge | 1.265 |
| 75% charge | 1.239 |
| 50% charge | 1.200 |
| 25% charge | 1.170 |
| fully discharged | 1.110 |
| at 75 degrees | |