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Sizing Page
The size of the battery bank required will depend on the storage capacity required, the maximum discharge rate, the maximum charge rate, and the minimum temperature at which the batteries will be used. When designing a power system, all of these factors are looked at, and the one requiring the largest capacity will dictate battery size. Our System Sizing work forms take many of these factors into account.
One of the biggest mistakes made by those just starting out is not understanding the relationship between amps and amp-hour requirements of 120 volt AC items versus the effects on their DC low voltage batteries. For example, say you have a 24 volt nominal system powering a load of 3 amps, 120VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah). However, in order to determine the true drain on your batteries you have to divide your nominal battery voltage (24v) into the voltage of the load (120v), which is 5, and then multiply this times your amp hours (12 ah). So in this case the calculation would be 60 amp hours drained from your batteries - not the 12 ah. The easiest way to quickly determine the total battery amp hours required is to first determine total watt-hours required by all loads, and then divide by the nominal DC system voltage. This resulting number will indicate the amount of amp hours needed to operate all loads for a given period. However, additional amp hour capacity would typically be added for more "reserve" capacity or to prevent complete discharge. Using the above example, 3 amps x 120 VAC x 4 hours = 1440 watt-hours divided by 24 VDC battery environment equals 60 amp-hours; the same answer as before, but another way to get it.
There are other factors for determining the full extent of the battery drain, such as temperature, start-up factors, etc., but this should help you get a more complete picture on how to size your low DC voltage batteries when powering 120/240 volt loads using an inverter. Our System Sizing work forms take many of these factors into account.
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Temperature has a significant effect on lead-acid batteries. At 40°F they will have about 75% of rated capacity, and at 0°F their capacity drops to about 50%. An exception to this general rule would be the Concorde PVX battery, which is not as sensitive to these temperature extremes.
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 solar power system should have sufficient amp hour capacity to supply needed power during the longest expected period "no sun" or extremely cloudy conditions. In wind systems allowance for "no wind" days should be included. A lead-battery should be sized at least 20% larger than this amount. If there is a source of back-up power, such as a standby generator along with a battery charger, the battery bank does not have to be sized for worst-case weather conditions.
Series Wiring
Series wiring refers to connecting batteries to increase volts, but not amps. If you have two 6 volt batteries like the Trojan L16 rated at 350 amp hours, for example, by connecting the positive terminal of one battery to the negative terminal of the other, then you have series wired the two together. In this case, you now have a 12 volt battery and the rated 350 amps does not change. If you were to series wire four L16's you'd have 24 volts at 350 amps, and so on.
Parallel Wiring
Parallel wiring refers to connecting batteries to increase amps, but not volts. If you have two 6 volt batteries like the Trojan L16 rated at 350 amp hours, for example, by connecting the positive terminal of one battery to the positive terminal of the other, and the same with the negative terminal, then you have parallel wired the two together. In this case, you now have a 6 volt battery and the rated 350 amps increases to 700 amp hours. If you were to series wire four L16's you'd have 24 volts at 350 amps, and then parallel wire these four to the four other that are in series, then you'd have a 24 volt battery at 700 amps.
Five
basic wiring types
Using these wiring examples a complete battery bank
might have any number of total batteries to achieve required reserve
capacity.
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 80 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 sulfation of the positive plate and a permanent loss of capacity.
Sealed deep-cycle lead-acid batteries are maintenance free. They never need watering or an equalization charge. They cannot freeze or spill, so they can be mounted in any position. We especially recommend sealed batteries for remote, unattended power systems, but also for any client who wants the maintenance free feature and doesn't mind the extra cost associated with these batteries. The Concorde PVX series (Sun-Xtender) is an excellent choice.
Sealed Gel Cell (gelled-electrolyte) batteries are relatively maintenance free, however unlike a high quality sealed lead-acid battery like the Concorde PVX extra care must be taken to insure a Gel Cell battery is not charged above 14.1 volts for a 12 volt battery, for example. Over charging a Gel Cell even once for a sustained period can really shorten it's life and even ruin it. Any charge source or charge regulator used must have user adjustable settings for sealed Gel Cell batteries to insure charge voltage does not exceed a safe limit. If your application dictates a sealed, gelled battery the Deka-East Penn MK series is an excellent choice.
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Wet cell lead acid batteries like the high quality Surrette require periodic watering and equalization. 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.
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.
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 possibly you too.
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. Typically, we'll recommend an equalization charge at least once a month. Do not equalize sealed or gell type batteries. With proper care, lead-acid batteries will have a long service life and work very well in almost any power system.
Measuring battery condition
Connect a voltmeter and measure the voltage across the battery terminals with the battery at rest (no input, no output) for at least three hours. These readings are best taken in the early morning, at or before sunrise, or in late evening. Take the reading while all loads are off and no charging sources are producing power.
The following table will allow conversion of the voltage readings obtained to an estimate of state of charge. The table is good for batteries at 77·F that have been at rest for 3 hours or more. If the batteries are at a lower temperature you can expect lower voltage readings.
You can see that when your voltage reading is about equal to
the battery "nominal voltage" your battery is about 60% discharged.
Battery State of Charge Voltage
Table
|
Percent of Full Charge |
12 Volt DC System |
24 Volt DC System |
48 Volts DC System |
|
100% |
12.7 |
25.4 |
50.8 |
|
90% |
12.6 |
25.2 |
50.4 |
|
80% |
12.5 |
25 |
50 |
|
70% |
12.3 |
24.6 |
49.2 |
|
60% |
12.2 |
24.4 |
48.8 |
|
50% |
12.1 |
24.2 |
48.4 |
|
40% |
12.0 |
24 |
48 |
|
30% |
11.8 |
23.6 |
47.2 |
|
20% |
11.7 |
23.4 |
46.8 |
|
10% |
11.6 |
23.2 |
46.4 |
|
0% |
<11.6 |
<23.2 |
<46.4 |
The following chart reflects state of charge vs. specific gravity of the electrolyte in each cell. A hydrometer is used to determine specific gravity.
|
State of Charge |
Specific Gravity |
|
100% Charged |
1.265 |
|
75% Charged |
1.239 |
|
50% Charged |
1.200 |
|
25% Charged |
1.170 |
|
Fully Discharged |
1.110 |
|
These readings are correct at 75°F |
|
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