The size of the battery bank required 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. 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°F they will have 75% of rated capacity, and at 0°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-acid battery should be sized at least 20% larger than this amount. If there is a source of backup power, such as a standby generator with a battery charger, the battery bank does not have to be sized for worst-case weather conditions.

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 starting batteries in automobiles, are designed to supply a large amount of current for a short time and to stand mild overcharge without losing electrolyte. But they cannot tolerate being deeply discharged. If they are repeatedly discharged more than 20% 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% of their capacity so they are a good choice for PV 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 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. Sealed deep-cycle lead-acid batteries (gel cells and absorbed glass mat) are maintenance free. They never need watering or an equalization charge. Sealed batteries 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.

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 let them stand "dead" for an extended period of time. 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. 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. 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. 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 gel-type batteries.

With proper care, lead-acid batteries will have a long service life and work very well in almost any power system. With poor treatment lead-acid battery life will be very short.

We strongly recommend the use of an amp-hour meter with all battery systems. They can be found here.

Battery Condition @ 77°F

Nominal Battery Voltage

12V

24V

48V

Over 15

Over 30

Over 60

14.4 to 15.0

28.8 to 30.0

57.6 to 60.0

12.3 to 13.2

24.6 to 26.4

49.2 to 52.8

12.4 to 12.7

24.8 to 25.4

49.6 to 50.8

11.5 to 12.5

23.0 to 25.0

46.0 to 50

12.7

25.4

50.8

12.5

25

50

12.2

24.4

48.8

11.9

23.8

47.6

11.6

23.2

46.4

11.4

22.8

45.6

10.2 to 11.2

20.4 to 22.4

40.8 to 44.8

Hydrometer Readings

The chart below shows battery state-of-charge at various specific gravities. These readings are correct at 75 degrees F.

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