Battery Bank Information

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Batteries

Virtually all power generation systems require some form of energy storage. For grid-tied systems, the utility accepts surplus power and gives it back when needed. A battery bank is required for systems that need to function without the grid, either all of the time or during an outage. In these systems, the solar array or wind turbine charges the battery bank whenever they are producing power, and the batteries supply power whenever it is needed.

The most common battery technology used for renewable energy systems is lead-acid, in which lead plates are submerged in sulfuric acid. Lead-acid batteries (while bulky) are relatively inexpensive and readily available compared to other battery types. Lithium-ion battery technology encompasses a range of battery chemistries that use various lithium compounds to store electrical energy. These batteries are much smaller and lighter than equivalent lead-acid batteries and can be charged and discharged faster and to greater extremes without suffering damage, decreasing the capacity required. Despite being significantly more expensive than lead-acid batteries, they can be the right choice when transportation costs and/or storage is at a premium.

Battery Size

To properly design a battery bank, you need to account for the storage capacity required, the maximum discharge rate (the sum of all the loads which might be run simultaneously), the maximum charge rate (the current output from the solar array or wind turbine though the charge controller), and the minimum ambient temperature at which the batteries will be used. Whichever of these factors requires the largest capacity will dictate the size of the battery bank. 

The storage of a battery, the amount of electrical energy it can hold, is usually expressed in amp-hours (Ah). Using one amp for 100 hours means 100 Ah have been used. A battery bank in a PV power system should have sufficient capacity to supply needed power during the longest expected period of cloudy weather when the solar array cannot keep up. If there is a source of on-demand backup power, such as standby generator with a battery charger, the battery bank does not have to be sized for worst-case conditions.

Standby or Cycling Batteries

Lead-acid batteries come in a wide variety of sizes and types, but the most important designating is whether they are made for daily cycle service or standby service. Automobile starting batteries are not used for any renewable energy systems.

Standby power batteries are designed to supply power to loads for occasional use. They are optimized to supply moderated to large amounts of power only during utility power outages, and float at full charge most of the time. they are designed to use a minimal amount of energy to stay fully charged. They are not made for deep daily discharges or their useful life is shortened. Standby batteries are usually sealed for reduced maintenance. Flooded standby batteries have become rare. 

Deep cycle batteries, on the other hand, are designed to be repeatedly discharged by as much as 80% of their capacity and are therefore a better choice for off-grid PV systems. Although they are designed to withstand deep cycling, these batteries will have a long life if they cycles are kept shallower. Deep cycle batteries can be either flooded or sealed gel or AGM.

Many sealed batteries will perform well in either cycling or standby service. Flooded batteries are mostly restricted to deep cycle service. Sealed lead-acid batteries, gel cells and AGM (Absorbed Glas Mat), are often referred to as maintenance-free because they never need watering or an equalization charge. This makes them well-suited for remote or unattended power systems. However, sealed batteries require accurate regulation to prevent overcharge and over-discharge.

Caring for Lead-Acid Batteries

Always use extreme caution when handling batteries and electrolyte (sulfuric acid). Wear appropriate personal protective equipment, including electrical-and chemical-resistant gloves with sleeves, goggles, and acid-resistant clothing. "Battery acid" will instantly burn skin and eyes and destroy cotton and wool clothing.

Lead-acid batteries should always be recharged as soon as possible. The positive plates change form lead axide when charged to lead sulfate when discharged. The longer they remain in the lead sulfate state, the more of the plate remains lead sulfate when the battery in recharged. The portion of the plates that become "sulfate" can no longer store energy. 

Batteries that are deeply discharged and the only partially charged on a regular basis often fail in less than one year. Check your batteries on a regular basis to be sure they are being charged. Use a hydrometer to check the specific gravity of your lead-acid batteries. If batteries are cycle very deeply and then recharged slowly, the specific gravity reading will be lower because of incomplete mixing of electrolyte.

NOTE: Battery warranties do NOT cover damage due to poor maintenance or loss of capacity from sulfation.

Check the electrolyte level in wet-cell, or "flooded" batteries, at least once every 3 months and top-off each each cell with distilled water.

Do not add water to discharged batteries!

Electrolyte is absorbed when batteries are discharged, so if you add water at this time and then recharge the battery, electrolyte will overflow and create a safety hazard. Keep the top 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 the case and potentially ignite a fire or an even larger explosion if the batteries are not properly vented.

An "equalization" charge should be performed on flooded batteries whenever cells show a variation of 0.05 or more in 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 wok very well in almost any power system. Battery warranties do NOT cover damage due to poor maintenance or loss of capacity from sulfation.

Battery Wiring Diagrams

The diagrams below show typical 12VDC, 24 VDC and 48 VDC battery wiring configurations. Batteries can deliver extremely high current. Always install fuse protection on any positive wiring connected to batteries.

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