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 batteries whenever they are producing power, and the batteries supply power whenever it is needed.

Battery Technologies

The most common battery technology used is lead-acid, in which lead plates are used with a sulfuric acid electrolyte. The electrolyte can be fluid or absorbed in fiberglass mats (AGM), or gelled. AGM and gel batteries are together known as VRLA (Valve Regulated Lead Acid) and are sealed, do not require water addition, and do not emit gases when operated within specifications. Lead-acid batteries are relatively inexpensive and readily available compared to other battery types. New advanced lead acid batteries have carbon additives in the negative plate to prevent sulfation at partial states of charge (PSoC), and are still much less expensive than high technology batteries.

Lithium Ion batteries can handle large charging and load currents. They are also lighter weight and compact for their power and energy capacity. One advantage of Lithium Ion (Li-Ion) batteries is their long life even when cycled heavily, and without needing to be brought to a full state of charge each cycle. This makes them particularly suitable for short to long duration use in self-consumption systems where net metering is unavailable or utility rate structures otherwise disincentivize energy exports during peak solar production hours.

Aqueous hybrid sodium-ion batteries have significant safety and environmental advantages over traditional batteries. They are made from non-toxic materials and have an aqueous electrolyte that is non-flammable. They have the ability to cycle for many years at any state of charge, making them suitable for systems that need to take advantage of charging when available, and do not need to be fully charged like lead batteries. They are ideal for long duration applications such as off-grid systems, or larger capacity self-consumption systems. These batteries are very robust, but are similar in size and weight to lead-acid batteries and must be sized carefully to ensure appropriate current for loads or charging.

Standby or Cycling Batteries

Batteries come in a wide variety of sizes and types, but the most important designation is whether they are made for daily cycle service or standby service. Automobile starting batteries should not be used for renewable energy systems.

Standby power batteries are designed to supply power to loads for occasional use, and are preferred for grid-tied solar systems with battery backup. They are optimized to supply moderate 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 frequent deep discharges and have a limited cycle life but often very long calendar life when kept in float conditions. AGM batteries are most common for standby power applications as they are less expensive, have low self-discharge and require little to no manual maintenance.

Deep cycle batteries, 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. Even when designed to withstand deep cycling, most batteries will have a longer life if the cycles are kept shallower. Deep cycle batteries can be either flooded or sealed lead acid variants or, increasingly, newer chemistries like lithium-ion or sodium-ion.

Caring for Batteries

Maintenance requirements vary by battery chemistry and configuration. Additionally, some maintenance tasks, such as adding water or equalization, require on-site manual operations and/or oversight, while charge regulation, voltage checks and related measurements can be automated via sophisticated charge controllers or battery management systems, which are a de facto requirement for lithium-ion batteries.

Sealed lead-acid batteries, gel cells and AGM (Absorbed Glass Mat), are often referred to as maintenance-free because they don’t require 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.

Lead-acid batteries should always be recharged as soon as possible. The positive plates change from lead oxide, 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 is recharged. The portion of the plates that become “sulfated” can no longer store energy. Batteries that are deeply discharged and then only partially charged on a regular basis often fail in less than one year. The new lead-carbon batteries substantially reduce sulfation. Always use temperature compensation when charging batteries to prevent over or under-charging. 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 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 tops of your batteries clean and check that cables are tight. Do not tighten or remove cables while charging or soon after charging! 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. Use a hydrometer to check the specific gravity of your flooded 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. An equalizing charge will help mix the electrolyte.

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 work very well in almost any power system.

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. Similar precautions apply to other battery types – always read and adhere to manufacturer safety recommendations when handling batteries. For any type of battery, be sure to remove any metal jewelry and avoid shorting the battery terminals.

Battery State-of-Charge

Battery state-of-charge (SOC) can be measured by an amp-hour meter, voltage, or by specific gravity. Some care and knowledge is required to interpret state-of-charge from voltage or specific gravity readings. We recommend amp-hour meters for all systems with batteries. An amp-hour meter is like a fuel gauge for batteries and provides all the information needed to keep batteries charged. At a glance, the user can see system voltage, current, and battery condition.

Battery voltage will vary for the same state-of-charge depending on whether the battery is being charged or discharged, and what the current is in relation to the size of the battery. The table below shows typical battery voltages at each state-of-charge for various battery conditions in flooded lead-acid batteries. Voltage varies with temperature. While charging, a lower temperature will increase battery voltage. Full-charge voltage on a 12 VDC battery is 0.9 VDC higher at 32 °F than at 70 °F. While discharging, a higher temperature will increase battery voltage. There is little temperature effect while a battery is idle, though higher temperatures will increase the self-discharge rate.

Source: Ralph Heisey of Bogart Engineering.

Battery Voltage at Various States of Charge

Battery condition at 77 °F

Nominal battery voltage
12 VDC24 VDC48 VDC
Battery during equalization charge> 15 VDC> 30 VDC> 60 VDC
Battery near full charge while charging14.4 – 15 VDC28.8 – 30 VDC57.6 – 60 VDC
Battery near full discharge while charging12.3 – 13.2 VDC24.6 – 26.4 VDC49.2 – 52.8 VDC
Battery fully charged with light load12.4 – 12.7 VDC24.8 – 25.4 VDC49.6 – 50.8 VDC
Battery fully charged with heavy load11.5 – 12.5 VDC23 – 25 VDC46 – 50 VDC
No charge or discharge for 6 hours – 100% charged12.7 VDC25.4 VDC50.8 VDC
No charge or discharge for 6 hours – 80% charged12.5 VDC25 VDC50 VDC
No charge or discharge for 6 hours – 60% charged12.2 VDC24.4 VDC48.8 VDC
No charge or discharge for 6 hours – 40% charged11.9 VDC23.8 VDC47.6 VDC
No charge or discharge for 6 hours – 20% charged11.6 VDC23.2 VDC46.4 VDC
No charge or discharge for 6 hours -fully discharged11.4 VDC22.8 VDC45.6 VDC
Battery near full discharge while discharging10.2 – 11.2 VDC20.4 – 22.4 VDC40.8 – 44.8 VDC

hydrometer is very accurate at measuring battery state-of-charge in flooded lead-acid batteries if you measure the electrolyte near the plates. Unfortunately, you can only measure the electrolyte at the top of the battery, which is not always near the plates. When a battery is being charged or discharged, a chemical reaction takes place at the border between the lead plates and the electrolyte. The electrolyte changes from water to sulfuric acid while charging. The acid becomes stronger, increasing the specific gravity, as the battery charges. Near the end of the charging cycle, gas bubbles rising through the acid stir the fluid. It takes several hours for the electrolyte to mix so that you get an accurate reading at the top of the battery. Always try to take readings after the battery has been idle or slowly discharging for some time.

This table shows the battery state-of-charge corresponding to various specific gravities for a battery bank in an ambient temperature of 75 °F. Some batteries will have a different specific gravity density by design so check with the manufacturer.

Hydrometer Readings at Ambient Temperature of 75˚F

State-of-chargeSpecific gravity
100% charged1.265
75% charged1.239
50% charged1.2
25% charged1.17
Fully discharged1.11