Battery System Design
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.
The most common battery technology used is lead-acid, in which lead plates are used with a sulphuric acid electrolyte. The electrolyte can be fluid (flooded), absorbed in fiberglass mats (AGM), or GEL. 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), while remaining 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 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 grid-tied short to long-duration use in self-consumption systems where net metering is unavailable or utility rate structures otherwise discourage energy exports during peak solar production hours. They are not well suited to off-grid power systems at this time as they require very specific charging parameters and a high-quality Battery Management System (BMS).
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 optimised 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 AGM or GEL or, increasingly, newer chemistries like lithium-ion.
Caring for Batteries
Maintenance requirements vary by battery chemistry and configuration. Additionally, some maintenance tasks, such as adding water or equalisation, 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 an absolutely necessary requirement for lithium-ion batteries.Sealed lead-acid batteries, GEL cells and AGM, are often referred to as maintenance-free because they don’t require watering or an equalisation charge. This makes them well-suited for remote or unattended power systems. However, sealed batteries require accurate regulation to prevent both 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 sulphate, when discharged. The longer they remain in the lead sulphate state, the more of the plate remains lead suphfate when the battery is recharged. The portion of the plates that become “sulphated” 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 sulphation. 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 sulphation. Check the electrolyte level in wet-cell, or “flooded” batteries, at least once every three 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 equalising charge will help mix the electrolyte. An “equalisation” 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 equalise VRLA batteries unless recommended by the battery manufacturer! 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 (sulphuric 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 andavoid shorting the battery terminals.