Charging guidelines for renewable energy
By Daniel R. Seidel and Bruce Habeck
Renewable energy applications that depend on battery power must be ready to perform at all times. To ensure the highest rate of performance, there are a few key steps that should be followed.
As with any product, it is very important that batteries are fully inspected upon delivery. If any noticeable damage to packaging is observed, the shipment should be refused, and the vendor should be contacted immediately. DO NOT install damaged product.
Key factors that affect a battery's ability to provide the rated capacity and its full life are: System Design, Storage, Temperature, Depth of Discharge (DoD), Charging and Maintenance.
Regardless of the lead battery technology or brand, proper design and installation should be performed by qualified professionals to ensure systems perform as expected. All electrical equipment should be reviewed for compatibility, and set to manufacturers' recommendations. Documented routine maintenance, reviewed on a regular basis, will assist in detection of any potential system deficiencies that can be corrected at an early stage, saving time, money and frustration.
The majority of lead battery capacity/life issues can be traced to improper charging. The battery charging system must be set properly. An undercharged or overcharged battery bank will affect battery performance and life, as well as the performance of the entire system.
In renewable energy applications, especially off-grid, the practice of oversizing a battery system, based on intermittent and unreliable sun/wind conditions, can result in capacity loss that is not always immediately noticeable. By the time capacity loss becomes evident, the battery is severely sulfated and may not be recoverable.
Typically, inverters/charge controllers charging lead batteries utilize 3-stage charging: Bulk, Absorption and Float, with an optional Equalize stage. It is important that the charging system (charge controller/inverter) be set up correctly upon installation.
All bulk, absorption, float and equalize settings should be verified to be within the battery manufacturer's recommended settings. Consult individual battery Installation & Operating manuals for inverter/charge controller setting recommendations. Default settings should not be presumed to be correct.
Current is applied to the batteries at the maximum safe rate they will accept until the voltage rises to a near full charge level. The battery voltage rises because the charging current that is provided by the charge controller/inverter is replenishing the energy removed during discharge. The charge current is flat (constant) as the battery voltage is rising. The maximum allowable charge voltage and current allowed by the battery manufacturer should be used to ensure that the most energy is returned within the bulk stage.
The inverter/charge controller will attempt to hold its output voltage constant while the battery continues to absorb charge. The rate at which the battery continues to absorb charge in this mode gradually slows down. The amplitude of the charger current gradually decreases. The charge current declines and the battery voltage stays flat (constant).
Inverter/charge controllers will use either time or current to determine the length of the absorption stage.
- Time regulated absorption is based on a predetermined time after the battery has completed the bulk stage. It is recommended to set the absorption time to the maximum time setting possible to take advantage of all available sun regardless of season or weather-related issues. With this method, the availability of sunlight will determine the absorption time.
- Current regulated absorption uses the charge current to determine battery state-of-charge. When charging in constant voltage, the current will start to taper when a battery/battery system reaches the absorption voltage setting. The point at which the current stops tapering is referred to as the stabilizing current. This is an indication that the battery is fully charged, and the current being drawn is only needed to keep the battery at the set voltage. This minimum or stabilizing current will change based on the charge voltage setting. The battery manufacturer should be consulted for current settings. Using this method, the battery will determine the absorption time.
The voltage at which the battery is maintained after being charged to 100% SOC (State-of-Charge) to maintain capacity.
A charge at a level higher than the normal float voltage, applied for a limited period of time, to correct inequalities of voltage, specific gravity, or state-of-charge that may have developed between the cells during service.
For lead battery systems located in an uncontrolled temperature environment, Temperature Compensation must be used to optimize performance, longevity and safety.
IEEE (The Institute of Electrical and Electronics Engineers) 1,2 suggests batteries be checked on a monthly, quarterly and yearly basis. Battery manufacturers' maintenance recommendations should be consulted as well. A maintenance log should be maintained. Periodic voltage checks will help verify that batteries are being fully charged and operating properly. If any conditions are found that are out of specifications, corrections should be made.
For more detailed information on these topics and additional expertise on Renewable Energy Systems, please review our Renewable Energy Technical Manual found at www.mkbattery.com
1 IEEE 450 — Recommendation Practice for Maintenance, Testing and Replacement of Vented Lead-Acid Batteries for Stationary Applications
2 IEEE 1188 — Recommended Practice for Maintenance, Testing and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications
Daniel R. Seidel is Senior Technical Support Engineer, East Penn Manufacturing (www.eastpennmanufacturing.com) and Bruce Habeck is Global Director — Energy Storage Business, MK Battery (www.mkbattery.com)