Like sunlight, photovoltaic panels produce a current that varies depending on clouds, time of day, and number of panels in the array. On bright days the batteries would receive a higher voltage than what is recommended and as a result would boil the electrolyte out of the battery cells. When clouds shade the array, the voltage would drop down below the battery S.O.C. (state of charge), the end result would be no gain in battery power.
Enter the intelligent charge controller. This piece of electronics allows for the charging of the battery bank in varying light conditions to maximize solar gain and battery replenishment.
These devices use MPPT technology altering the voltage and current optimizing charge capability. MPPT stands for maximum power point tracking. This allows the controller to determine the maximum power available and adjust voltage and amperage to reach this maximum.
Optimizing power is not the only trick up the controller’s sleeve; it has the ability to charge in four separate stages. The “bulk” rate of charge utilizes higher voltage and modulates the amperage to quickly take the battery bank to 75% S.O.C. Once the battery bank is at this level the controller holds voltage at a preset constant voltage. Known as the “absorption” rate, this brings the battery bank up to 95%. The third stage known as “float” literally trickle charges the battery to 100% without overcharging and damaging the bank.
The fourth stage of charging “equalization” is designed to extend the batteries’ life span and storage capabilities. Generally a specific gravity of 1.255 to 1.275 is advised across all cells but therein lies the maintenance issue, if the battery bank has been undercharged for long periods, low on electrolyte, or never fully charged, sulphating will occur. To prevent this condition the electrolyte specific gravity is monitored. Measured with the battery at rest if there is a variance of .02 SG between cells an equalization cycle should be performed.
Rule of thumb, equalization is recommended between 60 and 180 days depending on system load.
Depending on the model of the charge controller, an equalization cycle can be manually started or programmed into the electronics.
This cycle will keep the batteries power supply capabilities at the maximum and greatly lengthen the life of the bank. The charge controller can also be set to disconnect auxiliary loads in low battery conditions, log data, from both the battery bank and the solar array, such as kilowatts of power produced and kilowatts of power used.
They can also be interfaced with a laptop to log data over a long period of time gathering information which can help determine the economic viability of your system. In short, charge controllers are vital in the correct operation of your off grid or backup system. The advance in electronic technologies have developed equipment that will get the most out of available sunshine, keep your battery bank at its highest capacity and protect it from overcharge or sulphating of the plates.
Lorne Oja is an energy consultant, power engineer and a partner in a company that installs solar panels, wind turbines and energy control products in Central Alberta. He built his first off-grid home in 2003 and is in the planning stage for his second. His column appears every second Friday in the Advocate. Oja, who lives in west Central Alberta, can be contacted at lorne@solartechnical.ca