Some components, like charge controllers and system monitors, can display the battery’s temperature by using a battery temperature sensor (BTS) mounted on the case of one of the batteries. The BTS’s cable is plugged in to the inverter/charger or charge controller and automatically adjusts the charging voltages in relationship to the battery’s temperature. As the battery temperature increases above 77°F, the charging voltage is automatically lowered. At lower temperatures, the charging voltage is automatically increased.
Positioning and adhesion of the BTS to the battery’s case is important as the BTS will determine the battery’s charging voltage levels based on the battery temperature. If a BTS reads a lower temperature than the actual battery temperature, the charge controller or inverter/charger will overcharge the battery, leading to excessive gassing, increased water usage, and excessive battery temperatures, substantially decreasing the battery life. Note that a BTS may not work well on batteries with steel or plastic double cases. In these cases, the installer will need to consult the manufacturer and may need to make provisions to attach the BTS to the inner battery case.
During a battery assessment, a more accurate measurement of each cell’s temperature can be accomplished by inserting a glass thermometer directly into the electrolyte of each cell. Measuring each cell’s temperature also allows for an interbattery comparison to determine if some cells are operating at higher temperatures than others. The resulting average temperature can then be compared to the reading from the BTS to determine its accuracy.
It is important to also check and record all of the inverter and charge controller settings, such as the charging voltages (bulk, float, and equalize); charging time (hours); and charging rate (charge amps). These values dictate how the battery will be charged: at what voltage; for what duration; and at what amperage. These parameters must be set for the specific battery type and size used (see the “Case Study” sidebar for a detailed example).
Depending on the application, other setpoints may also need to be evaluated to see how the battery is being managed on a daily and monthly basis. Understanding how settings for the low-voltage disconnect (LVD), load shedding, alarms, and backup charging sources impact the operation of the battery is often also required when completing a detailed battery assessment.
Performing an assessment several years after a system has been installed can be easier if the system has a data logger that has been recording information. If there is no online data logging system, then the data will need to be downloaded from the system’s components using memory cards or via a connected laptop computer. Some system monitors can provide the daily minimum and maximum battery SOC and voltages, and some can tell you how long it has been since the battery was equalized. This information can be helpful in understanding how a system has been operated and also can be used to estimate the remaining life of the battery based on the number of DOD cycles or the cumulative amp-hours that have been removed from the battery over the system’s life.
The data collected and its format varies between manufacturers. Assistance of the inverter and controller manufacturers is usually required to analyze and draw conclusions from the collected data.
If the system does not include a data logger, the manually written operator logs are sometimes available. They may reveal how often the battery was watered, when the generator was run, or how frequently the battery was equalized. This information can be useful in developing recommendations for the system’s future management.
Carol Weis is a NABCEP-certified PV installer and ISPQ Master PV trainer. She writes curricula and teaches national and international PV classes to technicians and end users. She has worked as a licensed electrician and solar installer in Colorado, and was part of Solar Energy International’s PV technical team for 15 years.
Christopher Freitas is an engineer and project manager for international RE projects. He was a cofounder of OutBack Power Systems and was the director of engineering at Trace Engineering.
The Improving Health Facility Infrastructure project is funded by the U.S. Agency for International Development (USAID) and implemented by Tetra Tech ES. The data used in this article was made possible through support provided by the USAID Office of Economic Growth, Trade and Agriculture under the terms of Contract No. EPP-I-00-03-00008-00. The opinions expressed herein are those of the author(s) and do not necessarily reflect the views of the USAID or Tetra Tech.