Monitoring VRLA State of Health

This is the ninth in a series of units that will educate the reader on the part played by a battery in an uninterruptible power supply (UPS) system. Measuring the health of a valve-regulated lead-acid (VRLA) battery means more than just taking a voltage reading.  A cell or battery can have the desired voltage when it has been sitting on float charge, but it might not have enough stored energy to support the critical load for more than a few minutes or even seconds. IEEE 1881[i] defines two terms that are of great interest to battery owners and operators: state of charge (SOC) is “the stored or remaining capacity in a battery expressed as a percentage of its fully charge capacity.”  SOC is akin to “energy,” that is, “What is the voltage output of this battery at this moment?”  A battery can be fully charged, but because of age or other factors it might not be able to hold up the load for the desired time. state of health (SOH) is “a measurement representing the present state of a battery’s available capacity or remaining service relative to rated capacity or specifications.”   SOH is akin to “power,” that is, “How long can my battery support my load?”  It adds the element of time and is useful in predicting the “life expectancy” of the battery. While these two terms sound a lot alike, there is a difference.  The term “state of charge” is often misunderstood and misused when the speaker is really referring to “state of health”.  SOC tells you the capacity of a battery at the time it is measured (e.g., 95% of its rated capacity).  SOH is more predictive, telling us the expected service life (usually expressed in units of time or number of cycles) remaining in the battery before [...]