Many methods use voltage to determine SOC, although it’s not the most accurate measure. The voltage of a battery at rest can tell us SOC, but in an RE system, they are nearly always charging or discharging, so attaining this rested state is difficult.
The most accurate way to measure SOC on flooded batteries is by checking the electrolyte’s specific gravity (SG). Hydrometers are the most common tool used to measure SG, but handheld refractometers can also be used. Be sure to choose one that is accurate to at least three decimal places, and follow the instructions for your model. The “SOC” table gives approximations, but the actual specific gravity and voltages will vary for battery makes and models.
An amp-hour meter is a more common and fairly accurate way to keep track of SOC. These meters require a shunt to measure the current going into and coming out of the batteries, and some can keep track of more than one charging source (PV, wind, generator, etc.). Some have built-in alarms, generator start relays, and data logging capabilities. With proper setup, they can give you a much better picture of the SOC.
Every battery has its own charging specifications. Chronic under- or overcharging is one of the most common ways to shorten the life of a battery bank. Undercharging can cause sulfate crystals to build up on the plates, reducing the battery’s capacity and shortening its life. Overcharging leads to excessive gassing, lowered electrolyte levels (which cannot be replaced in sealed batteries), and more wear and tear on internal plates.
Generally, manufacturers give both voltage and current charging specifications. Check with the manufacturer for their recommended maximum charge rates. Most PV arrays are not large enough to supply that much current, so it is not usually an issue, but generator and utility charging can be. Sophisticated chargers have programmable maximum charge current.
RE system batteries should be charged with a three-stage charger. During the first stage (bulk), all available charging current is sent into the batteries until they reach a specified voltage. Once they reach this voltage, they are about 80% charged, and the second stage (absorb) starts, and the current decreases just enough to keep the voltage stable. The absorb cycle has either a time and/or current end point (see “Absorption Time” sidebar). When this is met, the batteries should be full. The charger then enters the “float” stage with a slightly lower voltage and a trickle charge keeps the batteries full. Occasionally, the battery bank will need to be equalized to remove imbalances between batteries and cells. This is accomplished by intentionally overcharging the bank. See “Methods” in this issue for details.
Reliable, programmable charge controllers will ensure batteries do not get overcharged, and a low-voltage disconnect (LVD) protects them from being overly discharged. Most residential inverters have a built-in, programmable LVD, but beware of smaller units with a set LVD. These often trigger at a very low voltage (down to 10.5 V), not to protect your batteries, but to protect the inverter. An inverter LVD can only protect the battery bank from AC loads, not any DC loads. For small systems, some PV charge controllers add this function—for larger systems with DC loads, a dedicated controller may be needed for LVD.
Another common problem is a lack of adequate charging capacity. Batteries should be brought to 100% SOC at least once a week—and more often is better. Be sure your charging source can deliver enough energy to replace daily usage and to catch up after any periods of days without input. For example, if you’ve designed a battery-based PV system to have three days of autonomy, but your PV array only produces enough energy to replace 1 to 1.5 days of energy use, you’ll need another charging source to “catch up” after three cloudy days or your batteries will spend too much time in a discharged state, shortening their life. This is one of the main reasons why many off-gridders have backup generators.