Using specific gravity to determine SOC is messy and time-consuming, and can’t easily be automated. Voltage readings are woefully inaccurate. So what’s left? Coulomb counting.
A coulomb is the amount of electric charge that is transported in 1 second by 1 amp. Devices that count and total coulombs are called amp-hour meters or watt-hour meters. A shunt—a high-power, precision resistor that is not affected by temperature—is used. The meter measures the voltage drop across the shunt and, using Ohm’s law, calculates the amps and/or watts going into or out of the battery bank. The meter also tallies how long this current is moving in either direction, giving you amp-hours or watt-hours.
More sophisticated amp-hour meters may use multiple shunts, so you can separately monitor your RE energy inputs—like from a PV array and a wind turbine. Many include a battery temperature sensor, which improves accuracy.
Coulomb counting is not infallible, as charging efficiency and self-discharge will both change as a battery ages. But the technique is very convenient and gives you a darned good estimation of battery SOC. Reading an amp-hour meter is so easy that anyone can learn to do it, and then start the backup generator before the SOC is low enough to cause battery damage.
Amp-hour meters are generally set up to display a simple “percent of full” reading, but they store much more data. When set to show amp-hours, the meter reads “0” when the batteries are full. As energy is used from the battery bank, the meter counts down in negative numbers; as the bank is charged, it counts up. A positive number indicates incoming energy that was not stored because the batteries were already full. Even the most basic of these meters stores some historical data, like the maximum depth of discharge since the last reset and the number of hours the battery bank was under a certain set critical voltage. This data can be used to troubleshoot a system or watch for problems.
There are a variety of options available with amp-hour meters, including their ability to read multiple shunts, and their capability for having remote displays, data storage, computer interfaces, and even Internet monitoring via smartphones or websites.
Internal meters. If your battery-backup grid-tied PV system is simple, your battery monitor will be simple, too. Stored energy from your batteries is used only during a utility outage, and grid energy is usually used to quickly charge them again after power is restored. Some newer battery-based grid-tied inverters already have their own metering for the battery bank, which you can monitor through the system status display and built-in computer interface.
System-integrated meters. Many newer inverters and charge controllers can be networked together using special hubs and routers for monitoring. Battery monitors can be easily added to that network, so that all of the data from every device can be read on the system status display. This puts the entire system’s performance at your fingertips. Even wireless links to the monitor are possible.
In some cases, you’ll have to purchase extra equipment, like hubs and displays, for this networking capability. Also, the communications protocols used by equipment manufacturers are proprietary, so if your inverter, charge controller, and battery monitor are made by different companies, you’ll need a laptop computer to integrate the data.
Stand-alone meters. These include the original amp-hour meters, and are the most versatile. No matter what sort of system you have, what resources provide your energy, or who designed your power system how long ago, you can monitor your battery bank with a stand-alone meter, and there are models that can monitor multiple shunts for more detailed data.