Nearly all residential-scale renewable energy systems require some sort of energy storage. In batteryless utility-interactive systems, the grid “stores” any unused energy as credits applied to future electricity use. But those of us living off-grid or with utility backup systems rely on something else for storage—batteries, those mysterious chemical-storage boxes.
Battery-based systems require more user involvement for optimal performance. If ignored, batteries can easily become the weak link in the system—and an expensive component to replace. But how do we know when batteries need our attention? We cannot tell how much energy is left in a battery simply by looking at it—a charged battery looks the same as a discharged battery. But special metering devices help us track what’s happening inside our batteries so we can keep them performing at their peak and maximize their longevity.
Energy is constantly being put into or extracted from batteries, and the dynamic chemical composition inside reflects a varying state of charge (SOC). Voltmeters and hydrometers (which measure electrolyte specific gravity, reflecting the amount of acid in the water) can give indications of how fully charged the battery is. But measuring battery SOC with either of these devices requires accessing the battery terminals or testing the battery acid. This is like going to your bank’s vault to count your money every time you want to check your balance. Thankfully, there’s a device that monitors energy in and out, negating the need to directly access the batteries: the battery monitor, your batteries’ personal bank teller.
Metering is typically dispersed throughout an RE system, like digital displays on charge controllers and inverters. But a battery monitor is in its own class as a system tool. It provides instantaneous values such as battery voltage and current, and also keeps a running tally of net energy into and out of the batteries. Like a bank teller recording your deposits (PV production) and withdrawals (household consumption), your battery monitor can provide you with a relatively accurate account balance (SOC).
Armed with this data, you’ll know when energy needs to be conserved. And the monitor information can accomplish multiple tasks—help educate others about the system, analyze energy consumption to help improve household efficiency, troubleshoot the system if needed, keep a record of cumulative energy production over time, and even log some data about the battery bank’s operating characteristics.
Battery capacity is measured in amp-hours (Ah), and a battery monitor’s main responsibility is to measure the current in amps through a shunt (see “Shunt” sidebar) over time, and then compute the SOC and other data that is needed. A single Ah is the equivalent of 1 amp of current flowing for one hour. It could be 0.5 A flowing for two hours or 2 A for a half hour, etc. A battery bank is purchased based on a predictable need for a certain amount of Ah needed over a certain period of time; a battery monitor constantly compares the Ah in and out in a ratio to the battery’s Ah capacity that was programmed into it. Subtracting the net Ah used from the battery’s capacity gives the Ah remaining, which is normally expressed as a percentage of full.
Let’s say you have an 800 Ah battery bank. If the battery monitor counts 300 Ah out (-300 Ah), and then 100 Ah back in (+100 Ah), the monitor will calculate a 75% state of charge:
[800 – 300 + 100] ÷ 800 = 0.75
In addition to monitoring the SOC, you can use your battery monitor to verify the power usage of appliances and experiment with how different loads affect battery voltage. And you can watch how the incoming amps rise and fall with the changing weather and throughout the day—a battery monitor is more than just useful, it’s actually interesting! Get to know your monitor—understanding what the monitor is showing you is as important as having the meter itself.
A battery monitor is only as accurate as the data programmed into it—garbage in equals garbage out. Programming is critical (battery capacity, battery pack voltage, etc.), as is the skill level of the installer who put it in. The correct shunts and data cabling are essential to successful operation. And always remember that the meter only gives you information, which is subject to your ability to interpret. What you do with that data is up to you—the meter will not water your batteries for you, clean the dust off their terminals, or prevent you from overdischarging the batteries. It should not be relied upon to manage your energy system—that is your job.
Battery monitors should be placed in a location that will both encourage attention and allow for easy access and operation. Mounting monitors in a visible and convenient location, such as in the kitchen or living room, makes them easier to use—they will sometimes become the center of attention with guests. Most monitors can be mounted in their own junction box or flush-mounted into the wall for an unobtrusive presence.
Battery monitors get their signal from the shunt through small-gauge, twisted-pair conductors (typically 16- to 22‑gauge wire). While high current flows directly through the shunt, these monitor wires only measure the very small voltage drop across the shunt. By using twisted-pair cable, electrical “noise” and voltage induction that can affect monitor readings are lessened. Another twisted pair connects to the shunt to measure battery voltage and to power the monitor. Although the shunt is mounted in series with the negative conductor (between the batteries and the inverter) and usually within the DC disconnect enclosure, the monitor itself can be mounted hundreds of feet away in a more accessible location. The distance depends upon the wire size and the degree of accuracy required—the longer the signal wire, the greater the chance of induced voltage, which affects readings.
Once the meter is mounted, a few basic user inputs must be manually programmed into the meter. First, program the Ah capacity of the battery bank into the monitor. Without this, the monitor cannot calculate the battery SOC. Be sure to specify the capacity at the battery’s most relevant rate. Taking into account Peukert’s Law, battery manufacturers list different Ah capacities for the same battery because a battery’s capacity is lessened the faster it is discharged. A battery’s capacity at a 20-hour discharge rate will be greater than at a 5-hour rate. For most renewable energy systems, the 20-hour rate best reflects how the battery will be operated.
The final manual programming tells the monitor when to consider the batteries fully charged. When the monitor detects that the high-voltage set point and the low-current threshold have been met, it assumes that the batteries have been fully charged and are now in float mode (where a low-current charge keeps the battery full). This automatically resets the “amp-hours from full” display, with the monitor reading “100%” or “Full,” and sets the “days since fully charged” number to zero.
With a few minor programming details—such as selecting the desired battery reminders/alerts—and a thorough reading of the manual, your monitor will be up and running. Oh look, the bank’s at 14.8 volts, I’ve got 18 amps coming in from the array, and it’s been seven “days since laundry.” Gotta go!
Khanti Munro instructs, coordinates, and develops curricula for the PV online program at Solar Energy International. He is an ISP Certified Instructor for Photovoltaic Courses and has been designing, installing, and educating about solar-electric systems since 2002.
Battery Monitor Manufacturers:
Bogart Engineering • www.bogartengineering.com • TriMetric & PentaMetric monitors
OutBack Power Systems • www.outbackpower.com • FLEXnet DC battery monitor
Xantrex • www.xantrex.com • TM500A, Link 10, Link 20, LinkLite & LinkPro battery monitors