Wouldn’t it be great if you could look at a battery and tell how many amps it was holding at any point in time? Unlike a fuel or water tank, a battery has no mechanical sender that can float up and down, and then convert the current volume to a readable number. With so much riding on the state of charge of our batteries, how are we supposed to guess how full they are?
Let’s look at our options.
Tanks of Amps
Batteries are tanks, of a sort. Inside the plastic box are cathodes and anodes immersed in an electrolyte that facilitates the transfer of electrons. But we can’t see electrons or stored energy. That’s where meters come in. In the most rudimentary sense, a voltage meter can tell the current battery voltage, which can allow us to make certain assumptions about the battery state.
But that doesn’t tell the whole story.
If a battery’s only purpose is to start an engine, like in a car, then a manufacturer can specify a certain size and type of battery capable of turning over an engine fast enough to start when it’s cold. The manufacturer can then install an alternator that will produce enough charge to recharge the battery, plus enough extra amperage to power the engine’s electrical needs. This system can work for years without any monitoring at all. When the battery loses enough capacity to start the engine, it needs replacement.
On the other hand, if a battery’s purpose is to run a boat’s house loads for several days at anchor, either without any charging or with intermittent solar charging, how are we to know when it is time to run the engine or genset to recharge? Lead-acid batteries (also called wet cell, sealed, AGM or gel) will output energy well beyond the point at which the discharge is doing them harm. The typical rule is not to use more than 50 percent of the stated amp hours if you want to come close to the manufacturer’s claimed lifetime for the battery.
Since we can’t see the capacity, how can we tell when we reach 50 percent?
The rudimentary way is with a voltmeter. A 12-volt battery has several important voltage points. A fully charged and rested lead-acid battery (meaning no loads and no charging for at least 15 or 20 minutes) should read about 12.6 volts. (Different battery chemistries may be a point or two higher; check the manual.) A battery is fully discharged when it reaches about 10.5 volts, but remember that you never want to take it that low on purpose. A more battery-friendly 50 percent discharged reading would be around 12 volts.
So, again, our useful voltage range is really only 12.6 to 12.0. You can extrapolate these figures for 24- or 48-volt banks.
But even voltage doesn’t tell the whole story. It doesn’t tell you how long you have until you need to recharge. To figure that out, you need to know how many amps are being used at any given time.
Enter the ammeter, which measures the current in a circuit. Originally, many ammeters were installed in-line with the expected loads or energy producer (alternator or battery charger). All the current flowed through the meter. This arrangement works fine if the load is relatively small, say, 25 or 30 amps.
Things get complicated, however, if the ammeter is installed in a place where you can conveniently see it, but that type of positioning necessitates a long wire run. And the run needs to be oversized to reduce voltage drop. It also means that if the ammeter fails, it can prevent any load downstream of it from working.
Another way of reading amps is via a device called a shunt. It’s basically two blocks of copper connected by additional pieces of copper of a known resistance. It is installed in-line with the load and has sense wires connected to each of the two blocks. The difference in resistance between the two blocks can be measured via the sense wires. That information is converted to display the amperage being used. Since all the loads run through the shunt, they do need to be rated large enough for all expected loads.
Shunts that monitor batteries are typically installed on the negative cable that supplies the house loads, as close to the battery as possible with no other loads connected between it and the battery. In practice though, sometimes decisions need to be made about larger loads such as thrusters, windlasses or other high-amperage loads on the house bank. There may not be a shunt available with a large enough rating to handle the house loads plus the high-amp load at the same time. Just note that if the high-amp loads are routed to bypass the shunt, the loads will not be counted by the battery monitor, which will throw off the amp count. For additional connectivity, some shunts specific to monitors are now “smart” in that they can connect directly to a CAN bus.
American physicist Edwin Hall discovered another way to measure amperage. When a current is applied to a thin strip of metal and is in the presence of a magnetic field perpendicular to the direction of the current, it produces a difference in voltage between the two sides of the strip that is directly proportional to the strength of the field. The Hall effect sensor is useful to track ampere use. The wire to be measured is run through the center of the ring of the sensor. Handheld multimeters with amp clamps use this same technology to measure amperage. One downside is that the sensors consume a small amount of energy, so they tend to be used more for AC monitoring than DC aboard boats.
Battery Capacity
It can be harder than it should be to know how much capacity your batteries should have. Starting batteries are more concerned with how much energy they can discharge quickly. The rating for this is called cold cranking amps, which describes how many amps a new battery can discharge for 30 seconds at zero degrees Fahrenheit. Batteries destined for the marine environment can be categorized with a marine cranking amp designation, which uses 32 degrees Fahrenheit as its base temperature. Neither of these ratings are particularly useful for deep-cycle house batteries when we want to know how many amps a battery can produce over a long period time.
Deep-cycle batteries typically have a figure listed for amperage capacity. Different battery manufacturers use different parameters in their ratings. Most use 20 hours as the unit, but the constant load can vary. Some use 5 amps, while others use 10 amps or more. This information is important if you’re comparing batteries or wondering why your battery capacity doesn’t seem to match the manufacturer’s rating. You also need that figure to set up a battery monitor.
Here’s a quirk about lead-acid batteries that a German scientist named Wilhelm Peukert discovered in 1897. As the rate of discharge increases, the battery’s available capacity decreases. This can make a big difference if you are counting amps and subtracting them from what you think the battery’s capacity is. Batteries are typically rated at a specific 20-hour draw at a certain temperature. For instance, a 100-amp battery may be rated to produce 5 amps for 20 hours at 77 degrees Fahrenheit. If any of your devices or a combination of your devices pulls more than that, even for brief periods, you’ll have less capacity than you thought.
More accurate battery monitors will have some ability to set and modify Peukert’s equation in their programming. This allows you to set the monitor for the specific way you use amps, to better guess how many amps are left. In reality, it is not uncommon for the stated available amp-hours to lose touch with reality, either showing too much or too little. The solution is to periodically charge the batteries fully with no other loads present, then reset or resync the total available hours. If the error occurs again in a short time, a modification of Peukert’s variable is in order.
Note that batteries are rated at a specific temperature. That’s because temperature also affects how they work and at what rate they should charge. For the longest battery life, a charger or alternator regulator with a temperature compensation capability can treat the batteries with the kindest charging regimen.
Logarithms for Happy Monitoring
We have devices to measure voltage and amperage, so how do we combine that information into a package that will simply tell us how much capacity we have left? Instrument manufacturers have tried a variety of solutions to bring us closer to this goal.
There is a style of battery monitor called a coulomb counter. A coulomb is a unit of electric charge equal to the quantity of electricity conveyed in one second by a current of one ampere. Named after French physicist Charles-Augustin de Coulomb, a coulomb counter acts like an odometer, keeping track of each amp that is used. The complication arises in line loss and charging inefficiencies. Plus, you have to accurately know the total battery capacity, which can change over time. This information becomes critical when dealing with measuring charging batteries that are discharging amps at the same time.
Lithium batteries bring their own set of benefits and complications. On the benefit side, these batteries are significantly lighter and much more energy-dense. They can also discharge more energy or accept charging energy much quicker. On the other hand, it is critical that each battery cell stays balanced with its neighboring cells. A battery management system is required to protect the integrity and safety of the bank. Lithium batteries also hold their voltage much longer during discharge, making voltage-meter monitoring for stage of charge basically useless.
Another complication comes from utilizing multiple charging sources. On boats with a combination of solar panels, wind generation, AC battery chargers and alternator regulators, all sources will try to compete to quickly charge the batteries. Problems can occur when one device sees the battery voltage is high from another source and chooses to ramp back output because of it. The battery voltage can register as high even if the batteries still need hours of charging.
Sophisticated Interconnection
What if our battery monitors, alternator regulators, solar controllers, battery chargers, inverters, smart shunts and battery monitoring systems could all talk to one another? Perhaps one could be designated as the master device and could send out instructions to the other devices to best protect and monitor the battery bank.
In fact, several of the big names in marine power devices offer products with this capability. CAN bus technology allows plug-and-play interconnection and multifunction display compatibility. Many units have Wi-Fi capability and can be controlled by apps via smartphone or tablet. The beauty of this design is that the software can be continually updated.
The downside may be in the complexity. The manuals for some of these systems can be hundreds of pages long, and the choices in how to set them up can be daunting. Remember, we’re after the holy grail of battery management: state of charge expressed as a percentage and displayed like a tank gauge. The simplicity of the tank gauge can be destroyed if it takes a computer programmer to set up and monitor the myriad parameters that are possible. The balance comes with the initial setup, which requires answering some straightforward questions: How big is your battery bank? How efficient is the charging? How big are the normal loads?
An effective system will then “learn” how the amps are being removed and replaced from the batteries, and will give us the information we most need. Success ultimately comes down to the sophistication of logarithms, the simplicity of the display, and the correct setup from the owner.
This article was originally published in the November/December 2023 issue.
