State of charge


Today’s onboard electrical systems continue to get more sophisticated and more complicated, making it increasingly important to keep track of the health and status of the electrical system. That’s where electrical system monitors come in. These units provide voyagers with information about the state of their boat’s electrical system in general and on the condition of their batteries in particular. 

The heart of every vessel’s electrical system is, of course, its batteries. Although battery technology is rapidly advancing as well, most vessels still use standard flooded lead-acid, gel cell or AGM batteries. These are still a cost-effective solution to storing electrical power aboard a boat. Although lithium-ion batteries are coming of age, they are still not cost-effective and have some serious drawbacks for use in boats. They also require special charging and monitoring when setting them. The monitors discussed here will focus on the more common lead-acid battery types. 

Batteries store energy like a fuel tank, but measuring just how much energy is left in a battery is not so easy. The fuel level in a tank can be measured with a gauge or sometimes just by taking a look inside. A skipper knows how much fuel goes into the tank and how much is used by their engines. This makes it relatively straightforward to calculate when more fuel is needed. 

Although electricity can be measured, it is not quite as easy to track and there are often many sources going in and out of the battery all at the same time. And while a fuel tank can be drained close to the bottom, it can be harmful to a battery to deplete it below 50 percent of its full capacity. This makes it even more important to know just how much “juice” is left in the batteries. Batteries will last longer and work better if not routinely over-discharged.

One alternative to using a system monitor is regularly checking the specific gravity of the electrolyte, but that’s only an option for flooded cell batteries and means messing with battery acid.

Understanding how much reserve energy is left in a battery and how much needs to be put back into it to get back up to a fully charged state is affected by battery type, age, condition, and even temperature. 

Volt and amp meters
It was not so long ago that the only tools available to keep track of a battery’s state of charge (SOC) and what was going in and out were volt and amp meters. A voltmeter can still be a good indicator of battery condition but it does little to tell just how much power is going in and out of the battery. Amp meters that track current flow can only indicate what is being used at a particular moment in time and do not track usage over an extended period of time. They also are not at all useful for judging the SOC or condition of the battery. At best, amp meters tend to be confusing; at worst, they will give false indications of what is going on. The only other practical way of knowing the state of charge was by measuring the specific gravity of the electrolyte in the battery. This, of course, is not easy to do and is often messy. It also is not possible to do with sealed batteries. Fortunately, as technology has advanced so have electrical system monitors. 

Modern system monitors work by ether totaling amps in and out of the battery bank or by measuring voltage combined with amp usage to calculate the overall condition of the battery bank. Only one monitor, the Balmar Smartgauge uses voltage alone to calculate SOC. The most common monitors use amp counting combined with known battery type, size and condition. They then apply built-in software or firmware to calculate SOC. With these units the initial setup is important to the accuracy of the results. When properly installed, a shunt is used in the main negative battery cable to measure amperage flow. Some units will use a battery temperature sensor to monitor battery temperature as well.

A Xantrex Link 200 monitor.

To get accurate results, a monitor needs to know about the battery or battery bank it will be monitoring. Once again, initial setup is critical for getting accurate results. Almost all monitors will need to be programmed with the battery type and rated amp-hour capacity of the battery or battery bank. It is best to be a bit conservative when it comes to rated amp-hour capacity. One would assume a battery rated at 100 amp-hours would carry a 1-amp load for 100 hours but this may not be the case. 

Batteries usually are rated using a 20-hour test. This is the load the battery can deliver at 77° to 80° F for 20 hours before the terminal voltage hits 10.5V. Draw the current out slower and the rating will be higher; draw the current out faster and it will be lower. Most monitors can take this into account but it helps to under-rate a bit when initially setting up the monitor. 

Stratification and sulfation
Next, the age and condition of the battery is needed. This is where things can get a bit tricky because there is no easy way of really knowing the internal condition of a battery. As batteries age, several conditions occur that affect their performance. Among these is stratification, a condition where acid concentration is greater at the bottom of the battery than at the top. Additionally, sulfate crystals build up on the plates and, if left unchecked, cause a condition called sulfation, which will reduce the overall capacity of the battery. To aid in estimating for these conditions, there is something called a Peukert constant. This value is normally between 1.1 and 1.3. It can range from 1.05 to 1.15 for AGM batteries, 1.1 to 1.25 for gel cell batteries, and 1.2 to 1.6 for flooded batteries. 

The more accurate this information, the more accurate the monitor results. This does take some effort on the user’s end to make sure the unit is properly programmed. As batteries age it is important to adjust this information to reflect the battery being less efficient. How often this adjustment needs to be made depends on the use of the batteries, as well as the quality of the batteries. Some monitors will also come with a battery temperature sensor that needs to be properly installed as well.

A Blue Sea Systems M2 monitor measures three voltages and one current and supports programmable shunt ratios.

Once set up, the internal computer can then give a fairly accurate estimate of how much energy is left in the battery bank or how long it will take to recharge. Note that I said “estimate” rather than an exact number. It is important to understand that there are so many variables involved in batteries that it is difficult or impossible to be spot-on. Although the monitors can crunch the numbers, they cannot always know the exact condition of the battery bank being monitored. Most monitors are pretty good at getting within 10 percent of actual SOC, and they may be even better under ideal conditions.

Should changes be made to the system, such as replacing a battery, it is important to reset the monitor. Not periodically adjusting for age and not updating when batteries are replaced are two of the most common mistakes many owners make, and this will affect the results and accuracy of the monitors.

The only unit that does not use amperage or amp counting to calculate SOC is the Balmar Smartgauge. This system monitor works by measuring voltage and voltage drop rates in order to calculate battery capacity. The Smartgauge algorithm will “learn” the battery over several discharge cycles to compute the SOC. By using only voltage, the installation is easier and it does not require periodic updates to the internal program as it is constantly learning the batteries. The user only has to program battery type and begin using the monitor. The proprietary firmware will calculate the SOC; it is reported to be fairly accurate without a lot of user input. A possible drawback to this unit is that there is no amp in or out reading, a feature some users may want.

Charging and discharging
One weakness of all the monitors is tracking the SOC when the battery is being charged and discharged at the same time. This tends to throw off the accuracy of the monitor to a greater or lesser extent. With most cruising boats, the batteries are often being used while being charged at the same time. If the boat is in a marina it will likely have a shore-powered battery charger; while underway, the engine alternator will be charging the batteries and while at anchor, solar and wind generators could be supplying power — all this at the same time power is being used. Although most monitors try to compensate for this, it will result in some inaccuracy that the user has to take into account. Better results are had with more standard charge and discharge cycles, but this does not mean a monitor is not useful for a voyaging boat — far from it, as most cruisers are careful not to drop below the 50 percent level. A monitor properly set and allowed a bit of safety margin will help with staying well above that. 

There are several things to consider when thinking about just what type or brand of battery monitor to select for your needs. Not all monitors provide the same information or display that information in the same way. Most will have digital displays but a couple will only have graphical displays, and some like the Mastervolt will display both. Even the numeric units do not all display the same information the same way. Some will only show SOC as percentages while others will give remaining amp-hours or time left at current load. Blue Sea Systems VSM 422 monitor also displays fuel and water tank levels along with the SOC information. If you like things simple and just like to know your battery condition at a glance, the graphic displays with a bar graph might be the best bet. But if you like to get a bit more in depth, a unit that will display data as numbers might be best. 

The next thing to consider is the number of batteries or battery banks the unit is capable of monitoring. Most units will monitor two or three banks while some will be limited to just one. A word of caution here: Although some monitors will have input for two or more banks, they may really only track one bank while reporting voltage on the others. Read the manual carefully to make sure you understand any limitations. Most boats will have at least two batteries or battery banks — one for house loads and the other for engine starting. It may seem a bit limiting to have a monitor that will only handle one bank, but in most cases it really is not necessary to monitor the start battery as it only has limited use.  A simple voltmeter is all you need for a starter battery. It is the house bank that is of most importance when it comes to tracking SOC.

The Balmar Smartgauge uses only voltage for determining battery capacity, making installation easier. It “learns” the battery bank over several discharge cycles.

Alarms and other features
Many battery monitors will also have some other useful features. Most come with an alarm and/or switching function. A low-voltage alarm could prove useful when anchored out, and low-voltage switching could be set up to turn off heavy loads (such as an inverter) should the voltage drop below a set level. This is a programmable feature on many units that will allow the user to set the switching or alarm levels to their desired voltages. This could prove helpful for protecting batteries from being damaged by discharging below the recommended 50 percent capacity. Again, not all monitors work alike with this function so it pays to review the monitor’s specifications carefully if this feature is desired. This switching function could also be used to turn on a charging system if the voltage drops too low as well.  

Several monitors have interfaces that can be used to connect to a computer for further recording of battery condition and use. This function may not be useful to all users, but for the tech heads and technicians trying to locate battery problems it could be a desired feature. Most of the monitors that have this function will provide the interface and software needed for an additional cost. Some units will allow interfacing to NMEA 2000, which in turn could be used to display on plotters or other equipment. A few monitors such as the Victron BMV series can be interfaced with their global remote that will send messages and alarms via cellphone texts — a useful feature for those away from their boats for extended periods. 

Over the last 20 years, electrical system monitors have come a long way from the simple volt and amp meters, but the future still holds some improvements. Electrical systems for boats in general are improving with better basic wiring systems and integration with other systems. Electrical panels are becoming more modular and with that will come ease of installation for system monitors in general. As battery technology continues to improve, so will the devices used to monitor them. Batteries will begin to have built-in sensors that will let users to not only monitor the whole battery but individual cells, allowing them to spot problems early. Firmware improvements will allow the monitors to learn the batteries and the way they are used, giving even more accurate information. 

System monitors can and do help extend battery life and offer the user a better idea of the useful condition of their batteries. Users expecting results within a tenth of a percent will be disappointed, but most of us can expect and be happy with accuracy within 10 percent and often better. Results will be best when the user follows the installation instructions carefully and completes the installation and setup correctly. Once set up, some user input and updating will help keep the monitors “honest.” Like most things on a boat, you cannot just install and forget them.

Contributing editor Wayne Canning is a surveyor, engineer, delivery skipper, marine writer and equipment installer. His website is He lives on board his Irwin 40, Vayu, in Florida. 

By Ocean Navigator