Monitoring Your Marine Battery Bank

Two long-time players in energy systems charge to the top spots.


Two recent tests graphically demonstrated the importance of monitoring state of charge in a boats batteries: Tracking Batteries, in the May 2015 issue and AGM Battery Test Update, in the August 2015 issue. These tests demonstrated that a sealed batterys capacity will be reduced over time, if its never brought back up to a full state of charge. But just how accurate are the monitors we use to gauge our batteries states of charge?

It has been 16 years since Practical Sailor looked at battery monitors, so we thought it was high time to take a look at the latest offerings in this area. Certainly, for many cruising sailors, battery banks are among their most important onboard resources, and knowing when you might be running low on electrical fuel is as important as keeping track of potable water and diesel fuel.

Marine Battery Monitor

Photos by Ed Sherman

The need for accurate monitoring is especially important when you consider that over the last decade, we have seen some important improvements in battery technology as well as other areas of onboard electrical systems. For example, LED lighting systems have finally evolved so that they not only use very few amps to provide high-quality interior light, but they also actually meet international standards, as is the case with running lights. The net result is systems that dramatically reduce the amount of power we need to keep things illuminated.

On the other hand, many cruisers now expect to have such amenities as air-conditioning and refrigeration on board-features once considered luxuries-which has increased their appetite for amps significantly, in spite of product vendors considerable efforts to design in lower power consumption. The bottom line here is that keeping track of your batteries states of charge is especially important for maximizing battery-cycle life. Many boat owners today can easily have several thousand dollars invested in batteries by keeping close track of their batteries state of charge.

What We Tested

For this report, Practical Sailor studied and compared monitoring systems from five different suppliers: Blue Sea Systems, CruzPro, Scad Technologies, Victron Energy, and Xantrex . Among these vendors, we looked at offerings ranging from a simple, red warning-light voltage monitor (Scad) that automatically shuts down heavy electrical loads before a dead battery occurs or turns on a generator to get a battery charger up and running when needed, to very sophisticated adjustment and profile settings (Victron) that can work well with lithium iron phosphate (LiFePO4) battery technologies. Of our test units, the Victron monitors were the only ones to even mention lithium iron technology and make a recommendation for adjusting the units profiling to fit lithium technology and discharge/recharge regimens. It appears that the other vendors really have not tested their products with that battery chemistry in mind.

Its important for readers to understand the limitations of any battery monitoring system. These devices are quite good at monitoring amperage and voltage and keeping track of these values down to 0.1 volts in either case. But when it comes to determining state of charge or hours of use remaining, there are many factors that can introduce some margin of error. This is something our testers had to take into account as well when they established a protocol. (See accompanying How We Tested on right for details on the testing.)

Battery chemistry and internal construction, age, temperature, and rate of discharge and recharge efficiency all play into the degree of accuracy we can expect from a battery monitor. Several of the vendors in our group have given these matters considerable attention and make recommendations to periodically synchronize or calibrate the monitor with the batteries being monitored. Victron, for example, offers an optional temperature sender to provide that data input to their monitors. Some of the vendors did not offer any guidance for calibration, presumably because calibration is limited, if at all possible.

Why does temperature matter? Monitoring battery temperature and building that data into a micro-processor that is sending out a voltage reading is useful for those seeking extreme accuracy. There is a direct correlation between temperature, current (amperage), and voltage. As batteries heat and cool due to both ambient temperature variations and electro-chemical reactions in each cell of the battery, their internal electrical resistances also vary. We know from Ohms Law that as resistance changes (mathematically) both voltage and current will change. In the case of battery monitors that essentially present a voltage reading, monitoring temperature in real time can more accurately depict voltage, and therefore, state of charge.

Why is calibration so important? As your batteries age, a re-adjustment of the charging parameters is really quite necessary in order to maximize the accuracy of your monitor.

When selecting a monitoring system for your batteries, you may need to consider how many battery banks the system can effectively monitor. The Value Guide that accompanies this article will help with these decisions. The CruzPro VAH 110 can provide data for three separate banks of batteries, with its focus being the house bank. The Xantrex unit we tested can only handle two banks.

It is also important to understand that in most cases, the house bank is your primary concern, so complete, comprehensive analysis is provided for the house banks, but the monitoring of auxiliary banks (as the installation instructions will typically describe the secondary banks) will be more limited.

Blue Sea Systems M2

The Blue Sea Systems M2 was the only monitor in our group that utilizes an extremely bright OLED (organic light emitting diode) display. This makes the display easily visible under just about all lighting conditions. The monitor includes an integrated relay that can turn loads on or off, based on the state of charge of the batteries and features a summary screen that displays pertinent information in a single screen.

Of all the units in our test field, the Blue Sea Systems installation guide was probably the simplest to follow. The M2 can work with a battery bank that has up to 5,000 amp hours of capacity and will work with 12-, 24-, 36-, or 48-volt systems. The instructions identified all of the currently available battery types as being compatible; however, they did not mention anything about LION technology.

Bottom line: We liked the ease of use and the visibility of the M2, but the instructions describe only initial setup and made no mention of any synchronization or periodic calibration requirements, so we have questions about its long-term accuracy as the batteries its connected to age.


CruzPro sent us three different units to check out, we tested two of the three: the VAH 65 and VAH 110. We did not test the VAH 60, which is very similar to the VAH 65 unit; the essential difference is the amount of amperage each can handle. These units are limited in their voltage-handling capabilities to either 12- or 24-volt nominal systems and in battery-bank capacity up to 3,000 amp hours. The units offer NMEA 0183 interface capability, which allows you to monitor battery voltage remotely through your multi-function display (MFD) or remote display. They also have a programmable voltage alarm to alert the owner when voltage is too high or too low. Additionally, these units can be calibrated to automatically turn a battery charger on and off at user-selected points.

All of the CruzPro units offer a wide range of adjustability for altering Peukerts Exponent, the fundamental algorithm used by all of our test subjects. The problem is that the methodology involves a great deal of trial and error. The default exponent used for calculating is set at 1.25. This seemed to work well for the flooded-cell, deep-cycle battery we used during our tests, but this exponent is not the same for different battery types, and it will change as the batteries age.

The CruzPro units also offer adjustments for what they refer to as amp-hour drift. CruzPro describes this drift as what happens over extended periods of time when the boat is not in use. During this period, small calibration and measurement errors accumulate and cause the meters amp-hour value to drift away from the actual amp hours available. Again, the method to deal with this is trial and error.

Instructions are provided in the owners manual, but they are a bit subjective (not to mention complicated), in our view. Certainly, some fiddling with the calibration will probably bring the monitor closer to its ultimate accuracy, but just remember that it will likely need readjustment as the battery ages. Our guess is that most people wont bother with these tweaks as they go along, and frankly, the documentation that comes with the product is just too vague to really be helpful. CruzPro explained that the user manuals are short and to the point, because they have found that people tend to not actually read longer, more detailed manuals.

Bottom line: While a techie might get many hours of pleasure tinkering with the CruzPros functions, in our opinion, this line of meters is more complicated than it needs to be for the average boater. Its complicated enough that we would recommend a professional installation in most cases.

Scad Technologies

The Scad Technologies monitor we tested is basically a simple voltage alert system. It works only with 12-volt systems. The unit offers both an audible and visual alarm and is programmable for both high- and low-voltage values. Additionally, the unit can be wired to shut off a high-current load, such as a DC refrigerator, 30 seconds after the visual alert or 15 seconds after the audible alert is activated. It can also be wired to turn on a generator when the low-voltage alarm gets triggered.

Bottom line: Essentially, this is an inexpensive backup monitor, or an energy-saving switch to preserve battery state.

Victron Energy

Victron sent us two similar units, the BMV 700 and the BMV 702, which adds features like battery temperature monitoring, basic monitoring of an auxiliary battery, and monitoring of mid-point voltage on larger battery banks.

The comprehensive owners manual does a good job explaining the monitors limitations and the variability in battery monitoring systems in general. It also discusses the Victron monitors programmable functions that help to ensure a higher degree of accuracy. Programmable functions include battery capacity, charged voltage, tail current (a percentage of the battery capacity), charged detection time, Peukerts exponent, charge efficiency factor, current threshold, time-to-go averaging period, zero current calibration, and a synchronization function. All of these functions are explained in great detail in the owners manual with help provided in making initial decisions based on the battery technology in use.

These units have the highest capacity of all the units we tested except the Xantrex LinkPro and can be used to monitor a battery bank with up to 9,999 amp hours of capacity. They can work with 12-, 24-, or 48-volt systems. The BMV units also store history data, which can be used later to evaluate usage patterns and battery health, as well as help in tweaking as the batteries age.

In all, we liked the documentation provided with the Victron energy units over all of the other products in our test group. The fact that the units come with a CAT 5 cable to connect the gauge to the shunt helps to significantly reduce the chance of making a potentially damaging error during installation. Basically, all of the wiring needed to install the unit that is not a part of the boat is provided with the kit.

Finally, the Victron monitors were the only units in our test group to provide information on using them with LION batteries.

Bottom line: The Victron BMV monitors earned the Best Choice pick. They will hold particular appeal to owners with high-capacity battery banks who are leaning toward Lithium Ion. If youre a savvy do-it-yourselfer, the BMVs lend themselves to DIY installation.

lead-acid battery


The Xantrex LinkPro we tested is quite feature-rich. The voltage limits are for 12- and 24-volt systems only, which cover most applications. Its amp-hour range goes to 9,999 like the Victron units, so connecting to large battery banks will not be an issue. The unit can be equipped with an optional temperature sensor like the Victron BMV 702 model, which we would always recommend as one more data point to improve overall accuracy.

Also like the Victron units, the LinkPro provides a history menu, which testers really liked. The data it provides can help boat owners analyze their on-board electrical habits, both good and bad. A unique feature with this model is the ability to auto-synchronize with the connected batteries. This synchronization function helps to eliminate the need for tweaking adjustments as the battery ages and will go a long way toward ensuring that battery maximum cycle life is achieved.

Bottom line: Also a Best Choice pick, the LinkPro is one of the most popular battery monitors on the market for good reason. The synchronization feature is especially helpful for the non-techie.


After spending a month working with these units and really digging into all of their features and setup requirements, we came up with two winners: the Victron Energy BMV monitors and the Xantrex LinkPro monitor. These monitors offer the highest level of sophistication available today, and we feel they will do the best job in ensuring that you can maximize your onboard battery investment.

For further tips on installing a battery monitor, Practical Sailor contributor Rod Collins of Compass Marine, who carried out the PSs 2015 battery tests, has an illustrated guide on his website.

Marine Battery Monitors
Monitoring Your Marine Battery Bank
Marine Battery Accuracy
Monitoring Your Marine Battery Bank

The degree of accuracy we can expect from a battery monitor depends on a number of factors: battery type, age, rate of discharge, and recharge efficiency, to name a few.

Several of the vendors in our test group recommend that you carry out periodic calibration in order to ensure that the readings are accurate. Victron’s monitors, for example, can be programmed for a range of scenarios. Other makers offer very little guidance on calibration.

1. The Victron units were the most compact and will require very little behind-panel depth for installation.
2. The Xantrex LinkPro offers a temperature monitor sensor option, which we feel is an important addition to enhance accuracy of readings over time.
3. The CruzPro display is large, and the data is in large font. This will make it quite useful for those needing to check the screen from a distance.
4. The Blue Sea Systems LED display will be crystal clear even in the brightest sunlight. In terms of display, it was very well suited for being mounted in the cockpit.
5. The Victron shunt was the only one in our group that supplied a CAT 5 cable to connect the really fine-gauge wires linking the shunt to the instrument display. This simplified installation considerably.

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.


  1. Have you ever looked at the SIMARINE PICO system? I’d be interested in your opinion of the technology and the company…


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