Despite the installation of solar panels, I still fear that my batteries will discharge when I am at anchor or in a marina without electricity.
Not only will household equipment not work and engines not start, but flattening batteries can shorten their lifespan. So how should you monitor them?
There are two solutions. The first is a simple voltmeter that will measure the charge level of the batteries. The second option is a marine battery monitor. A battery monitor will measure voltage, but will also measure the amount of stored electricity.
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The best way to imagine the difference is to think of a glass of water. If you have an absolutely full small glass, the voltage will be high, but there is not much water in it.
A larger glass filled with water will also register the same high voltage, but a battery monitor will tell you that there is more water in the glass.
Voltmeters are inexpensive, indeed many boats have a voltmeter built into the chartplotter. Marine battery monitors are more expensive depending on the manufacturer and the installations they offer.
How do marine battery monitors work?
The central part is a shunt, a low resistance connection between the negative pole and the rest of the electrical devices connected to the battery.
Since the resistance is known and the voltage drop across the shunt can be measured easily, this allows the number of amps flowing in and out of the battery to be calculated using Ohms Law. This indicates that Amps (I) = Volts (V) / Resistance (R).
A shunt can measure 100 A and the voltage drop for 100 A can be 50 mV. Using Ohm’s law, its resistance is V/I = 50 mV/100 A = 0.0005 Ohms. The software translates the measured 0 to 50mV into a current of 0 to 100A.
The important thing when connecting the shunt is that nothing is connected to the battery side of the shunt.
Everything, including solar panels and battery chargers, must be connected on the load side, otherwise the marine battery monitor cannot measure input and output accurately and therefore may give incorrect readings.
I chose to install a Victron BMV-712 which has a digital display and using Bluetooth can connect to a smartphone or tablet. Victron also manufactures models that only have a digital display and, more recently, a “smart shunt”.
I would have installed the smart shunt if it had been available at the time as it integrated everything into the shunt and gives details to a smartphone or tablet via bluetooth.
This means there is no need to drill holes or run wires through the boat, it will be quicker to install, as there are only battery connections and it is cheaper .
I also used a non-Bluetooth Victron marine battery monitor and all parameters and most information can be obtained using the digital display.
Whenever you work with the electrical system, the first thing to do is disconnect the mains (therefore there is no battery charge). Then disconnect the negative pole of the battery.
Mounting the Marine Battery Monitor Display
With the BMV-712, you have to decide where to mount the screen. I needed a 52mm hole with rear access. The screen mounting ring screws in the back and is reversible for different wall thicknesses.
If access from the rear is difficult, the display can be mounted using a faceplate into which it clips or in a surface-mounting box. The RJ12 wire should then be plugged into the back of the screen.
The shunt is connected between the negative battery terminal and all other connections. After that, the fused power wire from the positive terminal to the shunt is connected.
The screen to shunt lead should be connected to the rear of the screen, routed through the boat to the battery compartment, where it is connected to the shunt.
There is an additional port on the shunt. A lead wire is provided which allows the voltage of a second battery to be monitored.
I used it, but you can buy a temperature monitoring cable for the battery you’re monitoring. This allows for greater accuracy when charging.
After installing the shunt and connecting the display, you need to configure the system. Since my device had Bluetooth connectivity, I was able to do this (and update all the firmware) using a tablet.
Important information is needed like the size of the monitored batteries, their type and some other bits of data.
What is Peukert?
One of the data fields asks for the Peukert exponent. The higher the Peukert exponent, the faster the effective capacitance “shrinks” with increasing discharge rate. You should be able to get this value from the seller or battery manufacturer.
However, despite buying my batteries from the “largest online battery supplier”, they couldn’t supply them, and neither could the manufacturer! Fortunately the number is small and using the default should be fine for most lead acid batteries.
If you have other Victron equipment on your boat, you can connect them all together if you wish and monitor the whole system via the VictronConnect app.
Does it work ? The simple answer is yes! After summer and fall trips, I know the state of charge of my only home battery and an average night uses 25% of the battery capacity.
On a sunny day, the battery is recharged around lunchtime. On cloudy days, the battery may not be fully charged until nightfall, depending on the length of the day.
Once underway, the outboard will charge the battery at approximately 10% per hour. I can also use it to see if there is a large current consumption and take corrective action (for the refrigerator I installed additional ventilation).
Battery state of charge resolved… one more thing not to have to worry about!
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This feature first appeared in the March 2022 edition of Practical boat owner. For more articles like this, including DIY, money-saving tips, great boat projects, expert advice and ways to improve your boat’s performance, subscribe to Britain’s best-selling sailing magazine.
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