What's the charge?

Gilbert Park installs a Victron BMV-712 battery monitor

- Gilbert Park

Despite fitting solar panels I still worry about my batteries going flat when I’m at anchor or in a marina with no electricity. Not only will domestic equipment not work and engines not start but flattening batteries may shorten their lifespan. So how should you monitor them?

There are two solutions. The first is a simple voltmeter that will measure how full the batteries are. The second option is a battery monitor. A battery monitor will measure voltage, but in addition will also measure the amount of electricity stored. The best way to imagine the difference is to think of a glass of water. If you have a small glass absolutely full the voltage will be high, however there is not much water in it. A larger glass full of 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. Battery monitors are more expensive depending on the manufacturer and the facilities they offer.

How do battery monitors work?

The central part is a shunt, a low resistance connection between the negative pole and the rest of the electrical appliances connected to the battery. As the resistance is known and the voltage drop across the shunt can be measured easily this allows the calculation of the number of amps going in and out of the battery using Ohm’s law. This states that Amperes (I) = Volts (V) / Resistance (R). A shunt may measure 100A, and the voltage drop for 100A may be 50mV. Using Ohm’s Law its resistance is V/I = 50mV/100A =0.0005 Ohms. The software translates the measured 0 to 50mV to the current of 0 to 100A.