One of the things that I find most amazing about sailing is the simplicity of it. You pull a rope – ehrm sorry, a line – and a piece of cloth goes up, catches wind, and propels you forward. Easy. So much of our day to day life is determined and controlled by technology and electronics, it feels good to break out.

Not quite as simple

But unless you sail the simplest of boats, things are not quite as simple. Having a windlass is nice and keeps your back from breaking. That car stereo playing your favorite tunes while you are sailing into the setting sun makes the experience just perfect. Having some sort of GPS enabled device in the cockpit makes sailing on those long night passages just that much safer. And before you know it, you and your boat rely on a bunch of devices that need electricity to work.

That is the point when you inevitably start thinking about how to power all of these devices. Most boats have at least one 12 volt battery to keep the lights on and power our various gadgets. One of the main questions that will sooner or later arise is: How much power do I need? Where does this power come from? Running the engine in idle
just to keep the batteries topped up is not doing your engine any good. But maybe, if you just go out for a weekend, you don’t need to think about adding complicated charging kit such as solar panels or generators.

How much power do you need?

What you really need to do is to have a good understanding of how much power you consume on an average day sailing. One option to do so is to lookup the average consumption of every device that requires electricity, and add up their power requirements to a daily average. This is okay for a rough estimate, but depending on the complexity of your installation you might end up over- or underestimating your needs for power significantly.

This is where battery monitors come in. These are devices that constantly measure how much electrical power your system consumes, and how much it produces. Using this information, the device can give you a very accurate picture of the current state of charge of your batteries, how much load your appliances produce, and more. The newer generation of battery monitors are pretty easy to install. All they require is to hook up a shunt to the negative pole of your battery, and then all of your negative loads to the other site of the shunt. One cable from the shunt to the positive terminal of your battery, a small data cable to the monitor itself and you are done.

Installing a battery monitor

We’ve bought a Victron BMV-700 and installed it on two afternoons. Here is a list of very detailed installation instructions. We used this opportunity to cleanup our electrical installation and make it a lot simpler. We had all of our negative connections cramped into one massive battery terminal. That made it hard to understand what is connected where. We also realized that some of the cable that has been used is just copper wire, and not tinned copper wire. Where the wire had been exposed to salty sea air for many years, it had turned grey and green. This adds a lot of unnecessary resistance to our system.

To clean up this mess we decided to properly crimp all of our wires, and install a negative
distribution bus. This process turned out to be surprisingly easy. Using an inexpensive crimping tool like this and some heat shrink we tidied up all the connections, brought them to a distribution bus, and made a connection to the negative battery post.

It’s done!

Now we know exactly how much juice we have left in our batteries at any point in time. Fun fact: After installing the monitor, we saw that there was a draw of 20 mA even though everything in the boat was turned off. We started worrying about stray currents, faulty circuits and such … just to realize that we had accidentally had left the tri-color masthead light running! Oh well, you never stop learning!


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