Battery monitoring

On a typical boat, batteries are ignored, abused and unloved – and yet are asked to deliver maximum power at start-up. Is it any wonder they often last less than three years? Their life could be more than doubled with better maintenance.

The most important aspect of a long battery life is good management. You should never fully discharge any battery – over-discharge is said to be the single biggest cause of battery failure. Leaving the boat unused for long periods will undoubtedly result in self-discharge, but even a regularly-used boat can suffer from a flat battery if you ignore how much power is going in/coming out. Also, most batteries only provide a finite number of discharge/recharge cycles, so if you don’t know how many times the battery’s been charged you can’t judge it’s overall condition. Which is why a battery monitor is a good idea.

You may think the voltmeter tucked among the helm gauges is sufficient. After all, it shows how many volts the battery is delivering and (when the engine is running) how many volts are going back into the battery. But voltage is only a small part of the story.

Think of it like a water tap: The water in the pipe has a certain pressure, and as long as that’s neither too low nor too high we don’t worry much about it. But the volume of water is more important when trying to fill a container, and the tap gives you control over that. The flow dictates how quickly we can fill a bath, not the pressure. If the water pressure is the volts then the volume of water is the current, measured in amperes (amps).

And so it is with batteries: a low voltage certainly indicates that you have a problem but it’s not much help for overall management of your battery. And while a higher voltage tells you that the engine is pushing power back into the battery, in itself it does not tell you how quickly it’s charging.

What you need to explore is the current that’s flowing in or out of the battery. And this is where a battery monitor is useful. It can measure both voltage and current and, when pre-set with the capacity of your battery, it can also tell you the overall state of charge. You then know how long it will take to be fully charged or, if the engine is not running, how long before you run out of power based on the current drain from accessories such as lights, stereo or chartplotter.

On my launch I have a typical two-battery setup.