If you know what watt is what, you might be happy with conventional ways of keeping track of your batteries’ charge. Or, you may consider a battery monitoring system, writes Lawrence Schaffler.

Which is the most reliable health indicator of your boat’s electrical system? A volt meter or an amp meter? Should you have both? Well, unless you’ve an intimate knowledge of electricity, you might consider swapping both for a battery monitoring system.

Most yachts and launches with 12-volt DC systems have at least one volt meter, typically indicating the status of the start battery. Some have a second, dedicated volt meter for the house batteries or, occasionally, a meter that can measure both – you toggle between start and house batteries.


But what does a volt meter actually tell you? We put that question to Simon Jennings, managing director of HMB Marine Electrical in Auckland’s Half Moon Bay.

“Superficially, two things,” Jennings says. “First, with the engine running, it’s an easy, visual cue to confirm that the alternator is charging the batteries. If the alternator’s working properly the volt meter should read up to 14.4 volts. If it’s reading around 12 volts or lower, chances are that there’s a problem with the charging system.


“Second, it’s a simple – though often misleading – indicator of the health of the battery. A fully-charged battery, in good condition and without any load, reflects about 12.6-12.7 volts. Anything lower should be cause for concern.”

Few boat owners, he stresses, understand the significance of even small variations in a volt meter reading.

“A battery showing 12.4 volts on the meter is down to about 75% of its capacity – at 12.1 volts it’s only about 50%. If it drops below 12 volts it will struggle to turn over the engine. Anything below 11.5 volts is considered a flat battery. Because those variables are relatively small, digital volt meters are much easier to read than their analogue cousins. Determining the value of a tiny deflection on an analogue meter can be difficult. ”


Volt meter readings, he adds, can be riddled with pitfalls, and should be interpreted with care.

“For an accurate reading the volt meter must – in the first instance – be wired correctly. That means it must be connected to the battery’s positive terminal with a dedicated wire. Many DIYers simply tee-off a positive wire from the switchboard. That’s no good. The reading won’t be accurate because there are all sorts of resistances and voltage drops present in the switchboard – you’ll have a distorted reading.

“Furthermore, you should rely on a reading only two to four hours after the battery has been fully charged. The battery needs to stand for a period to allow the surface charge on the plates to dissipate. If you don’t wait, you’ll get a false reading.”

Amp meters

As with volt meters, an amp meter provides some easy visual clues about the performance of the boat’s electrical system.

It indicates that the charging system is working properly.

Depending on the condition of the batteries, the meter might indicate that the batteries are receiving, say, 30 amps – a value that drops off quite quickly as the batteries take more charge.

Interpreting small deflections on an analogue meter can be difficult.

More importantly, when the engine’s shut down, the amp meter reflects the load on the batteries. This is particularly useful if you are worried about the size of the load relative to the capacity of your batteries.

By isolating different circuits – fridge/lights/nav instruments/chartplotter – and monitoring the amp meter to measure the load drawn by each of these, you can manage the electrical system and battery life more easily. Some older-generation analogue meters may have a negative/positive calibration – the needle moves into the negative zone to indicate the amps being consumed.

NB: if you have switched off all circuits and the amp meter still shows a load, be nervous. There’s a good chance of a stray current. If so, it will be draining the batteries and may be contributing to electrolysis – a related but completely different can of worms.

Cranking amps and amp-hours

Just when you think everything makes sense, volt and amp meters get more complicated. One of the most plaintive cries you’ll hear around marinas is from boaties complaining that their volt meter shows well over 12 volts, but the battery refuses to crank the engine. Why is this?

Well, it’s because it isn’t the voltage that starts the engine – it’s the cranking amps. By way of illustration, consider this: you can easily create 12 volts from a handful of torch batteries connected in series (positive to negative) – but they won’t turn over a diesel engine. They also need amps – and plenty of them.

You may remember that your start battery carries a CCA rating. It stands for Cold Cranking Amps and indicates the number of amps available, when the battery is in optimum condition, to crank over a cold engine.

The cranking amps in the battery provide the required grunt and, depending on the charge/age/condition of the battery, the CCA value is diminished. Unfortunately, neither a volt meter nor an amp meter measures the available cranking amps in a battery. You’ll need a load tester, and few boaties carry one of those on board.

Similarly, neither volt meters nor amp meters measure available amp-hours. You may also remember that your house batteries are – or should be – deep-cycle batteries, designed to take repetitive charge/discharge cycles and their size in reflected in amp-hours. This is the number of hours the battery can deliver a specified amperage.

As with a start battery’s CCA rating, so the house battery’s amp-hour rating falls away as it ages – and even more quickly if the battery is maltreated. Neither volt meters nor amp meters nor a combination of the two can sense the level of available amp-hours.

Purists will point out that only a hydrometer – an instrument that looks like something from the Spanish Inquisition – can provide a true, accurate sense of a battery’s condition. It is used to test the specific gravity of the electrolyte in each cell of a lead-acid battery.

Apart from the fact that few boaties carry a hydrometer, let alone know how to use one, lead-acid batteries are becoming increasingly obsolete in recreational boating. They are being replaced by sealed, maintenance-free AGM and GEL batteries.

So where does this leave a boat owner interested in managing and monitoring his electrical system more effectively? Someone interested in learning more than the superficial values provided by a volt meter or an amp meter?

An ideal solution is a digital battery monitoring system.

Battery monitoring

Even if you have a thorough understanding of the mystical relationship between volts, amps, alternators, CCAs and amp-hours, quite a bit of maths is involved in trying to assimilate an accurate representation of your electrical system’s health.

“The good news is that a battery monitoring system does all the maths for you,” says Jennings, “whirring away quietly to deliver all the important parameters – volts, amps coming in, amps going out, remaining amp hours – in clear, easy-to-understand graphics. Furthermore, they not only provide the electrical information but, depending on the model, can also be used to monitor water, fuel and grey water tank levels.”

There’s also a more practical motivation for installing a battery monitoring system. It’s typically a single meter equipped with buttons for scrolling through menus for different information. Compared to an array of volt and amp meters, it demands relatively little space on your dashboard. Aesthetically, that’s a big plus, but the smaller footprint also means space for a bigger chartplotter.

As always, every solution has a drawback. Battery monitoring systems have two – depending on your skill set and the size of your wallet: installation and price. “Unless you have fairly extensive skills,” says Jennings, “installing a battery monitoring system is not really a DIY job – it requires introducing a shunt into the system.” Don’t ask. “I’d recommend using a marine electrician.”

Battery monitoring systems are available from a variety of manufacturers. New Zealand company Cruz Pro produces the fully featured VAH 110. VAH stands for Volts, Amps, Hours – while BEP Marine produces the DCSM 600, which retails for $814. Both are excellent examples of modern battery monitor systems.

Jennings says that it’s important to recalibrate the battery monitor systems every three months or so, requiring you to hit the meter’s reset button. “It’s another of the mysteries of marine electrics. For whatever reason the meter’s calibration gets out of whack and needs to be reset. It’s a simple job, but you should make a point of doing it regularly to make sure the readings are always accurate.”

Modern, maintenance-free batteries are pricey items and it’s worth investing in appropriate technology to extract the best possible performance from them. Nothing illustrates that point more effectively than the dull ‘click’ of a flat battery when you turn the key.