Boat batteries are expensive to buy in the first place, yet they can lose most of their capacity within a couple of seasons if they are mistreated. However, if you treat them well, a set of decent service batteries can last for five to seven years.

Boat batteries require proper treatment

Decent batteries are expensive, yet still vulnerable to mistreatment.


1. Monitor the battery state

Good battery management is vital to their longevity. Discharging even the best deep-discharge batteries below 50 per cent of their rated capacity will dramatically shorten their life, while regularly flattening the battery can destroy it in a few months. Don’t underestimate the effect of this: over-discharge is the biggest single cause of battery failure.

If you have the budget, sophisticated battery monitor systems will measure all the charge put into and removed from the batteries and can calculate when they will next need charging, assuming a constant rate of power drain. A more economic, though somewhat rough and ready alternative is a cheap digital voltmeter. With no load drawn, at full charge the battery voltage will be 12.6-12.7V, and when fully discharged it has just 11.8, while 12.2V represents approximately 50 per cent of total capacity. In order to preserve battery life it’s considered bad practice to discharge batteries beyond 50 per cent of overall capacity – so ideally they should be charged before the voltage falls to 12.2V.


Voltmeter: making boat batteries last longer

A simple voltmeter will give an indication of battery charge, as long as no load is being drawn.


2. Calculate your daily power use

This is one of the most important elements in ensuring battery life – if your batteries are too small then you will simply be taking out too much charge each day. Daily power use can be estimated by multiplying the current in Amps of each electrical device on board by the length of time for which you typically expect to use it in each 24-hour period. Ideally the service batteries should be sized so that their amp hour (Ah) rating is three times larger than the boat’s daily electrical consumption.


3. Reduce your power requirements

This can be one of the easiest and cheapest ways to extend battery life. It's as simple as not leaving lights on unnecessarily. Even better, changing to low-energy LED bulbs can significantly reduce consumption, as will ensuring the fridge has plenty of insulation – 4in (100mm) is a minimum for efficient operation. Similarly, a couple of bottles of frozen water placed in the fridge at the start of each trip will significantly reduce the power needed to keep the contents cool.


4. Add a solar panel

Your batteries will lose charge even if the boat is not being used – depending on the type of battery, there will be a self-discharge rate of around 1-3 per cent per month. However, a cheap (£10-20) small solar panel is all that’s needed to account for this, which will again help to prolong their life.

Small solar panel to top up batteries

A small solar panel will eliminate self-discharge of batteries.

A larger panel, charging via a regulator that prevents over-charging, is an excellent next step, as during daylight hours it will be constantly helping to put charge back into the batteries. This is true even in northern latitudes during the summer, when the long daylight hours can help to compensate for cloudy conditions. A further advantage of a larger solar panel is that, even if the boat is kept on a mooring with no access to shore power, the batteries will usually be fully topped up at the start of each weekend.

Large solar panel

A larger solar panel that charges via a regulator will replenish the batteries while you're away from the boat and help to provide daily power.


5. Fit a decent mains power charger

If you frequent marinas, plugging into their shore power supply is an easy way to reduce the drain on your batteries. In addition, a quality three or four-stage charger will ensure batteries are 100 per cent charged when you leave the following morning. Equally, for boats with access to shore power on their moorings a charger of a type that can be left permanently connected will ensure that the batteries are constantly pampered.


6. Add an alternator smart charger

With standard alternators, only a relatively small percentage of the theoretical output will reach the battery. For instance, with a 75Ah alternator, you may get 50Ah of charge immediately after starting the engine, but after just 15 minutes of engine running that figure will halve.

A smart charging regulator, such as the system from Adverc, prevents the charge rate from reducing so dramatically over time. Since it keeps the charging rate close to the initial figure, the alternator will re-charge the batteries in minimum time. This system has a further advantage in that it will charge batteries to 95 per cent of capacity – compared to 65-70 per cent for a standard alternator regulator.


7. Fit a larger alternator

If you’re still not able to put sufficient charge into the battery bank this can be one of the more cost-effective ways of increasing the amount of charge the batteries receive. When new, most marine engines had a choice of alternators, so it’s worth checking whether your engine was fitted with the larger size, or indeed whether the manufacturer uprated the sizes available for that engine at a later date. Alternatively, a number of aftermarket suppliers offer alternators with a significantly larger output that will fit a wide variety of marine engines.


8. Add a wind turbine or hydrogenerator

Before solar power became cheaper and more efficient over the past 15 years or so, wind turbines were a very popular way to put extra charge in to the batteries. There are still a number of owners that value their wind generators, although before opting to fit one it’s important to be aware of their disadvantages.

The most commonly cited is the noise they make in a strong wind. However, another drawback is that in many sheltered ports the wind speed will rarely reach the level at which the unit produces a decent output. Obviously this will vary with your sailing area – a Caribbean anchorage may enjoy reliable tradewinds most of the time, but a steep-sided West Country river will see many days with very little wind.

Hydrogenerators, whether the more traditional towed variety or the more efficient newer transom mounted types, can generate the entire power needs of a yacht when on passage. They can therefore be ideal for those that sail longer distances, but of course won’t help to charge the batteries while the boat is in port.

Written by: Rupert Holmes
Rupert Holmes has more than 70,000 miles of offshore cruising and racing experience, in waters ranging from the North Sea to the Southern Ocean and Cape Horn. He writes about all aspects of boat ownership and marine travel, including destinations, seamanship and maintenance, as well as undertaking regular new boat and gear tests. He currently sails around 5,000 miles per year and in the past couple of seasons has cruised from the UK to the Azores, as well as winning his class in the 2014 two-handed Round Britain and Ireland Race. He also owns two yachts, one based in the Mediterranean and the other in the UK.