There can’t be many activities tougher than cruising a sailing boat when it comes to testing battery longevity and efficiency.

Away from the convenience of a modern marina, where batteries can be charged via a shore power (240V) cable to an onboard battery charger(s), the primary means of charging batteries is by running the yacht’s engine and charging via its alternator. Modern yachts are normally equipped with efficient marine diesel engines and these are usually used routinely for departing from and arrival in harbours as well as in windless conditions at sea.

A secondary means of charging is possible and indeed recommended. During long periods at anchor and extended passages at sea, battery charging by engine alone may not be practical due to limited fuel carrying capacity and, furthermore, operating a marine diesel under minimal load can result in excessive engine wear. The most popular alternatives often seen on yachts fitted out for more serious passage making are wind chargers, solar (photovoltaic or PV) panels, or a towed water generator, or any combination of these.

Boat owners need to assess battery requirements and manage consumption very carefully. A properly planned 12 volt battery installation is imperative and the correct choice of batteries is vital. ‘Deep Cycling’ batteries, which are specifically designed for discharging and recharging, will give many ‘cycles’ over many years provided they are not frequently discharged below say 50% of their capacity. A dedicated engine starting battery is a different beast and designed to produce a high burst of power for a short time and it follows that a combination of the two types is the recommended set up for a sailing boat with an auxiliary engine.

A charging system which prioritises charging of the engine starting battery is sensible. This battery will have relatively low capacity and can be topped up before recharging the domestic batteries. A battery monitor is a useful instrument, enabling a high degree of visibility and control over consumption.

In calculating consumption, there are many factors to take into account. For example, a fridge operating 24 hours a day and/or perhaps a radar or short-wave radio transmitter on board will increase consumption markedly. A boat underway overnight with all instruments and navigation lights in use will use a lot of amps, whilst a boat at anchor with the crew asleep and just an anchor light glowing from the masthead will consume very little. The permutations are endless, but a detailed analysis and experience should establish a maximum power consumption or ‘draw’.

We reckon on a maximum draw, when we are on passage including nights at sea, of around 175 amp hours per 24 hours but I ought to add that our consumption does not include equipment such as a radar, water maker or short-wave radio which we do not carry on board. These could easily add up to a further 50 amp hours draw making a total of 225 amp hours. This is a rough estimate based on our experience and far more detailed information can be found online to assist with your own calculations.

Spirit of Penmar, our current boat, is a 39 foot (11 .7m) sloop and is equipped with a 35HP diesel engine providing a maximum 65 amp charge via its alternator. We have a wind charger capable of delivering 35 amps in a gale but averaging say 4 to 7 amps in a moderate wind. Also, we have a small flexible solar panel capable of producing a maximum 4 amp charge … when the sun shines!

We have two separate battery banks fitted, comprising

1. Four ‘deep cycling’ 105 Amp hour (Ah) Lifeline batteries wired in parallel, in total 420 Ah (12V) for the domestic requirements (everything on board other than engine starting) and;
2. For dedicated engine starting, a Red Flash 1100 with a small capacity of 43 Ah (12V), which cranks out a very high current (around 500 amps) for a short time.

The system was designed so that in an emergency the systems could be switched in parallel together to start the engine, making temporary use of the domestic battery capacity. All five batteries are sealed lead acid AGM and are tried and tested high quality batteries. We had a similar set up on our previous 29 foot (8.8m) sailing boat.

Why AGM (Absorbed Glass Matt) batteries? Well, we are no experts but the key reasons we would recommend AGM batteries for marine application are as follows:

1. AGM batteries are sealed and maintenance free and do not therefore spill fluids.
2. AGM batteries have excellent deep cycling performance which is critical for longevity on sailing boats where battery drain is not always immediately matched by recharging opportunities.
3. AGM batteries can be mounted in most orientations, provided of course they are properly secured.
4. AGM batteries are tolerant of ‘bouncy’ motion and vibration … essential in a sea going environment!

We have recently replaced our entire battery bank following nearly 11 years of use, including an Atlantic circuit, ocean crossings and a high proportion of time at anchor. The used batteries were returned to DMS technologies where lab testing showed that, after 11 years use, all batteries were showing signs of wear but were still averaging around 80% of their original factory capacity. This is quite remarkable and well exceeded our original expectations. We attribute this performance to the following:

1. Smart voltage regulated shore power (240V AC/12V DC) chargers were fitted, which are designed not only to charge the batteries but continue to trickle charge them to bring all the cells into balance. If this type of charging is not carried out the weakest cell in the battery will never reach full charge and battery life will be compromised.
2.  Although Spirit of Penmar has a lot of electronic gear, she was also fitted with a wind generator, a solar panel and, for long at-sea passages, a towing water generator so that every opportunity was used to recharge the batteries.
3.  Carrying 420 Ah capacity for the domestic load meant that there was a much larger reservoir of battery power than is normally fitted to a sailing yacht. This meant that the batteries were less likely to be completely drained, which would make them more difficult to recharge and could ultimately reduce the capacity.
4.  Upgrading to modern lower consumption instruments and LED lighting has decreased power consumption markedly.

In years past, yacht manufacturers and owners have tended to view batteries as a short life throw away item. However, with modern battery technology this no longer needs to be the case.

Geoff and Niki Phillips
SV Spirit of Penmar
September 2022