Rechargeable Batteries

Almost all cruising yachts nowadays run so many electrical and electronic items that the thought of going sailing without substantial battery power is about as ludicrous as going sailing without a rudder.

Requirements for electrical power fall into two categories; permanently fitted rechargeable batteries, commonly of the lead acid variety, and replaceable or rechargeable cells for items like torches, hand held VHF and GPS sets, mobile phones, etc. With the ever increasing demand for current it makes sense to ensure that maintenance schedules include these items, not just as a means of extending their service life, but also from a safety point of view.

Many yacht instrument systems can provide voltage readings

A digital multimeter will give accurate measurements for individual batteries.

Although some may be fortunate, or wealthy enough to have gel or AGM (absorbed glass matt) batteries which require little or no maintenance, conventional, and much cheaper, lead acid batteries do require a degree of care.

Firstly it is essential that the battery is not discharged below 50% of its capacity, or its ability to accept a charge will be dramatically reduced and will lead to early and complete failure. As a means of monitoring the state of charge situation, the addition of a volt meter, preferably a digital one, is probably all that most yachts need. Just never let the battery voltage drop below 11.1 volts, a reading that indicates half charge. If the fitting of an in line volt meter is not practical, simple hand held ones are available for only a few pounds. The readings for calculating the state of the battery are all taken with no load, i.e. everything switched off.

In order to ensure that the battery performs at its best, a few simple routines will help. Make sure that the connections to the battery terminals are tight and free of any signs of corrosion.

Checking the electrolyte level of each cell. Topping up should only be required occasionally

The application of hot water will help to remove the worst of the fungus that can appear, but to avoid it happening at all, apply a coating of petroleum jelly to the terminals before tightening. Other than that, the only other routine maintenance is to ensure that the individual cells are topped up with distilled water to a level just above the plates.

On the other hand, rechargeable cells such as nickel cadmium thrive on being charged and then fully discharged. In fact, if these cells are not treated in this way, they can develop a memory effect which can dramatically shorten their life. Some of the more exotic batteries now available such as nickel metal hydride, or NiMh for short, can be partially discharged and then recharged without creating this memory effect.

Many of the items of equipment that use these cells are also capable of operating on, and are often designed to be powered by replaceable dry cells which actually run at a higher voltage. A dry cell has a nominal voltage of 1.5 volts whereas the rechargeable types have a nominal voltage of 1.2 volts.

On board recharging facilities for portable items are almost essential. The battery charging unit is both mains and 12v capable.

This effectively means that the rechargeable type starts with a power disadvantage, and its output will reach critical levels much sooner than the disposable version. The only way of combating this situation is to either have a large supply of ready charged units available, or to have a suitable means of recharging on the move.

A number of small units are available at modest cost, which can be plugged into a cigar lighter socket, or alternatively wired into the ships battery. The main points to look for are the speed of charge offered by the different units. A one hour charging unit may sound attractive, but a slower charge rate may be kinder to some of the smaller rated cells.

Rechargeable cells are available in a wide range of capacities and sizes. Obviously the correct size is required, but the selection of capacity is less clear cut, although for all the difference in cost, extra capacity is probably well worth while.

Several mains powered charging unit can recharge different sizes at the same time

All these cells will self discharge over a period of time, and an important characteristic is that when a rechargeable cell is under load and it reaches the end of its capacity, the cut off point is very sudden, as opposed to a dry cell which gives some warning of its impending demise. This emphasises the need for a supply of ready charged spares. This in turn imposes the need to try to standardise the type and size of cell for as many on board items as possible. The thought of having half a dozen different sizes of cell and their respective chargers festooned about the boat could upset the routine on even the best run yacht.

Although many chandlers sell both chargers and the appropriate batteries, savings are often available by bulk buying at some of the specialist electronics stores, where a wider range of equipment may also be on offer.

	Gordon Buchanan

Gordon is the Scottish correpondent for Sailing Today. His writing covers most aspects of the maintenance and upgrading of cruising yachts and their equipment, including engines, electrics, rigs, and hull structures. He has over 50 years experience with boats, ranging from a Mirror dinghy to a 60ft ocean racer, and has raced at most levels, although sailing is nowadays mostly cruising. His recent cruises include Scotland to France, the Mediterranean, and a crossing from Spain to Ireland. Gordon has owned the same 32ft classic yacht for the last 23 years, and still races her from her base on the Clyde.