Power Management

Power management begins with the battery. Unless the battery bank is of the right type and capacity no charging system will compensate.

For small yachts this may mean just one battery that both serves to start the engine and to power the on board electrics. This set up will probably require either a charger using shore power, or where the yacht is on a mooring, a small wind generator/solar panel. Typically this might be 65AH (Ampere Hours).

The larger yacht will have a battery bank consisting of two or more batteries, usually wired through a switch so that they can be used independently, together or isolated. These battery banks are usually in the range of 130AH to 280AH. It is common practice for one battery to be reserved for engine starting and the other(s) for a domestic supply.

The latter should always be of the 'deep discharge type' capable of recovering from a relatively prolonged, steady drain, such as is the case on an overnight passage, which might have involved the autopilot, VHF, and berth lights as well as the navigation lights.

Whilst not strictly necessary, it is very common for the engine battery to be of this type so that the two are truly interchangeable and so that they will be properly matched if used together. Here it is worth pointing out that using a 'good' battery in parallel with a 'poor' battery will not produce an average of the two. The poor will pull the good down to it's own level.

The analogy with water is useful when considering 12 volt systems. In the case above, it is readily seen that linking a full tank to one that is leaking will soon have the full tank down to the level of the leaky one.

Power management consists of deciding what size of 'tank' or battery bank will fulfill the likely demand and how and at what rate that tank will be replenished. It is no bad thing to list the power draw of all electrical items on board and then consider a 'worst case' scenario, of which might be used simultaneously, and plan the battery size and chargers accordingly.

However for most yachts the space constraints limit the extent to which bigger or extra batteries can be fitted and the engine alternator, the only means of charging independently of shore supply. It is mainly for these owners, who want something extra to help 'top up' their existing battery situation, that the solar panels and vertical wind chargers will be of most use.

The larger yachts, or those who live aboard, will usually want fridges and other such luxuries, and will have battery banks in excess of 300AH They will need to consider combinations of all kinds of generating capability, especially the propeller type wind generator.