Calculating the House System Requirement

Now let’s put numbers to these requirements, starting with the ‘house system’ or ‘service system’ – the one that supplies smooth power over a long period. Before we can decide what batteries we need, we must decide how long we need power to last before we need to recharge the batteries, and we need to calculate how much current our equipment will use during that period. The unit we use to measure this is the Ampère Hour, usually shortened to Amp Hour and written Ah. It is the quantity of electricity used by continuously running a device that draws one Amp of current for a period of one hour – so a device that draws two Amps will use two 2Ah in a one-hour period, and a device that draws only half an Amp will use 0.5Ah in a one-hour period.

Normal practice is to assume that you will sometimes need up to 12 hours’ worth of consumption before recharging the house batteries, even if you usually avoid sailing at night or staying anywhere except on a marina with power available at each berth.

Either on paper or using a computer spreadsheet, draw up a table like the one below:

Now you need to find out the rated consumption for each piece of equipment. Each will have a plate or label somewhere (often on the back, where you can’t reach it easily) with this information. If you’re lucky, it will specify the current in Amps, but if it it only has the power in Watts you’ll have to divide this by the supply voltage (usually 12 Volts) to calculate the current. e.g. a 6 Watt item will draw 6/12 = 0.5Amp. Multiply the current by the maximum number of hours the equipment is likely to be running in the worst 12 hour period to get the number of Amp Hours required from the supply. e.g. an anchor light could be on all night, and so could nav lights, but in normal latitudes 6 hours should be ample for interior lights, and some equipment will be used even less.

Here’s a table filled in with a few typical values:

In the above rather incomplete table, I have made the following assumptions:

• The Fishfinder’s label specified a 12 Watt power rating without showing how many Amps it drew
• The Fishfinder would be used  for 2 hours during the day
• Interior lights would be used for 6 hours during a night
• Anchor and Nav Lights could be in use for 12 hours at a time
• GPS and Nav equipment could be in use for 12 hours at a time

In practice, the Anchor and Nav lights are likely to be in use during a different 12 hour period from the GPS and Nav equipment. Make your own judgement, but be sure to cover the worst typical case.

Calculating the Starter System Requirement

The battery bank used to start the main engine and any auxiliary motors is recharged immediately, so it’s not the Amp Hours that matter – it’s the ability to supply sufficient current to crank the motor.  Your engine’s technical specifications will tell you how much current the starter motor draws at various temperatures.

That’s enough for one session. The next post on this subject will discuss the characteristics of different kinds of battery, how the actual performance varies from the nominal performance, and how to choose batteries that are suitable for your needs.

Thirty or forty years ago, when I was passionately active in the sailing world, most yachts made pretty simple demands on their electrical supplies. Small craft carried little more than navigation and cabin lights, instruments to show wind strength and direction, and an echo sounder. Other gear such as radio transmitter and RDF often ran off their own dry batteries. Even if the motor had an electric starter, it was almost always possible to start it by hand.

Today, almost any craft that is intended for offshore passages will have a whole range of expensive equipment to assist with navigation, communications and search & rescue – in addition to comfort & lifestyle non-essentials such as refrigerators and entertainment systems.

Unfortunately, the important equipment is almost always the most sensitive – to airborne radio frequency interference caused by other equipment, to corrosion and electrical leakage caused by humidity and salt, to variations in supply voltage caused by other equipment, and to physical damage caused by vibration or by being hit or by falling to the floor.

What power does that equipment need?

Let’s start by looking at the power sources we need to provide so that all this expensive gear works as its designers intended. I’ll discuss the other challenges later, each in a dedicated post.

Some things need chunky power

Some electrical equipment needs lots of power for short bursts, and nothing the rest of the time. Extreme examples of this class are the starter for the boat’s main engine, and an electric anchor windlass. On larger craft, there will be one or more smaller auxiliary internal combustion motors for various purposes, including charging the batteries. Such equipment can tolerate a power supply whose voltage varies by quite a large margin and over a very short time – the critical demand is for a huge current at short notice.

Some things need smooth power

All electronic equipment, from navigation equipment to entertainment systems, is designed to be supplied with a Direct Current voltage that remains within very few percent of a nominal value, and does not change rapidly even within that very limited range.

The above requirements are incompatible

A typical modern car has only one battery and an alternator to charge it. Most boats still try to run with a similar setup, but they get problems. Why? The main reason is that normal people (other than courting couples, policemen and spies) don’t make a habit of sitting in a car for long periods with the engine switched off, using all the auxiliary equipment. A car derives its electrical power from the alternator as soon as it has replaced the charge used up in starting the engine.

Unfortunately, batteries designed to supply the occasional short bursts of heavy current required for a starter motor are bad at supplying much lower current levels continuously for a long time, and if you use them for that purpose they will be damaged. If you leave your car parked all day with the lights on a few times, or if you flatten the battery trying to start a faulty engine, you will soon need a new battery.

On-board power supplies

Stop treating your boat as if it were a car. Logically, there are at least two electrical systems, and the more you isolate one from the other the less problems you will have. For a start, if there is no electrical connection between the fridge motor and the supply to the autopilot electronics, that’s one less reason for it to suddenly decide to head north in the middle of a fast eastbound reach. (More than one Vendée Globe competitor experienced this during December 2008, but the cause was almost certainly something a little more subtle – the sophisticated pilot systems they use are very sensitive to radio frequency interference).

In my next post on the subject of Marine Electrics and Electronics, I’ll discuss what equipment to allocate to each electrical system, and how to specify the characteristics of the associated batteries.