Sail with New Freebooters

In the first post of this series, we saw that boat electrical and electronic equipment could be divided into two categories with incompatible power supply needs. We decided that this justified creating two independent electrical systems, one for equipment that needed a lot of power for a small number of short bursts each day, and one for equipment that needed a continuous supply at a tightly-controlled voltage.

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:

Item	Power (Watts)	Current (Amps)	Duty Cycle (Hours)	12 hour Capacity (Amp Hours)
TOTALS

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:

Item	Power (Watts)	Current (Amps)	Duty Cycle (Hours)	12 hour Capacity (Amp Hours)
Fishfinder	12.0	1.0	2.0	2.0
GPS/Nav		0.5	12.0	6.0
Interior Lights		5.0	6.0	15.0
Anchor Lights		1.0	12.0	12.0
TOTALS				35.0

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.