For 12 volt loads divide their wattage by 12

For inverter driven loads divide their wattage by 10

This gives the number of amps drawn.

We must therefore provide and store at least 91 amp-hours of USABLE battery capacity.

We build a little safety margin in by ignoring the fact for some time in the day the alternator is supplying the fridge and water pump.

The "normal" alternator regulator will only charge your batteries to something like 80% of fully charged – however large the alternator is!

So 80% - 50% = 30%

91 ¸ 30 X 100 = 303 amp-hours worth of batteries.

Thus 3 X 110 ah batteries should do.

For the higher fridge consumption needs 380 Ah

so 4 x 110 Ah batteries should suffice.

The output of an alternator (amps) on a base of time is shown below:

This tail of float charge is of an unknown length.

At first sight the unknown length of float charge makes it impossible to relate the alternators quoted (maximum) output to average output, but consider just the shaded portion.

An ammeter is required to be sure about the time represented by the shaded portion for any individual boat, but it will often be in the 2 to 3 hour range.

Over that 2 to 3 hours the alternator output will be about 50% of the rated output. So a 55 amp alternator delivers:-

55 ¸ 2 = 27.5 Amps average.

1. Various authorities say you need to put back between 10% and 40% more electricity that you take out to get back to the same state of charge.

This depends upon battery condition, charge rate, temperature and probably what you are being sold.

I will assume 30% is realistic. This means that the 59Ah discharge will require about:-

91 ¸ 100 x 130 = 119 Ah of charge

(149 Ah - fridge)

2. You will often be told that you should only recharge batteries at 10% of their rated capacity and although the higher the charge rate and the more often you use it will affect the life of a battery. In fact the 10% relates to constant currant charging, whilst we use a form of constant voltage charging.

I need to supply 119Ah of charge at an average rate of 27.5 amps so:-

119 ¸ 27.5 = 4.33 hours

(149 ¸ 27.5 = 5.4 hours)

I know I must run for at least 4.33 hours a day to recharge my batteries - unfortunately the IWA only allows 3 hours running at their National Festival!

NOTE:- These long charge times may make the assumptions about average charge wrong. The average may well be lower than 50% of rated output.

Ways forward:-

1. Alter the use pattern so the engine runs long enough.
2. Reduce the consumption.
3. Fit alternative charging (Land line/solar/wind)
4. Fit larger alternator. Note:- At around 90 amps single V belts start to suffer reduced life.

Calculations using an 80 amp alternator:-

Recharge time:-

119 ¸ 40 = 2.9 hours

(149 ¸ 40 = 3.75 hours)

The alternator delivers about 65 amps for a few minutes as it warms up and then settles to its 55 amp output.

This is a split charge system with 3 x 110Ah domestic batteries and 1 x 90Ah starting battery giving a total capacity of 420Ah.

55 ¸ 420 x 100 = 13.4%

It is worth noting that the engine battery on twin alternator boats will be fully charged very quickly so unless you parallel both alternators you are wasting potential charge. This can be done with nothing more than a split charge relay joining the positives of both battery banks.

With 80 amp alternator:

80 ¸ 420 x 100 = 19.1%

With a "vehicle" alternator voltage regulator  and typical engine running times you will have difficulty to charge it above 80% of fully charged. You will lose 20% of battery capacity this winter, 20% more next winter, and so on.

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