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 I recommend reading this whole section to help you learn more about how to determine your power requirements. You can quickly access specific information by clicking on the provided links.

Selecting a battery

Determining power requirements

Ensure adequate charging

Ensure correct voltage regulation

If you need to ask the crew a specific question please email the CREW.

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  Selecting a battery

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There is quite a variation in battery chioces depending on what your choices are , but you can break it down to Starting batteries (For use starting the engine then being charged straight away by the engine) and Deep cycle batteries (which can be cycled down and recharged without failure).

Then in each group you can choose a Wet cell deep cycle which when properly cared for is the best value for money.Then there is Gell cell and AGM(absorbed glass matt) batteries.These have a couple of advantages over wet cells.

1)They have a very low self discharge rate if left unattended for several months at a time.

2)They can better handle being rapidly cycled down and rapidly recharged ie.with the alternator, than wet cells.They can take high charging amps but make sure your alternators charging voltage is not higher than specified on the battery.


Determinig power requirements

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There are 4 steps to making this evaluation.

A)Figure out your power requirements.

B)Provide necesary battery capacity.

C)Ensure adequate charging.

D)Ensure correct voltage regulation.

I will base requirements for 24 hours of power without charging.

You need to calculate your power usage over 24 hours .The table below will give you the idea.

Equipment Rating                 hours use             total A/H

6 lights 1.5 amps ea              12 hours                 18 A/H

1 refridge comp 5 amps        10 hours                  50 A/H

Nav lights 1.5 amps               8 hours                   12 A/H

2 fans total 2 amps                5 hours                   10 A/H

Vhf radio 2 amps                   5 hours                   10 A/H

                                                 Total amphours=100A/H

Amps = Watts devided by Volts.

Example 24 Watts devided by 12 Volts=2Amps

For Ideal we times the above power requirements by 4.

I will breifly summarise why.

1)To maximise battery life you should not cycle your batteries below 50%.

2)A battery will charge reasonably quickly to around 80% but the last 20% takes a lot longer .So between 50% to 80% is practical usage which is 30% .I also subtract another 5% for a safety margin so we are at 25%.This means we have to times 100A/H by 4.So we need a capacity of 400A/H Ideally.This is just a guideline ,you may not be able to do this because of space limitations or expense.Also if you are generally always running with the engine charging you will not need as much capacity.


Ensure adequate charging

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I will explain a general guideline for sizing an alternator .

Marine alternators should have a hot (200 degrees) rating.This is for used in hot boat engine compartments.This is the rating we need.First we calculate 25% of our battery pack ( we will use our above 400A/H pack for this example)so 25% of 400A/H =100.

Then we add 25% of 100 for safety so 125 plus any engine loads that could be running while motoring ie 25amps.So ideal total alternator capacity would be 100+25+25=150amp alternator (hot rated 200 degree output)

Like on sail boats you do most of your charging at anchor or while sailing(when engine rpm is around 1000) you may choose to modify your alternator pulley gearing to maximise your output.To do this you need to know the safe maximum alternator speed(usually about 10.000rpm)You try to match this as close as possible to your maximum engine rpm.For example if your maximum engine rpm is 3000rpm I would try to gear my alternator 3:1 so the alternator would be at 9000rpm.So at 1000rpm the alternator would be spinning at 3000rpm will probably be close to producing 75% of its output.Pretty effective for low rpm charging.There are two things to watch out for when doing this.

1)If your tachometer reads off the alternator it will affect the reading.See if you can calibrate the tachometer or if you have a gas engine you can have some tachometers read off the coil.Sometimes living with an incorrect tachometer reading is just a compromise we learn to live with.

2)When going to a smaller alternator pulley you put extra load on the belt.You may have to get a more expensive higher quality belt.If you still have slippage you may have to look into a double pulley/belt set up.


Ensure correct voltage regulation

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If you have a high performance alternator system you should also invest in a good three stage regulator to complete the package.It will maximise the use of different voltages to enhance battery charge time + battery life.

After word

I've tried to simplifiy as much as possible and not to leave important things out.If you don't feel comfortable with aspects of doing these things yourself, seek some help from someone who is more familiar with these types of systems.I hope I have helped you gain a better working knowledge of your boats electrical system.Have fun.