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Old 04-01-2011, 14:25   #1
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How Many Batteries Are Needed ?

Hello - I just bought my first boat, a Grampian 26. I'll be traveling down the ICW soon. I'll be anchoring mostly. The boat has a mercury 9.9 with an alternator for charging. How many batteries will I need to power the lights and such in the evenings? Thanks.

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Old 04-01-2011, 15:05   #2

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You'll need to know the output of the engine's alternator (which may be a low-power generator not an alternator) and the drain from the nav lights and other systems on board. Generally a small outboard can just about keep up the running lights and not a whole lot more.

You might want to invest in LED lighting, which only seems damned expensive until you realize that it can save you hundreds of dollars in batteries and fuel costs because it is so much more efficient.

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Old 04-01-2011, 15:34   #3
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Before you can answer the question you'll need to know your daily amp consumption. On a 26' boat with minimal draw it should be in the 50 AH range per day.

Add up everything you use per 24 hour period, i.e. - lights (10 AH), chart plotter (10 AH), Radio (5 AH), etc. etc. etc.

Once you have your "Amp Budget" then choose batteries that provide double or more in reserve capacity. Two six volt Trojan T-105s ($100 each) give you 225 AH capacity and that should do the trick on a small boat with no fridge or other electricity hogs. More AH usage???? More batteries.

Then figure out how to recharge them. Doubtful your Ooutboard alternator will do the trick. A KISS wind generator ($1,000) will give you about 45 Amps a day in steady (10-15 kt) winds. A Kyocera 135 Watt Solar panel ($350) with Blue Sky Solar Boost ($250) will do about the same in 5 hours of good sun.

A Honda EU2000 generator ($950) should charge two T-105's in an hour with a 30 Amp charger. These gens are tiny, quiet and sip fuel. Expensive, but a nice thing to have......

You may also want to buy an oil lamp and some battery powered nav lights for when the gasoline runs out, the sky clouds and the wind dies.
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Old 04-01-2011, 16:08   #4
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Not sure I agree with the above figures. If it is conceivable that he uses 50 amps/day and has a 30 amp charger, it isn't too much of a stretch to guess a one hour period for charging isn't going to help much. Recognize that a 30 amp charger indicates only it's capacity. In real life, you get less under bulk charging which quickly tapers off at which point you might only be at 80% recharge of a 2 battery bank in the example above.

It can take hours to reach 100%. Not fully recharging limits the available capacity and more importantly, shortens the life of batteries.

A Honda genset might be your best bet but don't count on only 1 hour per day use.
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Old 04-01-2011, 16:15   #5
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I think an EU1000 would be all you need and a 30 amp battery charger. Make sure you get a good marine 3 stage battery charger or you will be replacing the batteries way to soon.

Be careful with the generator and I would suggest you install a CO detector in the cabin.
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Old 04-01-2011, 16:40   #6
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Our Honda 2000 would take a three hours to get close to replacing 50 amps with a 40 AH charger. What happens is it starts our giving 40 amps but tails off quite quickly so most of the time is spent at 25 - 15 amps before dropping into a float charge at 6 amps for the last bit. However, the Honda 2000 also gives us mains voltage and therefore:

Hot water via the calorifier.

Main electric for power tools including the hoover.

Occasional heat via oil filled electric radiators.

It will run from midnight to breakfast on a tank full keeping the yacht warm. the only thing you have to decide is who is going to fill the tank up at 7am, wifey or you, she normally wins and stays under the duvey.

In terms of power we use about 30 amp hours a day in harbour with lights, 4 cu ft fridge, laptop etc which is quite frugal. We get 10 AH back with solar 45 watt panel and generate the rest via the Honda. Therefore 2 x 110 AH batteries meet our needs whilst being cost effective.

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Old 04-01-2011, 17:28   #7
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If your Mercí outrboard actually has an alternator, suitable for battery charging (and not just powering lighting - possibly AC output), it would only be rated at about 6 Amp (at WOT). This wonít be adequate for practical recharging of a cruising house battery bank.
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 04-01-2011, 17:31   #8
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True, I miscalculated and you are correct. To put 50 amp hours into the house batteries would take longer than an hour with a 30 amp charger. (I actually rely mostly on the KISS and solar panels - but use about 75 AH a day when out and about). If wind and sun don't keep up (they usually don't) I run my engine with 90 amp alternator until the bank is charged. When the LED retrofit is complete I'm hoping not to have to waste fuel anymore.

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