How many amp hours should I have (within reason)

wakeup

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Hi I have a 28 footer single engine boat that is set up with two 110ah batteries wired in parallel for starting and domestic.

I have two fridges, windlass, radios, depth sounder that run off this setup. I am about to add a third dedicated starter battery with isolation switch and charging diode.

This should give me 220ah for domestic as above, plus 85 ah to start only.

I would like to sit at anchor over night and want to be able to run all the domestics without fear of dead batteries. Will this give me just enough juice or do I need a lot more lead?
 

rafiki_

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A quick calc says you should be able to run about 15 amps for 10 hours with a bit of contingency. This will give you 200 W continuous, which sounds OK with what you are planning. Anything that produces heat will start to ramp up the watts.

Enjoy your nights out.
 

Anthony

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Firstly, I would agree having a seperate battery bank for starting, and seperate from the domestics.

With two fridges you are really pushing having only two batteries (dont forget you only effectivley get half the usable capacity of a battery). I have 3 110ah domestic batteries and only 1 fridge, but really depends how long you spend away from shore power and not running engines.

As to the starter battery, its not the Ah but rather its CCA rating that you need to ensure is suffiecent for your engine.

Ants
 

VicS

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What you should really do is calculate your consumption in ampere-hours.

Work out the current taken by every bit of equipment from its power in watts if you do not already know it.
Estimate the time each item will be operating and drawing current, in hours, per 24 hours or just overnight if you feel that is adequate.

For each item multiply the current by the operating time to get its consumption in Ah.

Total all these to get the total consumption per 24 hours, or overnight if preferred.

Given that you will have two 110 Ah batteries for domestic use and that they should not normally be taken below 50% of fully charged you actually have 110 Ah to play with. If your overnight consumption is less than that (or only a tad more) you are OK. If your estimated consumption exceeds 110 Ah by much you need more battery capacity in the domestic bank.
True deep cycle batteries would help because they can be taken lower than 50% of fully charged without detriment, but they will be unsuitable for engine starting.
Leisure batteries would be marginally better for the domestics than starter type batteries but still should not be regularly taken below 50%

Keep your starter battery purely for engine starting. As said CCA is the important consideration as you will use very few Ah starting the engine.
 

wakeup

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Thanks all yes I will do the calc, I will have to dig out all the specs of the the various bits of kit first though.

Thanks for pointing out the 50% as you say I really only have 110 ah to play with so not as much as I first thought.

Yes I will get a proper start battery with CCA sufficient.
 

DavidJ

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Just to take an experienced based approach. Even though my Sealine is 37ft our overnight at anchor electric demand will be about the same. I have 3x 110Ah leisure batteries for domestic and 2x 110Ah leisure for starting the port engine. (maybe if it was a cranking battery I could get away with one but that is what Sealine fit as standard.
No problems keeping everything going at night without being particularly economic.
 

Wiggo

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You've got less than that, actually. Your alternator probably can't charge them to more than 85-90% of nominal capacity (though a decent battery charger will), so once you're at sea you may only have 220 x 0.85 - 110 = 77Ah of usable capacity.

I'm paranoid about domestic batteries and now have 560Ah on the house bank but I'm considering changing all the halogen downlights for LED ones to give me better power consumption (each halogen bulb draws almost 1A and from memory we have about 20 of them on board and the kids leave them on all the time).
 

Dave_Seager

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The two fridges are the main consideration when at anchor. I would not expect you to be using the windlass unless the engine was runnning. You might use the depth sounder and radio at anchor but their consumpion is unlikely to be significant against the fridges.

It largely depends on the type of fridges you have. I replaced and Electrolux which used 10A continuously (no thermostat) with a Isocool which only draws 5A part-time. Check the specifications for tyour fridges and then you can start calculating the AH you need.
 

BartW

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if you are just on the limit,
pay attention to the fact that fridges use a lot more power when it is hot.
Switching off one at night when the weather is hot might help to reduce consumption.
this was our own experience with a previous boat
 

jfm

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agree the other comments, except why do the calc? Just look at the domestic ammeter to see what you're actually drawing. That's what I do at anchor, cos it is easier than walking round to see what is switched on/off and doing the maths


beware fridges of course cos they are intermittent on/off, so when looking at ammeter you need to know if the motor is running or not. also with fridges there isn't much point working out the wattage cos the actual wattage is a function of how much heat leaks into the fridge and has to be pumped back out. On a warm day with lots of openings of the door it'll be near 100% of the fridge motor rating, but on a cold night with no door opening it'll be small
 

Bandit

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Fridges draw a lot of current, they are not essential be all and end alls. Two will be an horrific draw and for what cold orange juice for breakfast, you need juice in your domestic batteries for lighting, possibly nav or anchor lights and if in an emergency you have to lift your anchor ( electric windlass huge draw) and for when you move nav aids and radar. Its no fun moving out with flat batteries in fog your gps and radr soon kill the domestic battery, I know i have done it.

Switch off everything as soon as you can after you stop the engines.

Only run one fridge wiith essentials such as beer, tonic and white wine , ice, oh and maybe prescription medicines and maybe meat products at a push. turn this fridge off after you have eaten say 9 or ten at night it will stay cold until the next morning and you should not need really cold beer untill late morning.

Put fruit, veg and salad in the non essentials fridge that you stop when you stop the engines.

A good trick is to put a large freezer pack in the ice box that will give back the cold when you turn the fridge off.
 

ccscott49

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In UK two fridges will not flatten your domestic bank overnight, they would have to draw 5 amps each, all night (10 hours) to reach the magic 50% and if they did, (occasonally) so what, you are a motor boat, start your engine and the batteries will be charging. You also have to consider if your fridges shut down when the voltage drops below a set level, then you cant flatten your batteries.
A load of absolute tosh is spouted about domestic loads etc especially fridges and I think some folk are a bit behind the times and technology.
One tip I would add, is fit an intelligent alternator controller, to get the best out of your charging.
To another poster, teach your children, on pain of death, to switch lights off! Cheaper than LED bulbs!
 

jfm

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[ QUOTE ]
A load of absolute tosh is spouted about ... fridges

[/ QUOTE ]
Exactly Englander. It is just nonsense to say (in the same post!) that a fridge left on will discharge the battery whereas if turned off it'll stay cold all night. One can't have it both ways. A fridge motor only runs when the fridge warms up!
 

Bandit

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Read the post :

At anchor I assume electric windlass off domestic batteries.

You will need to run lights, radio cd from say 9pm to say midnight , lift your anchor in the morning and use navaids in the morning I assume.

Maybe he will need to run an anchor light over night?

Why do you need two fridges running all night on a 28 ft boat?

Say you arrive at 5pm and turn the fridge off at 10pm with a freezer pack in the ice box it will stay cold or cool.
 

wakeup

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Err because it is a cheapo French boat and there isn't believe it or not a domestic ammeter, which is why I have had a couple of close calls with a low battery bank prior to starting...So first fix is to get a dedicated battery for starting on separate circuit which I am about to do. Second is to make sure I roughly have enough AH in the soon to be domestic only and third is to fit some sort of battery monitoring system. The charger I have is pretty good although it doesn't currently have a display but the manufacturer does have a multi display monitor accessory that I will also fit.
 

wakeup

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Bandit appreciate you comments, but you need to understand 'my' type of boating. We often drop the hook after a shortish jaunt, the two fridges are divided nicely into food and cave, whilst one could be used as an ice box and it was until I converted it with a waeco unit, I can't be arsed to buy ice all the time and want to leave the 'cave' stocked ready to go and chilled. I also don't want to have to worry about switching the fridge or radios of at anchor and or worry about them. Ditto I want to leave GPS, Depth and fish finder on not to mention the sound system (low volume so not to disturb others at anchor)

I will at some point, probably next season move my boat to the med so I want to get all these issues fettled and sorted out before I shift her. Once in the med I will definitely need to fridges, and would even like to squeeze in an icemaker if I can find room. /forums/images/graemlins/cool.gif
 
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catalac08

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from my experience with similar battery drains - one night fine, 2 nights ok, 3 nights starting to get low batteries, 4 nights need to charge either genny or engine BUT this will take hours to put plenty of charge back into a 220AH bank. Suggest as other comments that you would be better off having a bigger battery bank if you intend to spend periods of time at anchor/away from shore power or genny charging which is noisy, fumes, takes hours. I sometimes feel very guilty spoiling a quiet anchorage by having to run a generator and try and get well away from others if I need to do this. I have 2 wind gennerators and can now do about a week-9 days on board but things still start to run down unless there is plenty of wind to keep batteries topped up.
 

ccscott49

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Just an aside, but if you find a 12 or 24 volt icemaker, let me know, or even one that will run off an inverter. They are horrendously power hungry and expensive.
 
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