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Old 09-08-2012, 11:58   #1
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House battery question

Hi All,

This may be covered in the parts of the Electrical Forum that I havenít gotten to (but, I did a search and found nothing) ; if so, I will stand by for the flaming I deserve. If not, here is my question.

How do you estimate power draw (before, you build the battery system)?

For example I would like to be able to have a microwave on board and be able to use it without melting wires or batteries or putting a hole in my boat.

Thank you
Scott
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Old 09-08-2012, 12:09   #2
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Re: House battery question

First comment, unless you have a microwave that works on 12V battery power you will have to have some way to make 120 V AC power (like the wall sockets in your house).

By the way, not sure of your level of knowledge so apologize if I am over simplifying.

Unless you are at a dock with a shore power connection you can get house power with a generator or inverter (converts 12V DC battery to 120 V AC house type electricity).

To figure your load you need to know the watts or amps used by everything you will run on the boat. Things that draw a lot: air conditioners, anything electric that gets hot like stoves, hot water, heaters, etc. refrigerators.

Add up all these and you have your answer. If you don't know what they use there are web sites for people buying home backup generators that give you pretty good estimates for power use for this stuff.
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Old 09-08-2012, 12:11   #3
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Re: House battery question

Keep reading.

The answer is always to read the rating plate on every device on the boat. Every light, instrument, fridge, whatever plugs into power. Take the number of watts, multiply by the number of hours it will be used during a 24-hour cycle, now you have kilowatt-hours per day.

Convert that to amp-hours (or use amps instead of watts from the rating plates) and that's how many amp-hours of power you need per day. Then you decide if you want or need to charge the batteries daily or every third day or whatever, and how deeply you want to cycle the batteries, 30% or 50% or 70%, whatever, and decide how many amp-hours of batteries you want to buy. Or can afford to buy, or can find room on the boat for.

All simple math and you'll even find energy budgets that other folks have posted showing their typical or actual usage if you keep on reading. That's the energy budget, but "not melting wires" is something totally different, you need to learn how to wire a boat so each circuit can carry the load properly. There are more threads on that.

Of course, sometimes it is simpler to just buy a book, and have the information all presented by one author in an organized fashion.
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Old 09-08-2012, 12:22   #4
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Re: House battery question

Thank you for the reply.
I guess I should have clarified a bit better. My thought was while at anchor not attached to shore power.
The other thing that helps a lot was the phrase that you used that I could not think of. “Energy Budget”

Thanks again
Scott
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Old 09-08-2012, 12:23   #5
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Re: House battery question

Indeed, you may be a newbie (or not), but the question is a good one. You will need to buy a book on this and read it. You will need to make a power budget, then figure out how you will be charging (how often, how inconvenient), and from there decide how much battery capacity to have.

To give you an example -- I have 420 amp/hours of batteries (at 24 volts; equivalent to 840 amp/hours at 12). I charge these with a 6.5kW generator and a large frame alternator on the main engine. I almost never let the batts get below a 50% charge (for the sake of longer life), and since you can't efficiently charge to more than 80% using a generator, I usually have 30% or 126 amp/hours to work with (like 252 amp hours at 12v), when at anchor for longer periods, or on my mooring.

This is about right for my style of life on board, which includes charging multiple laptops, running microwave and coffeemaker and toaster off an inverter, running a DVD player, and when at anchor, running instruments and anchor light all night. It means that I can go 24 hours without charging if the use is not unusually heavy (at anchor with instruments on all night, maybe less than that).

Your own power budget will be different, so you need to work it up. A good book on marine electrics will help you a lot. Good luck.
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Old 09-08-2012, 12:57   #6
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Re: House battery question

Quote:
Originally Posted by WhereAway View Post
Thank you for the reply.
I guess I should have clarified a bit better. My thought was while at anchor not attached to shore power.
The other thing that helps a lot was the phrase that you used that I could not think of. “Energy Budget”

Thanks again
Scott
Energy budgets are annoying to calculate, but are necessary if you want to survive at anchor.
The most accurate way to guage the consumption of an appliance is to measure the current it consumes and multiply this by the time you will use it. Its not always possible to do this, it may be something you are considering
purchasing , then someone from the forum will tell you the consuption of their similar unit, which you can plug into your electrical budget.

Don't forget the small stuff, it adds up, and battery inefficiency means you have to generate more power than you use to break even. (5-10%)

Be a bit wary of using the sticker on the appliance to determine consumption, many of these labels represent the worst case consumption and actual, average, power may be considerably less.
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