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Old 02-23-2016, 03:10 PM   #1
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Electricity Question

I am trying to understand marine battery ratings. Everything in theory, not counting start surge, reserve etc.

let's say I have an appliance rated as 1000w, I run it 1/2 of the time overnight(12 hours), so that's 6 hours continuously.
Let's suppose the battery is 12V:
1000w / 12v x 6hr = 500Amp Hour
so I need a 12V 500AH battery?

Again this is not a real case, just trying to understand the concept.

Is my calculation correct?
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Old 02-23-2016, 03:24 PM   #2
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No you need a 1000 amphour battery. Running deep cycle batteries much below 50% kills 'em quick. Get yourself a copy of Nigel Calder's "Boat Owner's Mechanical and Electrical Manual" and have all these questions and more answered for you. An essential tool for every cruising boat.
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Old 02-23-2016, 03:28 PM   #3
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if you're curious about your devices and any wattage used by them, get a Killawatt device and plug it into the wall, then plug the device into the back and let it run for a day or two.

Then it will tell you the power consumption of the device.

You didn't specify the voltage of the device. that matters.

You are mixing amps and watts which are different.
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Old 02-23-2016, 03:44 PM   #4
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Your math is not working for me.

On a 1000 amp hour battery it would be dead if you drew...

1000 amps for an hour
100 amps for 10 hours
50 amps for 20 hours
1 amp for 1000 hours

Keep in mind that deep cycle batteries are generally rated for a 20 hour rate. They will not be 100% linear if you deviate from the 20 hour rate.

Also remember that lead antimony or lead calcium batteries will have a shortened lifespan if you discharge them regularly above a certain percentage. Most generally agree that 50% is a good compromise between capacity and longevity.
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Old 02-23-2016, 04:05 PM   #5
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OK, Thank you guys. If I understand you guys correctly, when I go from pure theory to real life case, I need 50% reserve to not discharge the battery more than 50%. Even for a "true" deep cycle battery? In that case I wonder what's the point of rating that battery at 1000AH? Might as well just call it 500AH, Convoluted ...
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Old 02-23-2016, 04:12 PM   #6
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What Kevin said plus as batteries age their capacity reduces. last years 1000 AH battery may be only 900 today. The rating also assumes it is fully charged. That may not always be the case.


Appliances are usually rated at their maximum power drain but there is duty cycle to consider. A refrigerator for example does not run constantly once cold. It may use 15A for ten minutes then very little for a long period.
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Old 02-23-2016, 04:17 PM   #7
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Chicago, one issue is that a device rated at 1000w would use 1000w of power if run at its max capacity for an hour. So you are right, using Ohms law if your device ran at its max capacity for 6 hours it would use the 500 amps you came up with, and as others mentioned, you should have a 1000 amp hr battery bank to support that load before charging.

Most devices on our boat don't run at their max capacity for hours on end. I am thinking of refrigeration for example.
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Old 02-23-2016, 05:53 PM   #8
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Quote:
Originally Posted by chicagoq View Post
OK, Thank you guys. If I understand you guys correctly, when I go from pure theory to real life case, I need 50% reserve to not discharge the battery more than 50%. Even for a "true" deep cycle battery? In that case I wonder what's the point of rating that battery at 1000AH? Might as well just call it 500AH, Convoluted ...
Oh it will run down that much if new and accurately rated. It just won't live very long if it does that a few times. That's why cruisers install modern battery monitors and cut offs.
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Old 02-23-2016, 06:22 PM   #9
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What most people are missing is they are mixing up AMPS and Watts.

Volts * Amps = Watts

And a 1000 watt device at 120 vac will be far different than a 1000 watt device on 12 vdc
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Old 02-23-2016, 06:28 PM   #10
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Quote:
Originally Posted by stubones99 View Post
What most people are missing is they are mixing up AMPS and Watts.

Volts * Amps = Watts

And a 1000 watt device at 120 vac will be far different than a 1000 watt device on 12 vdc


There are two ways to work with electricity:

1) Understand it and figure things out yourself (few people can do this).

2) Do what manufacturers and the people who understand it do and don't think about it.
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Old 02-23-2016, 08:25 PM   #11
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Originally Posted by WesK View Post


There are two ways to work with electricity:

1) Understand it and figure things out yourself (few people can do this).

2) Do what manufacturers and the people who understand it do and don't think about it.
or Hire an electrician?
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Old 02-23-2016, 08:34 PM   #12
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I believe he came in with #1.

He calculated the amp hours correctly.

More to learn though as you have seen about the 50% derating for longevity.
DEEP CYCLE BATTERY FREQUENTLY ASKED QUESTIONS
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Old 02-23-2016, 08:54 PM   #13
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Yikes, watts and amp hours mixed!

So if you have an appliance that draws 1000watts, first, you have to know the voltage. Is it a 12v DC appliance or is it a 120 volt AC appliance? It matters. If its a 12v appliance, it will draw approximately 83 amps from the battery bank(which is a lot from almost any battery bank if it's for a sustained period of time) You can do the math, but a 1000amp hour battery bank could only sustain this kind of draw for about 6 hours before being depleted about 1/2 way which as has been mentioned is about as far as you should regularly discharge deep cycle batteries ( and less is always better for battery life)

If its a 120VAC appliance it will need to be connected to a device called an Inverter. An inverter is a device that converts DC voltage to AC. Inverters are at best about 90% efficient, so it would actually draw more and that would have to be factored in.

Also, as has been mentioned, batteries are normally rated at what is called a "20 hour rate" meaning if a given battery is a 200 amp hour battery, it is rated to be able to deliver its 200 amp hours over a period of 20 hours. That means 10 amps for 20 hours. If one draws power more quickly from the battery it should normally be expected to provide less than at the rated rate and this would be from a perfect, brand new battery. Each time a battery is cycled it permanently loses some of its capacity. Deep cycle batteries lose the least and the less the battery is drawn down before it is recharged reduces this as well. Battery manufacturers publish graphs that show their battery's performance at different discharge rates and different temps.

After all of this, you would want a way to recharge that bank within a reasonable period of time. Depending on the battery chemistry chosen, you would want at least a 100 amp battery charger for a 1000AH battery bank, but much bigger would be even better. As big as it sounds, even a 100 amp charger could take 8-9 hours to replace that 500AH of drawdown.

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Old 02-24-2016, 06:01 AM   #14
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ummmm....

Quote:
Originally Posted by kchace View Post
Yikes, watts and amp hours mixed!

So if you have an appliance that draws 1000watts, first, you have to know the voltage. Is it a 12v DC appliance or is it a 120 volt AC appliance? It matters.
Quote:
Originally Posted by chicagoq View Post
I am trying to understand marine battery ratings. Everything in theory, not counting start surge, reserve etc.

let's say I have an appliance rated as 1000w, I run it 1/2 of the time overnight(12 hours), so that's 6 hours continuously.
Let's suppose the battery is 12V:
1000w / 12v x 6hr = 500Amp Hour
so I need a 12V 500AH battery?


Again this is not a real case, just trying to understand the concept.

Is my calculation correct?
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Old 02-24-2016, 06:34 AM   #15
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"That means 10 amps for 20 hours. If one draws power more quickly from the battery it should normally be expected to provide less than at the rated rate and this would be from a perfect, brand new battery."

This statement is correct so a theoretical battery might need to be 1000ah , but in practice it will need to be larger if the discharge RATE is high.

Usually a discharge at C 10 (battery 20 hour rate capacity) divided by 10% is fine long term.

Also for batt figuring , remember the recharge will require a long time to get back to 100% as the last 15% goes in very slowly.

And with internal losses perhaps 140% of the amps used will need to be generated.
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Old 02-24-2016, 09:02 AM   #16
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Yes, Mr. Peukert raises his ugly head to complicate matters....
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Old 02-24-2016, 09:05 AM   #17
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So you've seen from the above posts that there are various reasons why a 500AH battery house battery bank is not adequate to provide for 500AH of usage between charges.

1. You don't want to discharge below 50%, to avoid battery damage and reduced longevity.

2. Recharging the last 15% or so at the top will happen slowly, and in regular cruising use might often not happen. (BTW, this could lead to shorter life for certain types of batteries, like AGM's)

3. Older batteries don't have the same capacity as new ones.

4. Using a 120V device through an inverter loses 10-15% in inefficiencies, so the battery drawdown may be more than you would have calculated.


The typical recommendation is that you need a house battery bank with capacity of 3X or 4X as many AH as your electrical devices draw between recharges.

If you're setting up a new house electrical system, upgrading an existing one, or even just replacing old batteries, it's a good idea to spend some time estimating your total AH use between recharges. Since this is typically an iterative process to get it reasonably correct, a spreadsheet program like Excel is a very handy tool to help you do this.
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Old 02-24-2016, 09:33 AM   #18
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Our house battery bank is in my opinion about the right size for our boat and our usage.

We have a 830 AH 12 volt bank and a 150 amp charger.

We have to run the generator 2-3 times a day to keep the house bank between 50% and 100%

Since we have an electric stove, and watermaker, and clothes washer, the generator is needed for other reasons beside recharging. So, we run the generator to do other stuff, and it keeps the batteries up.
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Old 02-24-2016, 11:52 AM   #19
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We have to run the generator 2-3 times a day to keep the house bank between 50% and 100%...Since we have an electric stove, and watermaker, and clothes washer.
Again, I know it depends but, with your cruising habits including lots of hook time, 2-3 times a day for how long if you are gone a week or more?
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Old 02-24-2016, 12:06 PM   #20
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Again, I know it depends but, with your cruising habits including lots of hook time, 2-3 times a day for how long if you are gone a week or more?
2-3 hours each time

A load of laundry takes 90 minutes. With two people we need to do two loads a day to keep up.
We need to run the water maker 2-3 hours a day
And make hot water for showers, etc ...

Our boat is a power hog. We draw around 40 amps DC much of the time.
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