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Old 14-08-2012, 09:44   #1
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Amp Hour Confusion

if i have a 250 amp hour battery and i'm drawing a constant 1 amp,how long will my battery last. does one amp draw every minute,hour, i just don't get it.
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Old 14-08-2012, 09:54   #2
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Re: amp hour confusion

Well, theoretically if you are drawing 1 amp continuously from a 250 amphour battery, then your battery will last 250 hours.

But.

Deep cycle batteries are rated on a 20 hour draw basis. So for your 250 amphour battery, that means that it can supply 250/20= 12.5 amps for 20 hours. But if you are only drawing 1 amp, then it will last much longer than above, maybe 350 hours.

To further complicate matters, you should never discharge a deep cycle battery completely. A good compromise for decent life is to stop and recharge at 50% discharge.

Clear!!!!

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Old 14-08-2012, 10:07   #3
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Re: amp hour confusion

amp is a rate, some number of electrons per sec going through the wire.

Amp * Hours is amps times hours, many people confuse this and write amp/hours which means amps divided by hours which is wrong.

Rate times time gives a quantity.

dmarchand's post is dead on.

John
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Old 14-08-2012, 10:24   #4
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Re: amp hour confusion

The amp-hour rating is just for comparison purposes, provided the manufacturer is being honest, which is another issue altogether.
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Old 14-08-2012, 12:34   #5
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Re: amp hour confusion

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Originally Posted by tuberider View Post
if i have a 250 amp hour battery and i'm drawing a constant 1 amp,how long will my battery last. does one amp draw every minute,hour, i just don't get it.

It's like miles per hour.
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Old 14-08-2012, 15:05   #6
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Re: amp hour confusion

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It's like miles per hour.
Not sure what you meant. Amps is like miles per hour. Amp-hours is like miles.
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Old 14-08-2012, 16:00   #7
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Re: amp hour confusion

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if i have a 250 amp hour battery and i'm drawing a constant 1 amp,how long will my battery last. does one amp draw every minute,hour, i just don't get it.
If you take your 250Ah battery and figured it had a Peukert of about 1.25, don't really know so just guessing, you would have 470.8 Ah's at a 1A draw before killing it to 10.5V.

If you are only discharging to 50% then roughly half the 470.8 or 235.4 hours before you hit 50% of your banks capacity.

Your 1A draw equates to 1 Ah for every hour the load is running or 235 hours of use before you hit 50% state of charge from a full state...
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Old 14-08-2012, 16:17   #8
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Re: amp hour confusion

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Not sure what you meant. Amps is like miles per hour. Amp-hours is like miles.
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Old 14-08-2012, 17:48   #9
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Re: amp hour confusion

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Originally Posted by tuberider View Post
if i have a 250 amp hour battery and i'm drawing a constant 1 amp,how long will my battery last. does one amp draw every minute,hour, i just don't get it.
You probably shouldn't read past djmarchand excellent reply as it gets confusing quickly.

To add another simple concept - A battery is a chemical reaction so you can't really compare it to "mph."

There are efficiencies of the reaction to consider. In general it is felt that about "half the a/h capacity" is useable. Batteries don't like to be run flat a lot.

Then there is the rule of thumb on charging efficiency that say the battery generally gets charged up to about 80% of capacity "most of the time"

So - 250 a/h battery should not be run below 125 amps and it generally doesn't get charged past 200 amps (80% of full) so you have about 75 a/h available on a daily basis.

High discharge rates are generally less efficient than slow discharge rates because of heat and other factors in the chemical reaction. So drawing that 75 amps in one hour may mean you don't get that 75 amps. Drawing that 75 amps over 75 hours means you likely will.

Very efficient charging systems and conservative power management can significantly impact these "rules of thumb."
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Old 14-08-2012, 17:58   #10
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Then there is the rule of thumb on charging efficiency that say the battery generally gets charged up to about 80% of capacity "most of the time"
This isnt what charge efficiency is

Dave
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Old 14-08-2012, 18:07   #11
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Re: amp hour confusion

Think of amp-hours as the amount of energy that a battery can store. And as I said, it's for comparison purposes only
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Old 15-08-2012, 17:26   #12
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Re: amp hour confusion

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This isnt what charge efficiency is

Dave
Technically you are correct.

Call it useable charge? The number can be higher with a more efficient and perhaps better charge system.

I don't think it useful to explain charge efficiency to the OP other than to say that it is similar to usage in that batteries can take a charge only so quickly for the same chemical reaction/heat/ etc. reasons.
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