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Help with calculating and understanding

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Posted by Bryan Martin on January 5, 2006, 9:24 pm
The unit I will be using is 12V 0.5A

12V X 0.5A = 6 Watts
6W x 24hours = 144 Watts per day
144W / 12V = 12 amp hours

Now given a battery with 110 amp hours
(http://www.defender.com/product.jsp?path=-1 |328|51495|306219&idQ496)

We should only draw the battery down 50% so we will actually only get

110 / 2 = 55 amp hours

12 amp hours needed / 55 amp hours available = 4.58 days
so with this battery we have enough juice to potentially run our unit for
4.58 days w/out charging.

Now if the above is not correct then please explain.  And the next question
is how would I now figure out approx how large of a panel I would need to
keep the battery charged.  I am really looking for the formula and a
understanding of how you got from A to B as oppose to "this panel should

Poor planning on your part does not constitute an emergency on my part.

Posted by =?UTF-8?B?4oCTIENvbG9uZWwg4oCT? on January 5, 2006, 10:25 pm

Wrong. 6W x 24hours = 144 Watt-HOURS per day.

Watts are a unit of power. A watt-hour (like a kilowatt-hour) is a unit
of energy.

Hope this helps.

Posted by RP on January 5, 2006, 11:20 pm

Colonel wrote:

That should be 6W x 24hr  = 144 watt-hours.


Posted by =?UTF-8?B?4oCTIENvbG9uZWwg4oCT? on January 5, 2006, 10:35 pm

This is really more a theoretical or academic answer than a practical
one but here goes:

Assuming the battery is 12V (which it's not) and STAYS at 12V (which it
won't), all you need to know is the draw (0.5A @ 12V) and the capacity
of the battery (110AH).

Use your algebra and cross out the units.

110AH/0.5A = 220 hours to completely kill the battery
110 hrs to kill the battery by 50%.

Of course, batteries don't discharge in a linear fashion, and ... and... and...

Posted by hallerb on January 6, 2006, 2:32 am
 allow lots of money for battery replacements.

better to upsize battery bank and discharge less.

even deep cycle batteries have tough tme with frequent deep discharges

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