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Battery Power Feedback

Submitted: Monday, Oct 01, 2007 at 23:38

Frankp

Article Overview - Battery Power
In this article we explain why you might need an additional power source independent of the starting battery installed into your vehicle and we look in detail at types of batteries, how and where to mount them, the role of isolators and the different View Full Article...

Hi folks,
In the "Device Usage" paragraph, the reference to "amp-hours" as units of current draw is incorrect. Electrical current is measured in amps. Amp-hours is a measure of battery capacity, not current drawn by a device. To use your examples, a 12V fridge does not draw 2.5 to 3.0 amp-hours. When running it draws 2.5 to 3 amps. If it runs continuously for one hour then it uses 2.5 to 3.0 amp-hours of battery capacity. If it runs continuously for two hours then it would use 5.0 to 6.0 amp-hours of battery capacity. And so on for the other examples.

The mis-use of units is confusing in an otherwise excellent page.

Cheers

Frank

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ThreadID: 50181	Replies: 3
Views: 916	FollowUps: 5
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Thread Summary

AnswerID: 264651 Submitted: Tuesday, Oct 02, 2007 at 06:53

hl replied:

Hi,
It is quite correct to state the power consumption in Ah. A fridge (let's say a WAECO) will actually "draw" about 4 amps while it is running, however, it will USE about 2 Ah per hour because it will cycle on and off. So, over a 24 hour period, it will USE 48 Ah. The real confusion will arise when one attempts to use the battery capacity as stated. A 48 Ah battery may be rated at 48Ah if it is discharged at a steady 1 amp. However, if the current draw is much higher, say 5 amps, then the capacity available may only be 35 Ah.
So, for real life applications it is wise to discount the stated capacity by at least 20%.

Reply 1 of 3

FollowupID: 526438 Submitted: Tuesday, Oct 02, 2007 at 08:03

Robin Miller posted:

Hi HL

Yes good idea, I like to include not discharging my battery to far as well and so as a rule of thumb I consider the useful power of my batteries as only 1/2 their rated.

So as I only run a single 48AH battery this equals only 24AH I count on for general camping purposes.

Robin Miller

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Robin Miller

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FollowUp 1 of 4

FollowupID: 526609 Submitted: Wednesday, Oct 03, 2007 at 00:52

V8 Troopie posted:

Quote "it will USE about 2 Ah per hour "

is like saying your car can do 100km per hour per hour.

Ah stands for 'Amp hours', it already has a time factor so it really makes no sense to quote it over time again.

I too noticed the misuse of Ah creeping into these pages, as an electrician it makes me cringe each time I see it used incorrectly.

Its simple, quote battery capacity in Ah, quote current drawn by items connected to it in Amps.

If you make a list of how long each item is connected to the battery, to get an idea what battery capacity is required, then you multiply each individual current drawn by the time its switched on, to get the total Ah the battery has to supply.
Keep im mind that one needs to put back MORE Ah to get the battery back to the stage it was before as there are losses occuring during the charge/ discharge process.
Klaus

FollowUp 2 of 4

FollowupID: 526695 Submitted: Wednesday, Oct 03, 2007 at 15:37

hl posted:

Only put the "Hour" there in case you missed the little "h"
cheers

FollowUp 3 of 4

FollowupID: 527500 Submitted: Monday, Oct 08, 2007 at 11:16

Mainey (WA) posted:

If a 100ah battery is 1/2 discharged is it then 50ah ??

Yes, it is in "numerically" because 50 is half of 100, my maths teacher told me that.

But - not in the amount of "time" it will run a 'fridge'
as the initial discharge "time" is very slow when compared to the available "time" at the end of the discharge cycle, which is much less, due to the lack of power available to draw from.

Another good reason to have a large 'capacity' storage battery system.

FollowUp 4 of 4

AnswerID: 264662 Submitted: Tuesday, Oct 02, 2007 at 07:55

Robin Miller replied:

Hi Frank

Maybe its been changed but it reads ok to me ?

It says "Typical amp-hour usage rate is " - and then goes on to state a figure in the same units (amp-hours).

This is a nice easy to understand way to talk about such things.

Robin Miller

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Robin Miller

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Reply 2 of 3

AnswerID: 264679 Submitted: Tuesday, Oct 02, 2007 at 10:12

Sand Man (SA) replied:

Frank,

You would be better to select About Us - Feedback and enter these details. The ExplorOz staff can then view and correct the error more effectively.

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HooRoo !

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Reply 3 of 3

FollowupID: 526466 Submitted: Tuesday, Oct 02, 2007 at 11:10

ExplorOz Team - David posted:

Sand Man,

Article feedback is actually the best method in these cases as there is an option to send only to ExplorOz however we reckon it is better to post it to the forum also so that a group response can be seen from the forum users when we look at the issue.

We also get copies in email of all article feedbacks. So in a fashion it works the same as Feedback but is targeted directly at the article.

You will be able to tell if a post was started from article feedback by the title in this case Battery Power Feedback, there will also be a link to the article.