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Old 06-09-2011, 01:07 PM   #1
2 Rivet Member
Evergreen , Colorado
Join Date: Jul 2010
Posts: 68
Great idea for testing batteries

Well - this might be obvious, but this seems like a great way to measure amp hours on batteries. I'm sure the gurus here can weigh in on the pros/cons of this method but it seems to be a good way to get a rough idea of the realistic amp hours available in your battery setup.

Size the Load to the Battery

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Old 06-09-2011, 01:21 PM   #2
Rivet Master
2005 30' Classic
Burlington , Ontario
Join Date: Sep 2008
Posts: 2,743
I'm certainly no electrical guru, but I think if I were to do this I'd want to include an ammeter in the load circuit to know how much current is really flowing.

Likely also, the ammeter reading will drop over time as battery voltage drops, so I think you'd need to take this into account if teh drop is significant.

To rely on the rating of a 60 watt bulb as being accurate is something I'm not sure about, and the other thing is that the inverter that is being used in the circuit also will consume energy due to its inefficiency - that is why it has the heat sink fins!

Just my 2 milliamps worth!


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Old 06-09-2011, 04:00 PM   #3
Rivet Master
1975 29' Ambassador
Reno , Nevada
Join Date: May 2003
Posts: 1,351
That article has a number of problems, some implied, some factual.

the basic idea is sound

but the need for such a test is questionable

and it could be harmful to the battery if appropriate care was not taken

Your RV battery bank has about 12 usable watt hours per pound of battery. The 'official' end of life point for the battery is at 80% or a bit under 10 usable watt hours per pound. Most RVers don't notice the battery being deficient until it gets well below that.

If you have a good converter, one that does multiple stage charging and has a maintenance mode that implements a sulfation inhibiting technique, then your outings will at least start out with the batteries with a full and complete charge. With experience, you will be able to gain a very good idea of the battery condition by how it fades on your outings.

Check the SmartGauge Electronics - battery metering tech notes and also look at the description about how that device works. It is a much more refined means to obtain battery condition using the same idea as the above link.

Also keep in mind that any battery capacity measure with a precision of greater than 10% to 20% is exceeding the accuracy of the phenomena involved.

As Brian notes, the 20 hour rating for battery energy used a load of about 1 watt per pound of battery. Larger loads than this will yield less capacity, smaller loads more. See the Smartgauge tech pages on Peukert to learn more about this phenomena.
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