Connecting Converter and Battery - Page 2 - Fiberglass RV


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Old 06-01-2012, 08:55 AM   #15
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Originally Posted by Roy in TO

OK, maybe I'm dumb. I read the specs for his fridge and got the avg amps per hour.

If I've got a battery rated at 100 amp hours and I don't want it to go below 50%, my understanding is that I can use 50 amp hours. Given that his fridge consumes about 1 amp per hour. Should I not be able to run his fridge by itself for about 2 days before I need to recharge the battery?
Roy, you are not dumb, you get an A- though, instead of an A. As Thomas explained, you need to take the Peukert exponent of the battery into account to get the exact duration under a particular load but your method gives a really good guess under "moderate" loads.

The amp-hour capacity rating is usually the 20 hour rate. That means your 100 ah battery can put out 5 amps for 20 hours until it is functionally dead (usually 1.75 volts per cell or 10.5 volts for a 12 v battery). Note this means that if the actual load is less than 5 amps you get a little more than the rated amp-hours.

The comment about 'watts' being being the correct unit is true but not relevant here as pretty much everyone here uses a 12v battery system.
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Old 06-01-2012, 10:01 AM   #16
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*snort* "English" units. "SI" units. "Common-Usage" units. "Units that someone cooked up in a marketing department" units. Trouble is, as far as I can see, is that there are a ton of semi-convertible measures out there that can give all sorts of results depending on how you squint.

As a mere computer geek and a geologist, I now bow out. If my dear old Dad was still alive and kicking (Bell Labs engineer for 50 or so years) I'd drag him in. But as the long distance charges to his current abode are rather steep, I'll just let it lie.
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Old 06-01-2012, 10:32 AM   #17
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*snort* "English" units. "SI" units. "Common-Usage" units. "Units that someone cooked up in a marketing department" units. Trouble is, as far as I can see, is that there are a ton of semi-convertible measures out there that can give all sorts of results depending on how you squint.

As a mere computer geek and a geologist, I now bow out. If my dear old Dad was still alive and kicking (Bell Labs engineer for 50 or so years) I'd drag him in. But as the long distance charges to his current abode are rather steep, I'll just let it lie.

We owe a lot to Bell Labs, originator of the transistor among many other things. With batteries, amps x volts = watts doesn't tell the whole story as the internal losses of the battery (heat) are greater at high current draws than at low current draws. So, it is not a question of units, but rather the fact that battery capacity is not a linear function.
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Old 06-01-2012, 10:41 AM   #18
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<snip> So, it is not a question of units, but rather the fact that battery capacity is not a linear function.
Absolutely! And that's the crux of the problem. I call these "bubble in the waterbed" issues. You push down the bubble here...and it pops up there.

And of course we have to add the variance that ambient temperature has on the battery. And which way you hold you head when you look at the trailer!

If only life were simple ! "Your Mileage May Vary" is oh so true!
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Old 06-01-2012, 10:45 AM   #19
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Wow, thanks for all the info guys. I'm having to go back and re read some of the posts multiple times because they look like gibberish to me! I thinkim getting the gist of it though.

Another quick question, looking at the Wire Awg Chart
American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies
I should be able to use 10awg wire? As its rated up to 55amps?

Again, thanks for all your help. 12v wiring almost seems harder than 110v!
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Old 06-01-2012, 11:02 AM   #20
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Wow, thanks for all the info guys. I'm having to go back and re read some of the posts multiple times because they look like gibberish to me! I think I'm getting the gist of it though.

Another quick question, looking at the Wire Awg Chart
American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies
I should be able to use 10awg wire? As its rated up to 55amps?

Again, thanks for all your help. 12v wiring almost seems harder than 110v!
10 gauge wire is OK between the controller, the battery and the fridge. If you use 10 gauge wire to run a circuit to charge your battery and run your fridge off the tow vehicle, it may be a little light. This is where it gets complicated. The voltage drop in the wire from the tow vehicle to the trailer battery is critical because if your trailer battery doesn't get enough voltage (14+ volts), it won't charge adequately.

The author of this article below is a little over the top, but he hits the key points.

Charging Trailer Batteries from the Tow Vehicle Alternator
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