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Old 10-19-2010, 08:07 PM   #1
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1993 36' Land Yacht
jackson , Tennessee
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amp hours

excuse my idiocy, but this newbie read that the batteries on my newly acquired 93 ly will power it for 210 amp hours. what does that really mean? can i run my heater overnight tonight and still expect it to start in the morning. if not i can run the generator. but that just seems like electronic overkill.

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Old 10-19-2010, 08:19 PM   #2
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How about some simple math...ya don't want to run them down more than 1/2 way or ya reduce the lifespan so your 210 is really about 100 amp hours. The heater will pull about 7 amps so if ya run it for 10 hours ya just used 7 x 10 = 70 amp hours (just for the heater...not counting the night use of lights, water pump when ya flush...) so ya used 70% of the battery.

Geez, we really gotta watch electrical usage!

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A honkin' long 34' named AlumaTherapy
and a 26' '63 Overlander, Dolly
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Old 10-19-2010, 08:22 PM   #3
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One amp hour will run something that uses one amp, for an hour.

The heater draws maybe 8 or 9 amps. So, if the batteries are completely charged, you can run it for, say, 24 hours or so before the batteries are completely dead. Most people don't routinely run them down more than halfway, because it shortens their useful life.
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Old 10-19-2010, 08:25 PM   #4
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thnx hiho. as a radio and tv talk show guy i live in the creative world. to me math is as inaccessible as ancient aramaic. so i guess i run the onan just to be safe, right?.
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Old 10-19-2010, 08:27 PM   #5
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Amp hour ratings on the common deep cycle type batteries used in RV's are based on a 20 hour rating...and can be used when comparing battery types for your specific use, etc...

One 'amp hour' is drawing (or charging) one amp, for one hour...charge your batteries at a 'true' 10 amps for one hour, and you've recharged '10 amp hours' of the battery's capacity, in theory, at least...

The 20 hour rating on many Group 27 type deep cycle batteries is about 105 AHrs...divide 105 by 20 = 5.25, which means, in effect, all things being 'normal', you can draw 5.25 Amps from the battery, and it will be 'usable' for a total of 20 hours - draw less and it will 'last' longer, draw more and it will 'last' a shorter time period...

This 5.25 amp draw (x 20) 'number' isn't really of much practical use, because the battery would be depleted to such a low level that it would take a long time to be recharged - also the 'deeper' you discharge your batteries each 'cycle', the less they will 'live' in years of service - many Mfg's suggest to only discharge your batteries to a 50% level during each use 'cycle'...

Now, more to your question...if you knew the 'real' amperage 'load' of the furnace's blower motor, you could, in theory predict the amount of time your batteries would be able to run it, using the Amp Hr rating, etc...

In truth, however, there are many variables in the 12 volt system - are the batteries at full charge?, what resistance loss is inherent in the wiring between the batteries and the furnace motor?, What is the temperature of you batteries (the colder they are, the fewer amp hrs they will provide)? How old are the batteries? - older batteries have diminished capacity due to the build up of sulfate on the internal plates...etc, etc...

I've found that only your particular situation and components will govern how long you can run your 12 volt stuff - a regular recharging of the batteries, the next day after an overnight use will help insure you have enough energy to run your stuff later -

A fully charged battery is a happy battery (and a happy RV user, too!)....
Ray & Pat; Morada, CA
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Old 10-19-2010, 08:32 PM   #6
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or i guess i cud just start it up and drive it 30 feet down to my car enclosure and plug it in. i just had that building rewired to a 30 amp system. think that will do it?
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Old 10-19-2010, 08:35 PM   #7
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jeez folks are so smart i feel like an idiot. you have compleely proved to me that AS owners are the smartest people on earth tnx.
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Old 10-19-2010, 08:38 PM   #8
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mex...i printed your post out at 16x20 and taped it to the front door. now i guess i can head for palm springs tomorrow.
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Old 10-19-2010, 08:40 PM   #9
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i'm going to do the radio show by iphone and tie line. believe can make a very good living on the road.
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Old 10-19-2010, 08:44 PM   #10
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Hi bukib. You have a motorhome, correct? It has two batteries one to run the coach and another to run the chassis. There is an isolator relay that only connects the two batteries while the engine is running. this allows the engine alternator to recharge both batteries. Once you shut down the engine the relay opens and isolates the coach from the chassis. If you did manage to drain the coach battery while using the furnace you would still have a fully charged chassis battery which would allow you to start the engine.
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Old 10-19-2010, 09:07 PM   #11
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1993 36' Land Yacht
jackson , Tennessee
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whoa i am way confident. it is a motorhome. it does have the dual battery system. with a projected low temp tonight at 52 degrees i will not suffer hypothermia. tnx. my wife just said if you ever need some pickled beets just email us.
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Old 10-19-2010, 09:29 PM   #12
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As Mexray said, amp hours is useful for comparing similar batteries.

but not very useful.

People get very confused when comparing amp hours of batteries with different voltages, for instance. Amp hours also suffer in that the energy in a battery depends upon how fast you discharge it and what the temperature is, how old the battery is, and some other factors. Trying to engineer battery capacity calculations within 20% is maybe fun math for some but rather meaningless.

One way to make sense is to use a proper energy unit, watt hours, like are used to buy your household electricity. Watt hours is simply amp hours times battery voltage.

For those batteries you commonly use in RV's, though, you can figure you have about 10 to 15 usable watt hours per pound of battery. Use more than that on a regular basis and your batteries won't last as long.
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Old 10-19-2010, 10:15 PM   #13
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An important thing to remember in this particular case is that your heater isn't going to be running full time. If it's only supposed to be down around 50F, and you don't leave the thermostat at 75F all night, the furnace might only actually run an hour or so in 10 hours.
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Old 10-21-2010, 07:43 AM   #14
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bundle up run heater in morning while drinking coffe i dont trust no propano senior

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