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Old 10-08-2016, 03:38 PM   #1
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Brand: Thor Motor Coach
Model: Hurricane 34J
State: Arkansas
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THOR #5647
Smile House battery life running furnace?

Hi, I'm new to the forum but have found it very useful whenever I have a question. I have a 2016 Thor Axis 24.1. How long can I expect the house batteries to run the furnace between generator charges? I like to camp in the winter and don't want to run the batteries down so that my generator will not start. Thanks for any help.
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Old 10-08-2016, 03:57 PM   #2
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If you have Harris batteries, and you probably do, I would not expect them to last long. I'm getting ready to change out my batteries for two deep cycle marine batteries and my coach is only a couple of months old. IMHO the Harris batteries aren't worth the lead they're made of.
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Old 10-08-2016, 04:11 PM   #3
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Aside from Harris batteries - the answer is of course 'it depends'...
How cold directly affects how much the furnace will be running.

These are not insulated like houses. Haven't done extreme cold weather - but has been in low 30's - frost a few times / snow on one occasion).
Burned through a LOT of propane rather quickly... so keep that in mind too.

I'd suggest monitoring the battery voltage in your usage conditions to get a better feeling of how long you can go...
And while you don't want to overly discharge the coach batteries - if you do mis-calculate - the 'Emergency Start' switch is a 2 way street... It will start the engine from the coach batteries - and - it will start the genny from the chassis battery.

Depending on how cold - also know what water lines might not be in heated space... That varies by rig - some heat a storage bay that contains plumbing... others are fully inside the RV... At least part of the drain system is likely exposed to outside temps.
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Old 10-09-2016, 07:13 AM   #4
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With stock Harris batteries, I ran down to 11.1 by 4:00am. The furnace would not ignite anymore. The blower does take a lot of power. Temp was only 40 outside so I was not a constant run.

Switched to two good deep cycles and still had 11.6 in the morning and everything is running great and I powered up the generator at 9:30am.
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Old 10-09-2016, 02:25 PM   #5
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Originally Posted by gauthig View Post
With stock Harris batteries, I ran down to 11.1 by 4:00am. The furnace would not ignite anymore. The blower does take a lot of power. Temp was only 40 outside so I was not a constant run.

Switched to two good deep cycles and still had 11.6 in the morning and everything is running great and I powered up the generator at 9:30am.
Note that according to "The 12v side of life", 11.58v is only 20% capacity left. For best battery life, you shouldn't exceed 50% discharge. That is 12.06v.

Google "the 12 volt side of life" for more info.
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Old 10-09-2016, 04:51 PM   #6
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I haven't tried it in my current RV but in my last I had two 6v batteries and went 2 days in below freezing temps (day and night) without a complete discharge in a 33' class A. If you are replacing batteries 2 6v deep cycles is your best bet.

Havnt had a chance to try the new RV but I'm guessing its much worse. I think in spring I will be changing them out.
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Old 10-09-2016, 05:06 PM   #7
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You have to remember that batteries are a storage medium just like your gas tank. Bigger and better batteries are more "energy dense" and store more power. Switch out the "junk" batteries for good deep cycle ones and you will get a longer run time for your 12v system. If you decide to buy batteries be sure to get "deep cycle" batteries which are different from starting batteries. Starting batteries provide lots of power for a short time (starting the engine), deep cycle batteries provide lower power over a longer period of time, they are more "energy dense."
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Old 10-13-2016, 03:19 PM   #8
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Originally Posted by tenbear View Post
Note that according to "The 12v side of life", 11.58v is only 20% capacity left. For best battery life, you shouldn't exceed 50% discharge. That is 12.06v.

Google "the 12 volt side of life" for more info.
A note here about the 12 volt side of life SOC chart it is incorrect and gives lower values then most battery manufactures list for their batteries.

Also measured our heater current and it draws 4 amps. Didn't measure the ignition or cool down cycles.
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Old 10-19-2016, 05:34 PM   #9
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Hey gauthig, what deep cycle batteries did you replace the Harris batteries with?
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Old 10-19-2016, 06:45 PM   #10
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I bought two Trojan SCS200 12 Volts.

I went back and fourth on the 6V vs 12v debate. But the cost was right and use for what I need. I do not do more than a 2 days completely unplugged. If I did spend more time off the grid I probably would have gone with 6V larger group size batteries.
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Old 10-19-2016, 08:04 PM   #11
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Here is a chart from the Trojan website:

Percentage of Charge Specific Gravity Corrected To Open-Circuit Voltage
6v 8v 12v 24v 36v 48v
100 1.277 6.37 8.49 12.73 25.46 38.20 50.93
90 1.258 6.31 8.41 12.62 25.24 37.85 50.47
80 1.238 6.25 8.33 12.50 25.00 37.49 49.99
70 1.217 6.19 8.25 12.37 24.74 37.12 49.49
60 1.195 6.12 8.16 12.27 24.48 36.72 48.96
50 1.172 6.02 8.07 12.10 24.20 36.31 48.41
40 1.148 5.98 7.97 11.89 23.92 35.87 47.83
30 1.124 5.91 7.88 11.81 23.63 35.44 47.26
20 1.098 5.83 7.77 11.66 23.32 34.97 46.63
10 1.073 5.75 7.67 11.51 23.02 34.52 46.03

I thought I would include > 12v as a demonstration to show it is not the voltage that matters i.e, 6 vs 12v since lead acid batteries are combinations of 2v cells.
So with a good brand deep cycle battery wether you get a 6v or 12v doesn't matter. They are the same basic battery one with 3 cells the other with 6 cells.

Trojan makes a very nice size 150 Ah, 20 h rating in 12v. Three of these and you have 450 Ah. They make a very nice 360 Ah 6v. Two of these and you have 360Ah. The chemistry is the same. The battery, Trojan, construction is the same. There is no difference between 2 6v and one 12v in a well regarded name brand battery. The weight of the battery in pounds for Ah produced at 12v is the same.

If you don't see the 20 Hour Ah rating but something else such as CCA it is not a deep cycle battery. There are combo batteries like the Harris which are a little sturdier, heavy duty, than a starting battery but not the same construction as a deep cycle. Starting batteries, combo batteries are designed to give a rapid discharge of high amperage current. Deep cycle batteries are designed to give up a slow long discharge of low amperage current irregardless of the voltage. The chemical engineer designs the battery to control the chemical reaction that he/she is seeking.

The determination of which deep cycle is best for you is determined by what your needs are and the space that is available to you. Then you have to be sure you have a charging system that is capable of maintaining these batteries.

BTW I seem to remember that I once calculated the Harris battery to be about 65Ah or so. So a 50% discharge limit is "ideally" 30 amps at 100% efficiency, which doesn't happen. You realistically probably have about 20 to 24 amps to work with for each Harris battery. Which is what most of you seem to be seeing, some less. As the battery ages, sulfonates and not properly maintained it slowly says goodby, adios.

I hope this helps, it's a combination of chemistry, chem engineering, physical space available, amps required per unit time (cycle), judgement and personnel preference.
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