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Old 03-18-2013, 11:16 PM   #1
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Replacement Batteries

I need to replace my batteries this spring. I built a carrier to hold two 27 sieries batteries. I can get either two 6 volt batteries or two 12 volt batteries. When I use the batteries I need a maximum of 25 amps for about 3 hours then it drops down to around 4 amps for the next two hours.

My question is which set of batteries will hold a charge (as close to 12 volts as possible) the longest?

Thanks,
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Old 03-18-2013, 11:57 PM   #2
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Dennis,
This is covered at length on various internet sites. One that comes to mind is the Trojan Battery Company site. The basic rule of thumb for RV's is that you want the maximum amp/hour batteries that you can fit into the available space. Be sure that you do not get something too tall for the space, since you need room for cables, etc. on top.

Getting to know 12V batteries is a complicated task, but when you finally understand them, you will never be without power when you need it.

Luke
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Old 03-19-2013, 12:02 AM   #3
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Battery manufacturers state that 2 -6 volt batteries is the most economical to use, both for longevity and cost.
Not sure how a group 27 battery corresponds to a couple GC-2 golf cart batteries for size but they have 232 amp hours at 20 amps draw. OP only needs 83 amp hours and that's well within the 116 amp hours that's available (50% draw down).
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Old 03-19-2013, 07:15 AM   #4
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I am a novice when it comes to battery usage. I was only concerned about running the lights and furnace blower over night. Now I am thinking of adding an inverter for the TV.

Just curious - why do you need 25 amps for 3 hours? That seems like a lot. Maybe there is more to what the batteries can do than I am thinking about.

Thanks
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Old 03-19-2013, 09:14 AM   #5
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Originally Posted by tuffr2 View Post
Just curious - why do you need 25 amps for 3 hours? That seems like a lot.
How many 12-volt amps are required to push an inverter that will run a 13,500 BTU AC?
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Old 03-19-2013, 09:45 AM   #6
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How many 12-volt amps are required to push an inverter that will run a 13,500 BTU AC?
A 13500btu AC draws about 2700w on start and about 1250w on run. 1250w @ 120v = 10.4A. PF not in equation.

The same power is required @ any voltage and there will be the power factor loss associated with the inverter so think about 125A constant draw on your 12v batteries with a surge of about 260A on start.

You need some very heavy battery capability to run a 13500BTU AC on a 12v supply, plus you will have current draw for other things.

Good luck with that option. I presume you factored in the Inverter requirement into your equation. I suspect you will need pure sine wave also
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Old 03-19-2013, 10:19 AM   #7
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Hummm - I never thought about running a 13,500 btu A/C unit with an inverter and battery power. I was conditioned to think you gotta have shore power or a genny. I was hoping maybe to run an 800watt micro wave for 3 to 5 minutes and a TV.

Are you guys sure you can run an A/C unit with just a two battery set-up or are you guys pulling my leg?

And is this what the OP is trying to do?
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Old 03-19-2013, 11:03 AM   #8
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Hummm - I never thought about running a 13,500 btu A/C unit with an inverter and battery power. I was conditioned to think you gotta have shore power or a genny. I was hoping maybe to run an 800watt micro wave for 3 to 5 minutes and a TV.

Are you guys sure you can run an A/C unit with just a two battery set-up or are you guys pulling my leg?

And is this what the OP is trying to do?
op said nothing about AC unit. The questuon was about 12 or 6v batteries.

there was another question about running a 13500Btu AC on Battery.

To find current draw add up power consumed and divide by the voltage.


In a pure resistive load that works but in loads with inductive and capacitive reactance, such as a in an AC motor there is a phase relationship so pure resistance formula differs slightly from the actual measured current but you can use power divided by voltage can be used for general applications.


You will see that for the same power consumed current might be 10A @ 120V it is 120A @ 12V for a pure resistive load.

Reactance components are not associated with DC circuits. The resultant vector in AC non pure restistive loads is called Impedance and this is the defacto resistance in AC ccts.

An element is a pure resistive load. An AC motor is not.

An 800w microwave would draw about 6,6A @120v and about 66,6A @12V. TV will depend on size and type. My LED 24 in TV consumes 45W but a CRT TV draws more than 10 times that amount.

there is also a power loss in the inverter itself so that must be considered.

hope this is helpful.
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