12v heater...how fast will it drain my battery?

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12v heater...how fast will it drain my battery?

Postby Scooter » Fri Nov 03, 2006 8:46 pm

I've seen a formula but darned if I can find it now. :(

Anyways, my plan is to use a little 150W heater for an hour or two each morning to knock the chill off before getting outta bed. Boondocking maybe two days in a row between charges.

Battery is a Maxx-29 Marine/Trolling/Deep Cycle/RV from Walmart. 875 Marine Cranking Amps, 675 Cold Cranking Amps. That's basically all I know about it. Here's a pic, if that helps.

Could I do this maybe 2-3 mornings in a row without sapping the batt?? Other than the heater, only power used would be a reading light 3-4 hours each evening. Have a terrible tendency to wake up around 4:00 every morning, at which time I could flip the heater on and get another hour or so of sleep while things warm up.

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Postby PaulC » Fri Nov 03, 2006 11:32 pm

Scooter, figuring that it draws 12.5 amp when all juiced up, it would take about 30 hours to flatten your battery. Where this falls over is in the fact that a radiant heater requires heaps more juice to deliver it's rated output. You could probably quadruple the power requirement and that would give no more than 6 hours usage, tops.
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Postby Scooter » Sat Nov 04, 2006 8:15 am

Correct me if I'm wrong. It sounds like you're saying the batt should last 30 hours on most 12.5 accessories but due to the nature if this type heater I could only expect around 6 hours (probably less).

Got the heater yesterday, and it dims the cabin lights slightly when the heat switch is turned on. That's what got me concerned. It's bound to be a battery hog.
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Postby madjack » Sat Nov 04, 2006 9:58 am

Scooter...the CCA of the battery is meaningless in this discussion(for me anyway)...it should have an amphour rating somewhere in the 75 to 110 ah range...50% usage of that is considered a dead battery so divide by 2 (110/2=55ah) now divide that by 12.5a(55ah/12.5a=4.4h)...so the math (theoretical) sez that a battery with a 110ah rating will give you 4.4hours of usage with that heater before needing recharging...due to inherent inefficiencies in the appliance and battery, it will be a little less than that...the formula you want is watts divided by volts = amps or amps X volts = watts or watts divided by amps = volts...
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Postby Dale M. » Sat Nov 04, 2006 2:16 pm

Madjack has it right... Its AMP HOUR rating that is the factor you need for use in calculations....

Also remember amp hour rating is rated over 20 hour period.... So if battery is rated at "75 AH" (amp hour), that means it will produce 3.75 amps an hour for 20hours before it drops below a sustainable rate of discharge, usually 10.5 volts.

Another consideration is as heat builds in cabin , heater will probably cycle off/on as thermostat tries to maintain desired temperature.

Once desired temperature is achieved the cycle time (off/on) will also extend/decrease battery time line.

Here is link to some great reference material on batteries...

http://www.uuhome.de/william.darden/

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Postby Sonetpro » Sat Nov 04, 2006 2:32 pm

Another consideration. One that prevented my from running a A/C off an inverter.
As voltage goes down-Current goes up. So it might use 12.5 amps at full charge and 20 amps at low charge.
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Postby PaulC » Sun Nov 05, 2006 5:25 am

There Ya go Scooter, I knew they'd all jump in once I put a few if's and maybe's up. :lol:

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Postby Scooter » Sun Nov 05, 2006 7:46 am

Thanks for the replies. :thumbsup: Thing don't put out much heat. May be able to get one or two mornings out of it, if used wisely. Would be just the thing for an old pickup truck with broken defroster and constant charge on the battery. http://tinyurl.com/ynhtll
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Postby 48Rob » Sun Nov 05, 2006 8:58 am

Scooter,

I have a small unit like you describe that I connect to a stand alone battery for a little heat in remote situations.

The battery is a 12 Amp hour, and I carry two.
Using a larger amp hour battery would make more sense, but they cost money, and are bulky and heavy.
The scooter batteries are small in size, and weigh just 7 pounds.

I don't dare hook the heater into my onboard battery bank, for fear I'd fall asleep and drain them!

The setup works well, but each battery only provides heat for about an hour before it is done.

Not very practical, but the batteries were free, and it is much safer than an unvented propane heater.

My next step is to connect a spring timer in line to prevent the heater from running more than a few minutes when I fall back asleep...

Rob

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Postby len19070 » Sun Nov 05, 2006 1:02 pm

Skooter
My experience with 12volt heaters is that they EAT BATTERIES. Regardless of what the formulas say.

What I do, and you have to be careful about this is prior to getting into the tear at night is run a candle lantern..mounted properly and safely.

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I don't sleep with it on either. Then prior to getting out of bed in the morning. I light it again.

I don't know what the watt or BTU rating is on a candle lantern but I bet its close to what a 150 watt 12 volt heater is.

Neither of these options, the 12volt heater or the Candle lantern are heaters. But I know the lantern will take the chill out of the air and the dampness.

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