Battery power question - Fiberglass RV


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Old 02-26-2006, 02:42 PM   #1
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I have a 13' Trillium with a deep cycle marine battery (hope thats the right way to say it). Can someone give a a really, REALLY layman's formula for figuring out how much power things will need vs how long my battery will go on it's own. Is it as simple as knowing how many amps a light or appliance or whatever uses/draws per hr and how many amps the battery can provide before needing to be recharged? Remember, I am the daughter of an electrical engineer and I have never recovered from him trying to help me with math! "What Sharon, how can you not get this, it's so simple, you just"........than he would start speaking another language.

Thanks!
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Old 02-26-2006, 03:32 PM   #2
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Quote:
Can someone give a a really, REALLY layman's formula for figuring out how much power things will need vs how long my battery will go on it's own. Is it as simple as knowing how many amps a light or appliance or whatever uses/draws per hr and how many amps the battery can provide before needing to be recharged?
Thanks!
Have you looked in the solar tutorial, over to the left? That seemed to be a simple explanation to me.
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Old 02-26-2006, 04:28 PM   #3
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hmmmm. from the example in the box, it does seem to be in line with what I was thinking. Ok, then is it the light fixture or the bulb or both that would determine how many amps it draws?
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Old 02-26-2006, 04:37 PM   #4
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Is it the light fixture, or the bulb, or both that would determine how many amps it draws?
I think it would actually be the [b]bulb that determines how many amps. But, while a flourescent tube draws less than an incandescent bulb, I have not ever seen a flourescent 12 volt tube that fits into an incandescent 12 volt fixture. So "[b]Both" might be the multiple choice test answer, here.
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Old 02-26-2006, 06:26 PM   #5
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Sharon:

The light most used in our trailer is a compact fluorescent bulb in a regular house-type light fixture. It is a standard 120v and we run it off a small inverter. It has done the job very nicely, but I just discovered a similar bulb designed for 12v at West Marine (forget the wattage, but it is typically in the 14-17watt range). I'm thinking of reconnecting the fixture to the 12v system and testing this bulb out. Less loss from the inverter. Maybe this is what Frederick is talking about, so they are available, at a price.
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Old 02-26-2006, 07:38 PM   #6
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...Is it as simple as knowing how many amps a light or appliance or whatever uses/draws per hr and how many amps the battery can provide before needing to be recharged?
Yes, and Yes to your two questions.

Here are some things to know:

1. Amps times volts= watts.
2. Rearranging things, Watts divided by volts=amps.
3. A 12v amp is not the same as a 120v amp.
4. But, just because we like a challenge, Watts are watts. They're handy because they make it easy to compare a 12v gadget to a 120v gadget.
5. Manufacturers recommend for best life you don't discharge their batteries past 50%. That means if you have a new 80 amp hour battery, you should use no more than 40 amp hours before recharging.
6. As batteries age, they loose their capacity. But, I don't know a rule of thumb for how much per year. Probably because it depends on how you use the battery.
7. Inverters (changing 12v to 120v) are only about 90% efficient.

An 18 watt bulb uses 1.5 amps per hour. (18/18=1.5 See no. 2) Incandescent or fluorescent. Fluorescent bulbs seem to put out more light per watt, so frequently they can be less wattage and do the same job. So a 12 watt fluorescent might do the job of an 18 watt incandescent bulb. That's why fluorescents are handy for boondockers. They use fewer amps per given light level. Although some cause a hum in the radio. Some boondockers change their bulbs down in wattage to save electricity. Perhaps keep only one higher one for reading.

Use an 18 watt bulb for 3 hours and you've used up 4.5 amp-hours. Add up how long every gadget is on (in hours) in a given day and divide the battery capacity by the total you used and you have an estimate of battery life. Don't forget the water pump on and off, the fan over the stove, a radio or TV plugged into 12v, etc.

Wattage is pretty much in all in the bulb. I don't believe that the transformer in found in some flourecent lights uses much electricity.

Running 120v gadgets through an inverter is a bear.

Although not a perfect analogy, I like to think of watts as work done. Imagine the work to be done is to toast two slices of bread. Let's pretend my toaster is a 1000 watt toaster. Lets pretend it takes 3 minutes to toast the bread. 3 minutes is 0.05 hours (3/60=0.05 hours)

Using big, beefy 120 volts that's 1000/120=8 amps(about) (no. 2 above). 8 amps x 0.05 hours is 1.2 amp hours.

You still need those 1000 watts (no. 4 above) if you're using wimpy, sissy 12 volts. With 12 volts you're using 80 amps (about)(1000/12v=80). 80 amps x 0.05 hours is 12 amp hours. Plus, you use about 10% more loss to the inverter efficiency (no. 7 above). That means you've used about 13 amp hours of your battery life.

80 amps drawn from your battery will need HUGE wires. I recommend you don't toast bread on an 120v toaster using a inverter!

When you see 12v appliances, they often are de-powered so they use fewer watts and therefore less amperage, but they take longer to do the job so the net effect is the same to your battery.

On the bottom or back of most gadgets is either the 120v amperage or the wattage. If necessary, convert the amps to wattage (no. 1) then divide by 12 to get the amps per hour from a battery. Add those into your total when calculating your daily usage.

More than you want to know but I got carried away. Mostly, what you suspect is true. The devil is in figuring out what you really use.
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Old 02-26-2006, 10:32 PM   #7
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Thanks for the replies. After I read them a few times (Steve) it will sink in. I appreciate the info!!

Sharon
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Old 02-27-2006, 07:17 AM   #8
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Steve L, I am surprised you didn't mention the new trend toward LED lamps for 12 Vdc operation. They can sugnificantly decrease the drain on the battery while boondocking.
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Old 02-27-2006, 10:04 AM   #9
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Good Job Steve, Thank you for the post
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Old 02-27-2006, 03:46 PM   #10
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Steve L, I am surprised you didn't mention the new trend toward LED lamps for 12 Vdc operation. They can sugnificantly decrease the drain on the battery while boondocking.
I'm into LEDs in a big way, but I don't think they're ready for prime time yet. I figured the thread was mostly about figuring out how long the batteries will last more than about boon docking tricks. I only mentioned fluorescent lighting because someone else brought it up before me. But when they did, it reminded me that sometimes I think people get the wrong idea about the fluorescent light. Sort of like:

Which uses more electricity?
A. 18 watt incandescent light
B. 18 watt fluorescent light

I think sometimes, somebody, somewhere, somehow thinks the answer is B.

Or the ever popular:

Which weighs more?
A. 1 pound of lead
B. 1 pound of feathers

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Old 02-27-2006, 04:17 PM   #11
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... I just discovered a similar bulb designed for 12v at West Marine...
Per, is this the Bulb-Fluorescent-Screw-Base-12V at West Marine? I had never heard of a medium screw base compact fluorescent in 12V before, and it could be the answer for my Boler range hood lamp. It is expensive compared to a household unit, but maybe not too bad for a 12V fluorescent fixture. If you try it, please let us know how it works out.
Quote:
Originally posted by West Marine
[b]BPESL13T/M12
13 Watt mini, equivalent to a 60 watt incandescent bulb
8,000 average life hours, 800 initial lumens, MOL 4.8"
Now, back to the original subject... this lamp would use about 13/12=1.08 amps at 12V, so every hour it is on would use up 1.08 amp-hours of the battery capacity. The "equivalent to 60 watt incandescent" comment just means that it is about as bright as a 60W regular bulb, not that it uses that much power.
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Old 02-27-2006, 05:08 PM   #12
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Lets see if I am correct a 2.2 amp CPAP machine 120vlt using an invertor
2.2amps x120= 264 watts
264 watts divided by 12=22amps per hour.
Wow I need more batteries to run if this is correct
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Old 02-27-2006, 11:50 PM   #13
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Can someone give a a really, REALLY layman's formula for figuring out how much power things will need vs how long my battery will go on it's own.
Using lights.......4-7 days.

Using lights, water pump, tv, and fan or a furnace with a blower......2-3 days.

===

A 'jump-it' (about $40) is handy as a back-up power supply in the trailer as well as starting the tow vehicle in an emergency.
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Old 02-28-2006, 06:19 AM   #14
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Lets see if I am correct a 2.2 amp CPAP machine 120vlt using an invertor
2.2amps x120= 264 watts
264 watts divided by 12=22amps per hour.
Wow I need more batteries to run if this is correct
That's correct. The back/bottom should show voltage and either required amps or required watts. If it shows 2.2 amps at 120 volts your're correct, not counting inverter inefficiency. At 90%, it's using 24 amps per hour.
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