Battery power question

[Sharon Herman]

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!

[Frederick L. Simson]

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.

[Sharon Herman]

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?

[Frederick L. Simson]

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.

[Per Walthinsen]

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.

[Steve L.]

...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.

[Sharon Herman]

Thanks for the replies. After I read them a few times (Steve) it will sink in. I appreciate the info!!

Sharon

[CD Smith]

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.

[Taylor Kissinger]

Good Job Steve, Thank you for the post

[Steve L.]

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

[Brian B-P]

... 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.

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.

[Taylor Kissinger]

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

[PineConeDon]

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.

[Steve L.]

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.

[Taylor Kissinger]

Thank you Steve, Now add up the rest and see, I think a second battery is in order or put the CPAP machine on its own

[Brian B-P]

...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.

For a timely example of this, the current Camping World internet-only specials include a normal-looking but 12V drip filter coffee maker. It is listed with a 22-amp current draw, which is a whacking load on your battery, but still only a wimpy 264 W (nominal, at 12V). Not surprisingly, this doesn't work well: see the scathing reviews which Camping World allows buyers to post... Koolatron™ 12V Coffee Maker - Reviews. I would quote the good bits of the reviews, but they're all priceless - have a look!

It takes a lot of heat energy to do anything useful, so in general I believe that the best plan is to provide heat by burning propane, and do other work (make light, make sound, turn motors...) with electricity, to make the amp-hours count.

[CharlynnT]

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.

Now THIS post I understood -- No math required!

[Gina D.]

OK Sharon, the geek speak stops here...

(engineers, can't live with 'em, can't live without 'em)



My REAL WORLD experience...when dry camping

I have a 120aH battery.
I have 2 18 inch flourescant light fixtures, and 2 8 watts incandescants. (Not all used simultaniously)
I have a wall mounted Cat heater that runs on propane, and a fan in it for exhaust.
I have a Fantastic Fan Endless Breeze fan that runs on a seperate 20aH battery, recharged by solar.
I have a 700 watt invertor that I USED to run a 120v DVD player off of, and my laptop. (I have since replaced the dvd player with a 12v model and occasionally, my satelite dish and receiver goes with me.. invertor is rarely needed anymore)

All else has a hand back up, or is strictly propane. (Water Pump, fridge etc, coffee done on stove..) A good old fashioned propane lantern is used outside.

I can run for 4 days without charging if I limit the TV to one DVD a nite, and only spend a couple hours reading.

With a 15 watt solar panel connected, I can run indefinately if I have REASONABLE weather. The panel still works in overcast conditions, just not as fast.

You CAN have your cake, and eat it too if you don't add microwaves and video games, air conditioning, electric dog polisher or turbo charged mouse chaser.

Now, the wonder of all this is.. if I do something stupid, like forget to turn off the porch light, an incadescant buld, I can run the battery into oblivion overnite.

The key is retraining yourself to conserve, and when you do, you'll find that most of the stuff you DID use wasn't needed and you don't really miss it.

I am now starting to replace my peripherals (Radio, flashlights etc) with Green items, hand cranks or solar charged. I am quite pleased with what I have so far, and will continue to do so as items become available.

Note: The hand crank items often have the ability to charge other items, and I can charge my dvd player with the flashlight I just got yesterday. I wonder how many cranks it is gonna take? *Holds arm in anticipated pain*

[rickster]

OK. So I am real new to this as well. I just bought a 79 trillium1300. It has the original equipment including a dead battery from what I can tell.

The "Central Power Pack" model CP-06 FK has a switch that lets me run on House current 120V or Battery.

The plug says "120V 60hz 10a Max 120 VAC" this is the only plug on the panel, I assume that this a plug that runs through the invertor and I can use but draws power from the battery. is that correct. What can I run off that plug? A PC, a DVD player???

The fuse says "12.5 VDC Max Fuse 10a type AGC 10"

The Panel Input says "120V 60Hz 15amps Conversion Input 1.2 Amps

Convertor Output says "12.5VDC 6 amps

OK what the heck does all this mean?

I have a deep cell Marine Battery that I am going to put in there to hopefully run my PC so I can watch DVD's. Maybe a light or two. Also, the pump that I want to put in for the water tank.

HELP What do I do. I read and reread all the stuff on this topic here, but still not quite sure how to approach the project. I guess I am looking for is, RICK, you need a battery, and you can or can't use the current invertor, which one can I use, how to hook it up, etc. Any help to get me started would be great. I don't think I am going solar for now, just want to charge my battery from the car if possible or charge it when I get back home off the house or charger.

Thanks
Rick

[rickster]

Now THIS post I understood -- No math required!

Gina,

Your breakdown is helpful. Why do you not need an invertor anymore because of satellite??

Do you have a brand that I should use for the invertor? I figure I am only going to camp two nites for the most part so should a fully charged marine battery work for that amount of time for a PC and a light.

Thanks

Rick