Need help with 120v --> 12v amp hours conversion formulas

[soundcolor]

I'm trying to create a nice little spread sheet to calculate the amount of time I can run my LED lights on various car batteries via an inverter.

But I can't get the results to match the predictions. HELP!

Here's the rought draght: http://www.soundcolor.net/amp_hours_calculator.xls

As I understand it, the general formula is Qantity * watts * 1.4 (inverter waste) / 12 Volts = Amp Draw * Hours Needed = Amp Hours needed.

(I'd like to be able to work the spread sheet a few different ways though... you know, backwards and forwards.

Batteries:

I have 2 160 amp hour batteries
2 120 amp hour batteries
and 1 10 amp hour battery (may get more...)

120 Volt items:

* (3) 30 LED Light bulbs: Green and Blue = .06 amps per LED = 1.8 each
* (2) 30 Light bulbs: Red = .05 amps per LED = 1.5 amps each
* (1) Big LED Light bulb: 4.6 amps
* (10) LED Christmass Light Strings = 1.8 Watts? each
* (1)15 Watt Light
* (1)10 Watt LED color changing Light
* (1) What's a small fan run? 15-20 Watts?

My central question is: how many of these things can I use and still get 5-7 full nights out of my batteries without a re-charge? That's why a spread sheet would be useful to play with it. So if anyone knows an existing tool like this please let me know. I'd be happy to post mine for other people when I'm done! THANKS!

[Lassen Forge]

No spread sheet, but that's a lot of draw. Plus car batteries are not the way to go to run stuff out there for an extended period - you need deep cycle batteries. Car batteries will not take the deep draw abuse and will cook themselves.

Just doing some math

You need 10 amps at 12 volts to get 1 amp at 120 volts (not counting inverter loss). for a 10 hour night, 5 nights, that's... 50 120 volt amp hours for a 1 amp draw (not much -a 100 watt light bulb), or (better sit down) 500 12 volt amp hours...

You either need to *way* pare down your electric consumption (unless I'm off, you got about a 9 amp/hour (120 volt - 90 amp/hour at 12V) draw with what you have listed) *or* get a generator. I run kerosene lanterns for lights, a genny for my electric demands, and don't worry too much. IF you set up a solar generator array or a battery charger, you can make up for some of it. But still, you're gonna hurt those car batteries.

[soundcolor]

Thank you bay bridge sue. I'm still not sure I have the formula down though. If I use only LED bulbs or christmass light strings, well, most of them are about 2.4 Watts. So let's just use that number... I figured I would have to reduce down to a limited number of these anyway.

Formula 1) What is the correct formula to tell me how long I can run ONE 2.4 Watt light using ONE of each of these 12 volt batteries: 160 aH, 120 aH, 10 aH? (So, 3 answers to this... and we can account for inverter waste by multiplying by 1.4... somewhere.... I think...)

Formula 2) Conversely, HOW MANY 2.4 Watt lights can I run for 9 hours for 3 nights using ONE of each of these 12 volt batteries: 160 aH, 120 aH, 10 aH? How many for 4 nights?

The 10 aH is for my bike and will be used only in transit. I'm hoping I can just get one more of these (for a total of two) and have 2 lights on my bike, though I realize I may need to go down to just one. They are deep cycle batteries by the way--my bad. Except for the little one.

I'm hoping I can get away with at least 6-9 lights at camp (two 160 aH batteries) and 3-7 lights at the art piece (two 120 aHbatteries)--without recharge for 6-7 nights.

THANK YOU SOOOOOOOO MUCH!!!!

[phil]

Amps = watts / volts

A 2.4W device run on a 12VDC battery means a draw of 0.2Amp. How many 0.2A
are in 160Ah, 120Ah, and 10Ah batteries is left as an exercise for the
reader.

0.2A x (9 hours x 3 nights) = 0.2A x 27 hours = 5.4Ah. How many 5.4Ah
are in 160Ah, 120Ah, and 10Ah batteries is left as an exercise for the
reader.

Inverter waste factors are anybody's guess. The manufacturer may
provide some information in the user's guide. Or maybe not.

Some recommend that your battery be rated for twice your needs, by the way.
This gives a margin of error so that you don't completely discharge the
battery, which is a bad thing. Many Web sites recommend never discharging
below 50%. This means if you need 5.4Ah, a 10Ah battery is likely too small,
especially if you factor in inverter waste.

[trocar]

I have found that getting away from the inverter is the best thing one can do. Inverters are far more inefficient than you can really imagine. Most things that run on AC actually convert the AC to DC anyway. So in effect you are starting with DC converting to AC only to convert back to DC. That’s really, really inefficient.

Though, I guess the Christmas tree lights are designed for AC. You most likely can find DC Christmas lights. Or hell, make your own. The LEDs are of course DC. A small fan can be AC or DC. So find a 12 volt DC fan. All this stuff can be found at auto, marine, and alternative power stores.

[phil]

I have found that getting away from the inverter is the best thing one can do.

That's absolutely true. In addtion to auto, marine, and alternative energy stores, RV outlets usually have an inventory of 12V fans, lights, and such.

Having said that, though, the wealth of supplies are 110VAC, with much more available -- plus, many of us can't afford to pay for a supply of 12VDC stuff which we'll use one week a year in addition to having a lot of crap already that runs on 110VAC. -shrug- It's all in what we can afford/ justify/ store.

For 12V stuff, see
http://www.allpowersupply.com/
which as 12V cabin fans for your yacht, 12V lamps, 12V blenders, 12V coffee makers, and such
http://www.westmarine.com/
and search for lights, fans, fridges, whatever you're looking for.

[gawul]

where would you find dc christmas lights, or how would you make your own?

[Elemental666]

not to jack the thread but I'm also interested in this and want to make sure I'm right... so for example, lets say I had a 120v/60hz appliance rated at 210watts.

Amps = 210/120 = 1.75A @120v = 17.5A @12v

using the 1.4 efficiency given by op = 17.5*1.4 = 24.5A
to run for 8 hours a day = 8*24.5A =196AH
doubled for battery health = 196*2 = 392AH

4 * 120AH batteries in series = 480AH? would give me one days use right?

[phil]

where would you find dc christmas lights

See
http://www.pgilighting.com/Merchant2/merchant.mvc
for links to battery-powered lights, most Christmas-themed. They aren't 12V, but they run on AAs or Ds.

Also look for "Holiday Living" brand color changing light set. Not specifically Christmas, but big balls of changing color running on AAs.

[soundcolor]

I could not find 12v christmass lights. I do, however, have a line on 12v light BULBS.

This guy will make them however you want them (12v, 120v, edison screw base or another type, with or without safty curcuits to protect the bulb, etc.):

Alpinetech
17831 Sky Park Circle
Suite A
Irvine, CA 92614
Phone: (949) 260-8042
Fax: (949) 476-8042
E-mail: [email protected]

However, because I couldn't find christmass lights in 12v and wanted to be able to use them in my back yard the rest of the year, I went for 120v. I found better pricing on a per-LED basis for 120v bulbs from http://www.led.lightwavesconcept.com/

FYI I have found for all LED's the blue, green, and red to be the most spectacular (in that order) and yellow and white to be dissapoining (in that order). I got the 30 LED edison screw style and was happy with the brightness, though 2 failed, which lightwaves replaced for me.

The least expensive 120v christmass lights I found were half-prices 2002 ultra brights from Daryl at Holliday Light source (800.762.2855) in Lubbock TX.

He has the new ones too, but the old ones were priced really good.