Electricity Help

[ThunderingWind]

I am having some trouble calculating battery amps needed.

Unit takes 120V AC, and turns it into 12v DC 4.5 Amps

How do I calculate how many "real" amps it would take to run the Unit for 8 hours
using a AC to DC converter attached to at 12v battery? The Converter can produce
upto 75 watts.

I know I am missing some needed numbers but I need to have the formula first.

The unit's power pack can't take 12v DC directly.

I have used a 400w DC to AC converter for nearly 12 hours on one of those 12 cell, 12v
golf cart batteries when I was on some mission trips to Haiti. Ran a drill and saw all day.

[RWSmith]

You are correct; we need another "key" for the formula, Ohm's Law, to solve your problem.

P = I • E

See: Ohm's Law

(I would tell you to Read the Merit Badge Pamphlet!!! -- Argh! Argh! Argh! Just kidding. Just kidding!

Frankly, I'm a bit (actually, very) confused by your post... how 'bout some more info, please? What -exactly- are your hooking up to what; and in what order? Will this be done indoors or out? (The ambient temp. may greatly affect run-time.)

What -exactly- is the, uh, "unit" and whatever else you're trying to use? What is the application (how & where it will be used)? Can you give me some specs on the battery and "converter" (Inverter?)? -- E.g., Brands and Model or Part Numbers? What I'm looking for here is what's called the Ampere-Hour, or "Ah", rating. (Note: This would be "mAh" on small batteries, like "D", "C", "AA", "AAA", "9-Volt", etc. )

===============

Note: Joule's First Law, Faraday's Law of Induction and Kirchoff's Laws, come into play (at the circuit level). But, I seriously doubt we'll have to go down any of those roads for this problem. If you can detail the above info., I can tell you whether or not we need to take any of these into consideration.

[ThunderingWind]

Trying to get a CPAP working while camping. Not thrilled to have to use it. Appearently I have had a problem since I was a teenager.

Input is 120v, 1.20 Amps, converted to 12v, 4.16 Amps using the transformer that came with it. I cannot directly input 12v from the battery so I am told and it would require cutting the power cord anyway - DON'T want to do that since I would not have a cord forthe normal AC input hook up when at home.

The DC to AC converter I have takes 12v, 8 Amps and outputs 115-120v, 100 watts, No Amps listed.

So, how do I calculate the actual Amp hour draw to see what size of 12v battery I need to use as my starting input to last 8 hours then sit on a charger all day (solar panel if we are base camping).

I would have read the MBP but I do not have it.

[RWSmith]

Ah... the dreaded 'CPAP-on-the-trail'. (I honestly figured this was it.)

With the addl. info. you just gave me --not the fact that it's for a CPAP; but, the numbers,-- I'll do some thinking and post a reply in a little while.

[PaulSWolf]

Input is 120v, 1.20 Amps, converted to 12v, 4.16 Amps using the transformer that came with it. I cannot directly input 12v from the battery so I am told and it would require cutting the power cord anyway - DON'T want to do that since I would not have a cord for the normal AC input hook up when at home.

(The formula P=i*e, that Bob gave in his response means, wattage = voltage*amperage. If you have 2 of the 3 values, you can determine the third, since by simple math you can also get voltage=wattage/amperage or amperage= wattage/voltage) so 120v* 1.2amps = 144 watts input \and 12v*4.16amps = 50 watts output from the converter.

The DC to AC converter I have takes 12v, 8 Amps and outputs 115-120v, 100 watts, No Amps listed.

So the converter takes 12v*8amps= 96 watts and outputs 100watts/120v = 0.833 amps

So that DC to AC converter you have won't output enough juice to power the CPAP, since it needs almost 50% more amperage output.

So, how do I calculate the actual Amp hour draw to see what size of 12v battery I need to use as my starting input to last 8 hours then sit on a charger all day (solar panel if we are base camping).

You need to get a converter that will output 144 watts minimum.

Then, to figure the amp-hours needed for the battery, just take the amperage the converter needs as an input and multiply by the 8 hours of draw to get the minimum amp-hours needed, and add a factor of safety to that (say 25% minimum).

[RWSmith]

Thanks, Paul. ( You saved me the trouble of having to break out my old No. 2 pencil. There was a time, but... I just can't do that stuff in my head anymore. )

===============

TW,

Paul is an Electrical Engineer, BTW. (I'm just an old Electronics Technician... IOW, I'd fix stuff he designed. Yuk, yuk.)

I am concerned though, that you're having to go from DC source to DC->AC conversion, then back through AC->DC transformer, and finally to the CPAP. Now, I'm not an engineer; but, even I know that's a mondo waste of energy in the conversions.

I know you don't want to cut your original power input jack... but, you could eliminate the messiness altogether by getting a DC-to-DC converter. There are kits, jacks, whatever you need to make it work, available from suppliers that have what you need to make it happen. There are others, but MCM comes to mind.

Just to double-check, is that final output power (to the CPAP) 12VAC @ 4.8A, or 12VDC @ 4.8A?

I know, for a fact, mobile power adapters are available for most CPAPS, esp. all the newer models (for mega-bucks). But, I'm sure we can get a perfectly good one on the cheap. I just need the full specs. on the transformer, including the input jack. Send me an e-mail, if you want.

[ThunderingWind]

Just to double-check, is that final output power (to the CPAP) 12VAC @ 4.8A, or 12VDC @ 4.8A?

12v DC @ 4.16A output on the CPAP "transformer"

The wire that come out of the "transformer" is hard wired to it. BUT it does have a standard plug end for electronics.

So I guess I need to find a DC to DC filter that has an ouput that can be set for 4.16A.

[ThunderingWind]

Then, to figure the amp-hours needed for the battery, just take the amperage the converter needs as an input and multiply by the 8 hours of draw to get the minimum amp-hours needed, and add a factor of safety to that (say 25% minimum).

So, if we were to remove the conversions from the problem, how do I measure the Amp hours of this machine?

Do I use the 4.16 Amps (as output from it's "transfomer") multiplied by 8 hours run time plus 25%?
That would be 42 Amps total (rounded up) and I could use a smallMotorcycle battery.

But something tells me I am wrong.

[ThunderingWind]

http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-CON-JAD2000-/58-12600
•Input Power: 12V DC
•Output Power: 1.5-12V(1.5/3/4.5/6/7.5/9/12) 0~600 mA
•Voltage Selector Switch

Is this what I need? Then just get a simple clip system for the battery terminals and one of those "add a 12V DC
plug kits?

[RWSmith]

http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-CON-JAD2000-/58-12600
•Input Power: 12V DC
•Output Power: 1.5-12V(1.5/3/4.5/6/7.5/9/12) 0~600 mA
•Voltage Selector Switch

Is this what I need? Then just get a simple clip system for the battery terminals and one of those "add a 12V DC
plug kits?

Nah... that's 0~600mA... as in, 0.6A.

I have literally dozens of various transformers, plugs, jacks, etc. I'll be happy to send to something. The CPAP is only going to "draw" as much power as it needs, which in this case is 50 watts. (4.16A x 12VDC = 50 watts.)

Frankly, you should eliminate the transformer, too; but, you really, really would want an "over-current" protection circuit. As long as the battery can "push" a minimum of 5A (or more) for about 10 hours, you'll be good to go.

Think of it this way... let's say you have a camper/RV that has a 60-watt interior light, which runs off 12VDC. The "Ah" rating you would need to run that one light bulb only for 10 hours would be about 5A x 10 hours, which = 50Ah.

So, a motorcycle battery sounds about right... for one night.

The question then becomes, can your solar panel really recharge the battery in one day? If so, fine. Even so, I'd step up to 100 Ah. Plus, I'd think more along the lines of a lawn mower battery... 300 Ah battery... that's six days, no problem and still half the size of a car battery.

Thoughts?

[PaulSWolf]

Thanks, Paul. ( You saved me the trouble of having to break out my old No. 2 pencil. There was a time, but... I just can't do that stuff in my head anymore. )
...
Paul is an Electrical Engineer, BTW. (I'm just an old Electronics Technician... IOW, I'd fix stuff he designed. Yuk, yuk.)

Your welcome, but actually, I'm a Traffic Engineer, with a degree in Civil Engineering, not an EE. My primary functions for most of my career (now retired) were designing traffic signals, and reviewing traffic control plans (signing and pavement marking plans, signal plans, roadway design plans, construction maintenance of traffic plans, etc.). I also did quite a bit of signal timing analysis for a few years, involving writing and rewriting high level computer programs on main frame computers.

So I didn't really design the equipment, just specified equipment designed by EE's. But even a dumb CE can calculate the values in the P=i*e formula.

[RWSmith]

Thanks, Paul. ( You saved me the trouble of having to break out my old No. 2 pencil. There was a time, but... I just can't do that stuff in my head anymore. )
...
Paul is an Electrical Engineer, BTW. (I'm just an old Electronics Technician... IOW, I'd fix stuff he designed. Yuk, yuk.)

Your welcome, but actually, I'm a Traffic Engineer, with a degree in Civil Engineering, not an EE. My primary functions for most of my career (now retired) were designing traffic signals, and reviewing traffic control plans (signing and pavement marking plans, signal plans, roadway design plans, construction maintenance of traffic plans, etc.). I also did quite a bit of signal timing analysis for a few years, involving writing and rewriting high level computer programs on main frame computers.

So I didn't really design the equipment, just specified equipment designed by EE's. But even a dumb CE can calculate the values in the P=i*e formula.

Oops. My bad. CE. I remember that now. But, you know... it was meant as, uh, 'tongue-in-cheek' more than anything else.

[ThunderingWind]

I have literally dozens of various transformers, plugs, jacks, etc. I'll be happy to send to something. The CPAP is only going to "draw" as much power as it needs, which in this case is 50 watts. (4.16A x 12VDC = 50 watts.)

Do not send me anything. Just tell me what to buy and where.

Think of it this way... let's say you have a camper/RV that has a 60-watt interior light, which runs off 12VDC. The "Ah" rating you would need to run that one light bulb only for 10 hours would be about 5A x 10 hours, which = 50Ah. So, a motorcycle battery sounds about right... for one night.

The question then becomes, can your solar panel really recharge the battery in one day? If so, fine. Even so, I'd step up to 100 Ah. Plus, I'd think more along the lines of a lawn mower battery... 300 Ah battery... that's six days, no problem and still half the size of a car battery.

Deep Cycle lawn mower battery it is. I hope I can find a Gel Filled one.

Since this is for when when a "car camping", I have not worries about carrying heavy weight for miles and miies and miles.

The solar panel is 145W from Harbor Frieght. I figured it will work at Summer Camp reasonably well.

Thank you for all the help. Look forward to seeing the transformer or whatever it actually called recommendations. I am enjoying learning somethign new about electricity.

[RWSmith]

Deep Cycle lawn mower battery it is. I hope I can find a Gel Filled one.

PERFECT! -- Deep-cycle and Sealed Lead-Acid (SLA), or even better, Absorbent Glass-Mat (AGM), are definitely best for this application. Gotta warn you though... these things get real expensive, real fast. (Will do some calculations to see if we can get down around 100Ah... see notes, below.)

The solar panel is 145W from Harbor Frieght. I figured it will work at Summer Camp reasonably well.

145W? Did you mean 45W? (Re: Harbor Freight Item #90599.) Do you have the Regulator that comes with this model, too?

Thank you for all the help. Look forward to seeing the transformer or whatever it actually called recommendations. I am enjoying learning something new about electricity.

N/p. It's fun for me, too.

BTW, I knew (in the back of my mind) there was some good reason for significantly increasing your battery's Ah rating. See "Peukert's Formula" and Calculating your Required Battery Capacity... especially depth-of-discharge.

Please send me (via e-mail) the Brand and Model of your CPAP (incl. any accessories you might already have) so I can spec. up the parts you'll need. Thanks.

[WVBeaver05]

TW - did you get it working? Sorry for not jumping in sooner but I've been really tied up with "real" work (I really hate it when work interferes with my Scouting). I am an EE and will help if you still need it.

YiS

[ScoutMomGWRC]

P = I • E

I just stopped in for the PIE!

[ThunderingWind]

TW - did you get it working? Sorry for not jumping in sooner but I've been really tied up with "real" work (I really hate it when work interferes with my Scouting). I am an EE and will help if you still need it.

YiS

For the short term need, I picked up a Diehard Jumpstart pack that has a 12v port and the 12v DC cable for my CPAP machine.
The Diehard pack was on sale plus I had Sear points to use so it was less expensive.

The unit states is it a 22aH battery. The CPAP is rated at 4.16aH. So technically that should give me only 5 hours. But I am getting a good 8 out of it. I do not use the heater on it at all but is built in.

I don't understand the Amp Draw at all here. I know I have to get one battery that will last 8 hours per night for 6 nights without a recharge by the time I get to summer camp or I can't go (Camp enforces tighter medical restrictions). I was told they will not allow me to take a rechargable unit to one of their buildings and plug in.

[FrankJ]

Maybe the heater is part of the 4.16 amps? The 4.16 amps might also a peak value. The battery rating would be the floor. A new battery should do better than rated. More batteries would be an option? A bit expensive. I saw a CIA display this weekend of a propane powered fuel cell. Maybe talk to some of your old army buddies.

Assuming the camp is a typical BSA summer camp, they are being short sighted. Maybe go up the food chain to find a reasonable person?

[WVBeaver05]

For the short term need, I picked up a Diehard Jumpstart pack that has a 12v port and the 12v DC cable for my CPAP machine.
The Diehard pack was on sale plus I had Sear points to use so it was less expensive.

The unit states is it a 22aH battery. The CPAP is rated at 4.16aH. So technically that should give me only 5 hours. But I am getting a good 8 out of it. I do not use the heater on it at all but is built in.

I don't understand the Amp Draw at all here. I know I have to get one battery that will last 8 hours per night for 6 nights without a recharge by the time I get to summer camp or I can't go (Camp enforces tighter medical restrictions). I was told they will not allow me to take a rechargable unit to one of their buildings and plug in.

This is great. You have already done one of the best things possible - eliminated the extra DC->AC->DC conversions that are wasteful and just leave places for problems to slip in.

Now is the time for Magic EE handwaving

Let's forget everything except what we know for certain - you now know that you can get at least 8 hours from your 22AH battery pack. We know that so we can (almost) forget all the numbers that came before (because they never really told us anything concrete - the 4.16A @12VDC quoted on the "transformer" is actually the rating and it doesn't tell us what the CPAP user - except with a good designer it is less than that - the CPAP doesn't run at the same power all during it's cycle, either - so it probably requires less than 4.16A even during the "worst" i.e. highest part of it's cycle...)

Now we don't have to rely on ratings and guesses. You have experimented and found that averaged over a night your unit requires less than 3A. You can use that value to calculate what you need then apply the safety factor that makes you feel comfortable. Just 6*22AH would give you a starting point. Since I don't normally get 8 hours sleep at summer camp you may be able to drop it down a little. We could talk for hours about ratings and peak vs average usage and duty cycles, etc. but in a very practical way you have done the actual experiment so we don't have to and we have a better starting point than the calculations would have (probably) given us.

I might try to go with 2 of the units that you have and find somone with some sense who will agree to let you charge one. I can't think of any reason that they wouldn't agree to that.

Do keep in mind that there are a lot of variables as pointed out in the referenced materials. Warmer batteries have higher capacities vs colder ones (but colder makes them last longer vs warmer) -- Older batteries don't perform as well as new ones. Changing the discharge rate will change all the characteristics. And, if you need to combine batteries, connecting them in parallel adds the capacities (e.g. 2 - 12V 22AH batteries connected in parallel will give you (almost) 44AH).

Let me know if you need more info, but you are well on your way to having the answer that you need.

YiS

[ThunderingWind]

It's all moot now. I have resigned my Scouting positions as of last night.