electrical translation???

Hey folks,
I've got this 5 in 1 power pack that is a jumper, light, compressor, 12volt power supply, and a 400 watt inverter. Does the 400 watt inverter mean that I can, when fully charged, run a 100 watt lightbulb for 4 hrs? or is it more involved with monkeymath than that?

Dave

Not that easy, is it ever..

You need to know the battery AH and how much current the converter draws delivering 400 watts. Then you can figure out how long the battery will last. My guess is about 5 minutes or less.

100 watt light bulb at 120 volts draws 0.8 amps. This equates to 8 amps at 12 volts, plus something to account for inefficiences in the inverter.

For the sake of argument, lets assume the inverter is 80% efficient, which means it"ll draw 10 amps from the battery with a 100 watt load on the inverter.

A reasonable size gel-cell battery may have something on the order of 10 - 20 amp-hours capacity, which means you could run the inverter with a 100 watt load for 1 to 2 hours. With a 400 watt load, you would be drawing 40 amps from the battery, and it would last somewhere between 15 and 30 minutes.

Keith

Dave, a 400 watt inverter supposedly can (if you believe the advertising and that 400 watts represents output????, its NOT a perfect device, it puts out less energy then supplied to its input remember, theres heat losses) at 120 volts, supply up to 400 watts which at 120 volts = 3.33 amps continuous AND WITHSTAND IT AND DELIVER IT AND NOT OVERHEAT OR OVERLOAD......... IE it can deliver 400 watts of power which is volts x amps if theres a resistive load.

BUT THATS ONLY HALF THE ANSWER It can take 12 volts DC and invert it to 120 VAC and deliver 3.33 amps at that voltage (assuming the 400 watts is its output rating) BUT ONLY SO LONG AS THE BATTERY CAN DELIVER SUCH. The battery only stores x amount of energy so once thats depleted NO TICKY NO LAUNDRY. A typical stored energy battery rating may be in Amp Hours and if you know that number and the inverter specs and losses, then you can compute for how long the inverter can deliver 400 watts of 120 VAC output power.

The 400 watts inverter rating is its power delivery measurement/capacity NOTTTTTTTTTT the Amp Hour energy storage ratinG of its internal battery.

Got IT?????????

John T

now gosh darn it john, these laws of electricty are really starting to bug me. who do we need to write to- to get them changed?? congress..senate....mythbusters??? i should be able to hook a big battery charger to a power inverter, to recharge the battery that i'm running the inverter off of, and the if i use the high setting on the charger, run the lights and the radio in my secret mens club cabin in the woods!!! it just aint fair i tell ya!!!!

Well let me interject this and see if you sort of understand. In a lot of math problems be it with a circle or with Zap-U-Trons they use PIE 3.14XXXX well if you think about PIE if you set up a computer to figure it out to the end that computer would die because there is NO correct answer to it. So in theory what you ave might do this or that depending on wire size. How long the wire is and the etc etc etc thing. So as I said Zap-U-Trons is not and probably never will fit in to that exact thing

(reply to post at 20:24:10 01/21/10)

Click to expand...

The other F factor is these:

First "ratings" have, can are, and will be "manipulated" by the advertisers. In other words, "they" lie.

This means that a 400W inverter "might be."

It means that XX amp hours is a "maybe."

BUT.....as others have said you have to allow for efficiency factors AND......

amp hour ratings of a battery are not "cast in stone." Google up "amp hours"----the traditional old way of figuring amphours was at the "20 hour rate."

This means that ANY amp hour rating is figured on this 20 hour fudge, and it ALSO means that as the current goes up---the math figures get more and more skewed---that is say, a 100 amphour battery when subjected to say, 20-30 amps is NOT a 100 amp hour battery, but must be DERATED at that high amperage draw.

SO what is the right answer?

It really comes down to.....

experience.

IF you have say, a trustworth RV dealer, etc, someone who is IN the everyday business of "knowing" and they say that this "model Y" inverter will deliver xxx reliably, and THEY have a good reputation, then maybe you can believe it.

If they ALSO say that you need ZZZ amount of "battery" REGARDLESS of what the battery "label" says then maybe you can believe that.

HERE is a sidedamstory fer ya. Years ago I installed E911 dispatch gear for "awhile."

We were at this newly remodeled dispatch, finishing up "stuff" when other contractors were doing stuff like "testing their UPS" The UPS in this joint had a battery bank about as big as a big coke machine. A telco guy and I were "back in the jail" (building had been remodeled numerous times) and we were in a small isolated telco closet.

WHEN THEY pulled the line power to test the UPS HERE is what we discovered:

Even though "important" stuff like the coffee pot was on the UPS,.......

There were no emergency lights outside of dispatch

the jail intercom system was NOT on the UPS

Neither were the electric door locks.

So there we were---in the dark--with my trusty mini--mag light

We finally found an analog outside phone line---no small deal in this digital world---and actually called dispatch from the telco closet.

GET US OUT OF HERE says we.

Hey, if you use the inverter to run a tyical 120 VAC to 12 VDC battery charger to keep that battery charged !!!!!!!!!!!!!!!! She should run forever FREE ENERGY RIGHT !!!!!!!!!!!!!!

It was them dudes named Watts and Volta and Ohms that made up these rules long ago, Im ONLY the messenger lol

Take care, Im headed to Craigs RV Park in aRCADIA fLORIDA for a weekend jam session woooo hooooooo

John T

No. Probably somewhere around 2 1/2 hours. Much depends on the true specs that are usually not given by the manufacturer.

You lose 5%-10% efficiency though the inverter and only get 75% efficiency from the battery. Then you have to figure the AH rate of the battery figured at a low of 11.8 volts. So, you need to know how many amp hours your battery can supply before if drops to 11.8 volts and your inverter shuts off.

The problem is, amp hour rates are not very useful unless more info is given, and it usually isn't. An AH is supposed to be expressed with the "rate of hours", and most companies leave that info out.

Some of the best power packs with built-in inverters have 28AH batteries that can run a 100 watt light bulb for 2 to 2 1/2 hours.

I have an exact formula for figured actual usefullness of inverters hooked to battery banks, but the formula only works with accurate data to input.

My dad has one of those little deals also. Whats its saying is it can handle 400 watts, (they usually have a peak rating on them also for loads that require more for starting up such as motors and TV's. We also have a little inverter generator that is 1000W, and when they say 1000W thats it, you hit that peak and you stall the generator or it shuts off the inverter with an overload fault.

Your best bet (what i have done in the past) charge it up, let it run your load until it dies and then you will know how long it lasts.

Just to clarify John's posts:

When you talk about operating an electrical device over time, that is "work". Work requires "energy", for all intents and purposes work equals energy. In the case of your hundred-watt light bulb, over one hour it will take 400 watt-hours of energy to operate. You'll note that you're charged for your power in units of kilowatt-hours. A kilowatt-hour is a unit of energy and it will do a certain amount of work.

Now a watt is a unit of "power". Power is the RATE at which you do work. (Specifically a watt is one joule of work per second, but we seldom use the unit of joules because a joule is very small.) So to use an analogy, power is to work as speed (miles per hour) is to distance (miles). If you take the product of power over time, you get energy. 100 watts times 4 hours equals 400 watt-hours. In theory, your 400 watt inverter should be able to operate four 100 watt light bulb forever as long as it is supplied with a source of energy. So how long you can maintain your light doesn't depend on the inverter, but rather the inverter's source of power. So you have to know how much energy is stored in the battery to determine how long you can operate the light. Battery energy ratings are given not in watts, but rather amp-hours. You have to know the voltage rating and the amp-hour rating to figure out how much energy is in the battery. Energy happens to be equal to voltage times current, so if we multiply the voltages time the battery's amp-hour rating, we get the battery's energy in watts. So if I have a 12 volt, 50 ampere-hour battery, it should hold 600 watt-hours of energy. IN THEORY, those 600 watt-hours will operate a 100 watt light bulb for six hours.

Now when you start operating a load from a battery through an inverter, you get all sorts of things that mess up your calculations. First, the amp-hour rating of a battery is always very optimistic and assumes a very light load that will maximize the rating. Also, the battery's voltage drops as it is discharged and will eventually fall below what the inverter requires to operate long before the battery is fully discharged. The last thing is the efficiency of the inverter, which is probably not very good.

Use some 12 volt 50 watt lamps. no need for an inverter.