Power Inverters, How Do They Work?

[Scorpio64 5,157]

A few months ago I bought an inexpensive 12V 400/800W power inverter from HF as a backup to my backup.  I tested it out using a heavy duty jump box and a 23W 120VAC CREE LED bulb in a clamp lamp.  It ran for about 9 hours without a hitch.  The light output seemed to remain the same throughout the test.

 

Nine hours on a meager 23W draw seemed a little short to me.  I mean, c'mon, the bulb pulls only .25A.  The box battery is rated at 14Ah.  I thought surely I'd get, at the very least, 24 hours out of it with a tree hugger bulb.

 

I also plugged in my Kill-A-Watt line condition analyzer before the endurance test.  It showed a steady 120VAC but when I looked at the frequency it displayed zero.  ZERO!?!?  WTH.  Even so, it still powered the bulb.

 

Lastly, after the gentle deep depletion, it took a full 24 hours to recharge the jump box and although it charged to 13.8V, the fully charged green light never came on.

 

So this is the point where I admit I know a little bit about electricity, probably forgot more than I remember..  Not exactly an expert but I do know what a transformer and a sine wave are.  What I don't know is why this itty bitty bulb and inverter sucked the life out of a deep cycle battery in such a short time.  It is irksome to me when I can't figure out stuff like this.  I know the inverter cooling fan will eat some of the juice and energy will be lost as heat and all that.  What bothers me is where did all the rest of the electrons run off to?  Is it just an inefficient inverter?  Is my expensive jump box lying to me about it's awesomeness? What gives?

 

 

[CMJeepster 2,779]

Did you check the flux capacitor?

 

[The brew guy 22]

There are numerous possibilities. You mentioned heat and the fan, inverters aren't 100% efficient and the cheap ones are even worse. You probably looking at less than 70% efficiency at the upper end of capacity (300-400 watts) and even less under smaller loads. That fan is going to run even with a 1 watt load.

Next, those gel cells don't last forever, you sure it's still a 14Ah battery. Running it all the way to zero isn't doing it any good, they usually aren't deep-cycle batteries.

[The brew guy 22]

And your killawatt thingy probably won't recognize the modified sine wave, its just a homeowners toy, not a real piece of test equipment.

[W2MC 1,699]

Without getting too deep into inverter technology; power conversion is never loss-less.  Your Harbor Freight inverter is probably running at 50% conversion efficiency, at best, and there is a significant draw regardless of the connected load. It's also placing a pretty good demand on that little 14 Ah gell-cell to produce it.  Gell-cells 'like' relatively light current draws for long periods of time, and don't 'like' being drawn-down too far.    

[mipafox 438]

Jump boxes are crap. Yes, we use them, they are useful, but they are still crap. We are not using them for their intended purpos, and neither are you. They do not have deep cycle batteries. Most lead-acid batteries are damaged each time they are discharged more than slightly. Even just letting them passively lose charge damages them. They will still hold voltage, but loose capacity. You probably don't have the watt-hrs you think you do. Get a couple better batteries and/or properly run and maintain them. One source of small, high-output deep cycle batteries is environmental equipment companies. They sell batteries to run pumps or computers in the field under all weather conditions, and nobody is going to take their crap if the batteries weaken after being discharged a few times. Because that is their expected service. Obviouly, you can buy marine or solar batteries. They will not be conveniently packaged. If you are creative, you can buy one that fits into your jump box. Be sure to account for any orientation or venting requirements.

[NJGF 375]

If you are drawing 1/4 a at 120v then it will require about 2 a at 12 v so getting 9 hours from a 14 ah battery is expected. You need a much larger battery to make this run longer or a smaller light.

[Scorpio64 5,157]

If you are drawing 1/4 a at 120v then it will require about 2 a at 12 v so getting 9 hours from a 14 ah battery is expected. You need a much larger battery to make this run longer or a smaller light.

This actually makes the most sense.  I forgot about the amperage conversion.  

 

Mipa, I already know about the differences in battery types.  This unit is supposed to have a deep cycle battery as it is advertised not only as a jump box but also as a backup power source.  Though I do admit, I may have damaged something by such a deep discharge.  it got down to 11V before the inverter gave up.  The unit is only two years old and was never discharged much below 13V before it was recharged.

 

I think my next step is to visit the marine supply store and pick up a couple of 12VDC fluorescent lamps. No sense in letting energy get wasted by the inverter.

[njJoniGuy 2,131]

... What bothers me is where did all the rest of the electrons run off to? ...

Thanks to the leader of our Choom gang, those electrons have been fundamentally transformed.

[djg0770 481]

This actually makes the most sense.  I forgot about the amperage conversion.  

 

Mipa, I already know about the differences in battery types.  This unit is supposed to have a deep cycle battery as it is advertised not only as a jump box but also as a backup power source.  Though I do admit, I may have damaged something by such a deep discharge.  it got down to 11V before the inverter gave up.  The unit is only two years old and was never discharged much below 13V before it was recharged.

 

I think my next step is to visit the marine supply store and pick up a couple of 12VDC fluorescent lamps. No sense in letting energy get wasted by the inverter.

 

There are 12 V LED's available.  I'd go for those over Fluorescent any day.

[Scorpio64 5,157]

There are 12 V LED's available.  I'd go for those over Fluorescent any day.

 

Even better.