# What is the load of an AC inverter on a battery?

Asked by JeanPaulSartre (5779) September 22nd, 2010

If I have a 9 Amp (peak) load (@ 1100 Watts, 115V) AC running from a 1500W inverter, and the inverter is powered by a (12V DC) 18Ah battery, will I get 2 hours (approximately, at peak) of run time, or does something in the inverter alter this draw in a way I’m not predicting?

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If this is a new inverter, figure it is only 80% efficient. If you are truly drawing 1100 Watts continuously then the inverter is drawing 1375 Watts.
You are feeding it with 12 volts Power = Volt x Amps So 1375W = 12V x Amps
Therefore Amps = 1375/12 = 114 Amps.
With an 18 Ah battery that means 18/114 = 0.15 hours or 9.5 minutes of full on operation. Not 2 hours.
You have some options.
1) Get 228 Ah of batteries. That is approx 6, 42 Ah car batteries.
3) Use a small 1500W gas generator.

Sorry for the bad news but the most of the laws of physics are difficult to break.

LuckyGuy (36138)

@worriedguy Awesome thank you! That’s the info I needed – I wasn’t sure which “side” I needed to get Amps from. It’s not completely terrible. I won’t be anywhere near peak in the project I’m working on (hopefully not even 10% most of the time) but I may want to consider a larger Ah battery or a few in series. Damn physics, always getting in the way of my magic.

@JeanPaulSartre You are still on the borderline at 10% load. That would only give you 95 minutes with the 18 Ah battery. Definitely you will need at least one car battery. The 42 Ah will give you 3.6 hours if you stay at 10%. Make sure to have a spare with you.

Also if you don’t have a generator you can use your car alternator. Typical GM car alternator puts out 1000 watts at 1200 rpm. lt is quiet and convenient if you can park close by.
Remember to size your cable correctly. There are easy to find charts for this. You should size it for at least 120 Amp wires to cover the peaks. The wire size will depend upon the distance. Think battery cable wire.

LuckyGuy (36138)

Thanks @worriedguy! It’ll definitely be (and has been) a learning project, thankfully that’s most of the point. I agree that it’s borderline – Probably will go with a car battery or two, as I don’t have a car. I’ll keep a close eye on wire gauges – thankfully all the runs will be pretty short. I’m planning on using high amp rated relays for the switches on the really big ones.

Remember, you only need high amp runs between the batteries and the inverter. Tht will be your fattest cable since it’s low voltage. After the inverter – where you have the 120 Volt AC – you can use “normal” wiring . 14 or 16 gauge house wiring or a good extension cord rated for 1500 watts.
If you are doing audio at 1500 watts than go with Monster cable.

LuckyGuy (36138)

You pretty much will need at least a car battery, and with a load like that and no alternator charging it, one may not do it. Let me double-check the amp-hour ratings on a few batteries. Oh, it should go without saying that you’ll need to use a newer battery; one that is in good condition.

jerv (31032)

Yeah, I’m using 12 gauge romex on the AC side @worriedguy, simply because it’s what I have – but it’s overkill.

It works on paper @jerv, but lots of experimenting will gain my confidence – yeah, I’ll definitely get a new battery maybe and ambulance battery…

I was originally considering using a couple batteries from an old power regulator, but one is warped which is, uh, concerning, and I don’t think that speaks volumes for what the other one has been through. In the course of this thread that “good” battery has gone from “maybe it’ll work” to “oh it’ll be a fine backup” to “test dummy.”

@worriedguy Very nice answer! I had no idea inverters were so efficient (“only 80%”)—I just assumed there were horrendous losses in conversion.

When you tap into your car’s alternator for 12V source, can you just connect to the battery terminals rather than the alternator itself ? <—possibly dumb question

And is it 12V or 13.6 V ? I’ve seen both figures for automotive voltage.

gasman (11296)

@gasman I believe car voltage is 12V off, but 13.6V when the car is running (with most accessories turned off.)

Makes sense, @JeanPaulSartre. Lead-acid puts out 12V but gets charged at 13.6. And accessories handle voltages anywhere in that range?

gasman (11296)

@gasman You can charge a 12V battery at 12V. Charging is more about Amps (how fast you can charge a battery safely.) I’m a little out of my league on this one but I believe that the stuff in the car causes the voltage to increase somewhat. So it’s 12V “unloaded” and 13.6V running, and maybe like 14V when you have the lights, radio, etc all on. Someone correct me if I’m way off.

Guys I’m back.
The alternator output can vary from 13.6 to 14.4V. An unloaded fully charged 12 V battery will read higher than 12 V. It varies between brand and the number of plates they have. It is common to see 12.6 volts.
As an example, look at your AA 1.5 volt batteries. A fresh set will read 1.6V or even 1.7V! As you use it, it goes down to 1.5 ~50% 1.4V ~15% and then 1.3 V ~5–10% stored energy. It kills me when people throw out a battery that does not work in their camera but will be perfectly good in a TV remote for another year.

The newer cars have variable rate charging. They are designed to change the rate of charge depending upon battery state and driving conditions. If you floor the engine the computer recognized you want power to the rear wheels and reduces the battery charging. It also reduces charging when the engine is cold or about to stall. It is not just RPM based any more.

LuckyGuy (36138)

Nice, thanks again @worriedguy.

A bit depends on the type of battery. NiCads only put out 1.2 volts per cell. Most car batteries are lead-acid though. Also, the voltage drop as they discharge various. NiCads hold a stay voltage whereas some other batteries drop in a linear fashion as they discharge.

jerv (31032)

@worriedguy: Wow—you mean the alternator output is controlled by the engine computer?! Who knew?

I was under the impression that NiCd’s, Li-ion, & NiMHs were “trickle charged” with a small, steady inward current; while lead-acid cells were “float charged” with a slightly higher constant potential difference. Is that right? Does it matter?
A little learning is a dangerous thing.—A. Pope.

Also, I learned long ago that it’s practically meaningless to apply a voltmeter to a battery unless it’s under some sort of load. Otherwise its internal resistance will give you a screwy reading.

gasman (11296)

As an ex-R/C car junkie, I am no stranger to charging NiCad packs in 20 minutes or less, and I’ve heard of a rather large Li-ion pack (for an electric car) that could be charged in three seconds if hooked to a suitable power supply.

Trickle charging is healthier as it doesn’t create the heat that quick-charging does; *very*important for most Lithium packs.

jerv (31032)

And even more important for a Lithium Polymer

@gasman Yes, the alternator/internal regulator combination is also controlled by the engine management system. As the ECM (or ECU if you are a Ford guy) got faster and more powerful, the engineers figured out that they could squeeze out better fuel economy, better performance, and lower emissions (the holy trinity of automotive engine engineering) by controlling the alternator. You could give it higher output power but only have it work under certain conditions. This makes the “ALT” or “BAT” warning light function more complicated. It knows the required voltage at all conditions and moves the threshold continuously. Most cars have it all integrated into the Check engine light.

Just for fun, hook a good volt meter on your car battery so you can see it from the driver’s seat and go for a drive. You will be quite surprised. The voltage is not constant.
And – VoilĂ ! – Better performance, better fuel economy, lower emissions.
Hug the next engineer you see.

@jerv You’re correct. I should have specified AA Alkaline batteries in my example.

LuckyGuy (36138)

Thanks, @wg—you are a wealth of information .

gasman (11296)

@gasman I worked on this stuff for many, many years – anything to save you gas.

LuckyGuy (36138)

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