# 4500 Watt inverter to power a laptop

1. Aug 26, 2007

### garrambide

I hope somebody can help me understand what is going wrong. I have a Gateway laptop with an ac power supply rated for up to 4.7 amps. I have an inverter rated for 4500 watts and 4.2 amps that I bought from Sam’s Club. It was made by Black and Decker. When I plug my laptop into the inverter I pop an internal circuit breaker that cuts off the electricity to the ac outlets. The cooling fan is still working and turning but the green indicator light is blinking indicating the breaker has tripped. I am told that I can calculate the Wattage required by multiplying the voltage 120 by the amperage 4.7. This gives me 564 watts. So I have come to the conclusion that something is wrong somewhere but I can’t figure out what. Any and all help would be appreciated

2. Aug 27, 2007

### chroot

Staff Emeritus
Think about that for a second: if your laptop really consumed 564 watts, it would be about as hot as six 100-watt lightbulbs! You wouldn't even be able to touch it, it would be so hot.

Your laptop's charger takes in 120 V and puts out (usually) 16 V. Your laptop consumes 4.7 amperes at 16 V. This is about 75 watts.

Your inverter is capable of delivering 4500 watts, so obviously you are not overloading the inverter. What's probably happening is that your laptop's charger has a lot of capacitance. When you plug it into the inverter, it draws a very large current for a short period of time, and that's blowing the inverter's (perhaps overly sensitive) circuit breaker.

What happens if you plug the laptop charger into the inverter BEFORE turning the inverter on?

- Warren

3. Aug 27, 2007

### garrambide

If the battery on the laptop is fully charged and I turn on the laptop and wait until xp has finished booting up to turn on the inverter it will usually last 60 to 90 minutes before the breaker trips. If electricity is interupted to the inverter anytime the breaker starts tripping right away and doesnt last for more than a few seconds before tripping.
The inverter is rated for 4.2 amps and the laptop charger is rated for 4.7 amps. I assumed the laptop was pulling too many amps (.5) than the inverter is able to supply. I guess I am just having trouble with the fact that the inverter is suppose to supply 4500 watts and yet is only rated to supply 4.2 amps. Oh well I going to connect a meter on it when I get home. Thanks for your time. George

4. Aug 27, 2007

### mgb_phys

As chroot said, the laptop is pulling 4.2Amps at 16V out of it's supply/charger so (ignoring losses in the PSU) it is only using (16/110) * 4.7 = 0.65A at 110v from the wall or the inverter.

5. Aug 27, 2007

### chroot

Staff Emeritus
If the inverter is rated for "4500 watts," yet trips after hours of use by nothing more than a laptop charger, then the inverter is broken.

As I've said, and mgb_phys has repeated, the "4.7 amps" used by the laptop charger is at a different voltage (and thus corresponds to a different amount of power) than the 4.2 amp rating on the charger. You cannot compare the amp ratings this way.

Either way, a 4500 watt inverter should be able to use way more thasn 4.2 amps from the mains. It sounds like it might be a gimmick.

- Warren

6. Aug 28, 2007

### Paulanddiw

Can you operate anything else, instead of the lap top, with your inverter. Will it operate an incandescent light bulb?

7. Aug 28, 2007

### Paulanddiw

What are you using to power the inverter? Are you using an automobile battery? or something else?

8. Aug 28, 2007

### Paulanddiw

4500 watts at 12 volts, e.g., a car battery, will require 375 amps, or so. The wires to carry such current usually look like welding cables. The connectors are usually a cast lead clamp that clamps to the battery post.

If you input wires are much lighter than this, the inverter may have trouble starting.

9. Aug 28, 2007

### chroot

Staff Emeritus
The inverter's not going to draw 4500 watts when nothing's plugged into it! It's supposedly capable of converting 4500 watts of power. If you plug a laptop into that draws 75W, then you're probably only consuming 80W of power from the battery.

- Warren

10. Aug 28, 2007

### Staff: Mentor

I'm guessing the 4500 watts is in error. 12*4.2= 50.4. With losses, maybe, that could be 45 watts.

What is the make and model number of this inverter and how much did it cost?

11. Aug 28, 2007

### garrambide

OOP I lied, that's what happens when I do things from memory late at night. It is rated for 500 watts not 4,500 sorry for the disinformation. I bought this inverter from Sam's Club for about \$40. It is a MAXX sst (soft start technology)made by Black and Decker. This is the second inverter of the same make and model that I have tried. Both behave the same way. They are plugged directly into the cigarette lighter. I have not tried to plug anything else into it but I am going to try it next.

12. Aug 28, 2007

### chroot

Staff Emeritus
Are you actually running the car's engine while the you're using the laptop? It's possible that the inverter has protection circuitry that stops it when the battery voltage is getting low.

- Warren

13. Aug 28, 2007

### Staff: Mentor

Good point. I have a Targus inverter for my laptop (90w?) that starts beeping at me after an absurdly short time on the car battery. Though usually when I'm using it it is winter, so I'm not sure what the voltage actually is...