Voltage inverter question

In My Humble Opinion
1

I got a new voltage inverter, 1000 watts continous 3000 peak. I got this for the rare event of a prolonged blackout to keep the fridge powered. I tryed it out and it works. Thanks to the little watt meter built in the fridge takes 1200 W during startup and 190 watts to run w/ the compressor, well within the capacity for this little wonder.

The question is what is the best way to use it. Should I run the car continously or Should I cycle the car on and off (and for how long each)?

How many watts (additional to the fridge) can I safely draw assuming I would like to power a few other items?

How many watts can the car power at idle (so the battery stays charged)?

2

An automobile alternator is at rest when idling. Engine speed must be raised to handle additional electical load, or the storage battery will slowly discharge.

Wattage is a function of amperes x volts, yet the watts of which you’re speaking are on the secondary of the inverter transformer, after the electronics.

Too many variables exist to allow calculation of duty on/off cycles.
It is possible that unless your inverter has overload protection, taxing the limit may cause internals to fry. Reread the data plate and/or paperwork to ensure that the inverter is ok with compressor loads-that is the high starting wattage which you see.

3

It doesn’t mention compressors, it does have things like microwaves adn submersible pumps, also a long list of tools. The plate of on the fridge says 7.5 amps max which should work out to 825 W. The inverter claims to be 90%efficient and it’s a long cable run from the car to the fridge, perhaps that’s why the gauge does show the starting surge at about 1200 W.

The full specs of the inverter are
output continous = 1000W
output surge = 3000W
AC RMS = 110C ± 10%
Output wave = filtered modified sine
DC input voltage = 10.5 to 15 ± .5 V
Low Bat shut down = 9.5v ±.5
freqnecy = 60
efficency = 90%
no load current = >1A
Over temp protection = 55C ± 5C
cooling fan = y
overload protection = yes
i/o short circuit protection = y
a/c output socket = GFIC

It does not say anywhere that any item should not be run off it except where the item exceeds the output. It also mentiones that it does handle the start up surge buy slowly ramping up the voltage during startup, which it implies is a problem w/ other units.

As for the RPM’s what would be a good range for running this thing to recharge the batts for a 4,6,8 cylinder engines?

4

Overall inverters aren’t real efficient. That 90% figure is at full load with good input voltage. At lower load or input voltages it might be more like 50%.

There’s no reason you can’t use the inverter up to its full rated 1000W continuous-duty capacity, PROVIDED you’re mindful of the startup spikes of motor-powered items. DON’T turn on 3 motorized things at once.

Trying to compute how to operate the car is pretty difficult unless you’ve got a calibrated ammeter in the charging circuit, something no cars have from the factory. So here’s another method that’ll get us near where we’re going.
Get a decent quality digital voltmeter and make some measurements as follows. You’ll need to record values to 0.1 volts accuracy.

  1. Put the meter across the car battery terminals with the car engine off and the inverter off. Make some firm connections here; we need excellent conductivity and repeatablility. If you have a top-post battery you can run a short (1/2") sheet metal screw right into the top of each lead post; be gentle, lead strips VERY easily.

  2. Start the car and run the engine at various RPMs, say idle, 1000, 1500, 2000, 2500, 3000. Leave the inverter off. Having an RPM gauge is essential; if the car doesn’t have one, you can get a shop tool for that at any car parts store. Measure the voltage at each point. You should see a step jump somewhere between idle & 1.5K as the alternator comes on line. Running just the car without the inverter with the engine RPM below that point will simply discharge the car battery.

  3. Turn the inverter on with some small steady load (200W of light bulbs) and repeat step 2. You’ll probably see a slightly lower voltage at idle and the jump point where the alternator kicks in will occur at a higher RPM. Once the alternator has kicked in, the voltage will be the same as it was in step 2.

  4. Rrepeat step 3 several times with increasing loads on the inverter, up to the rated 1000W. As the loads increase you’ll see slightly lower voltages at idle and the jump point continues to require every higher RPM. Again the alternator-on voltage should be the same as step 2. If, as the wattage load increases, you start to see the alternator-on voltage dropping, you’ve reached the maximum load the alternator can carry; more RPM won’t help.

Now you’ve got the data. I’d expect the unpowered batttery to be 10-11 volts and the alternator output above the jump point to be 13-14 volts.

For whatever load you’re running on the inverter, you need the engine RPM to be at/above the voltage jump point or you’re simply draining the battery into the inverter while maybe offsetting a smidgen of the drain with alternator power.

To make a pratical emergency system intended to last more than a couple hours, you’re gonna want to permanently wire a digital voltmeter into the car and put some sort of throttle on it so you can set the RPM and hold it wherever required. For the full 1000W inverter output, I’d bet the engine RPM needs to be 2000 for a V8 and maybe 3000 for a 4-cylinder. The engine size doesn’t matter directly, but there is a rough correlation between engine size, car size and alternator capacity. Alternator capacity is the real key variable.

Finally, many cars will overheat in warm weather if run at 2000-3000 RPM for a long period without moving. The radiator fan just isn’t sized to move enough air for sustained stationary high RPM operation.

So the idea of using the car as a 24-hour-a-day 1000W electrical source for a 2- or 3-day blackout is probably notso hotso.

OTOH, if you’re just trying to avoid food spoilage for 12-24 hours and not trying to run your house off the car, then the fridge can stay cold enough while running just a few minutes every couple hours, provided you DON’T keep opening the fridge door.

When meal time comes, make a list of what you’re going to need, open the door, get it all out ASAP and close the door. Forgot something? Do without. After the prep’s done, get everything organized and ready put back in the fridge, then open the door, stow it and close the door again. Then run the fridge off the car until the compressor quits and leave the fridge sealed until next meal time. No snacks or sodas.

If you do the fridge this way you can last indefinitely. And the car may only need to be started every 4 or 5 cycles of recooling the fridge. Just don’t run down the car battery or you’re really hosed.