Road Trippin' - Do I have a 300W Inverter that Doesn't Suck? 

200W on a 12V inverter is going to want to pull close to 20A, and voltage drop matters, as the input voltage drops, the amps have to go up farther. Connect it directly to a good battery with beef wires, then it's valid test, otherwise your method is useless.

They're all shutting down due to low battery voltage. You need much thicker wires (they're drawing more current than that wire is probably even rated for), and you probably need to have the engine running as well.

I have a 300W inverter as well (okay, it outputs 230V 50Hz, but watts are watts are watts). It came with a lighter socket plug and some battery clamps. The manual and even the sticker on the inverter says: 300W only when hooked up directly to a battery, on the lighter socket you're limited to 150W. You need really beefy wiring on the 12V side. On a friend's camper van, he wanted to be able to use his Saeco fully automatic coffee machine. So we installed a 2kW pure sine inverter. We hooked it up with 2 AWG cable directly to the auxiliary battery. It was able to make two cups of coffee on a fully charged 120Ah battery, then the inverter shut off due to low battery.

I think the 12v socket you are using is your downfall, You would be better off with a set of chunky terminals, or connecting direct to battery if possible.

True he need north of 20V DC to get the 300 watts from the inverter. However, his 15A fuse never blows so the inverters couldn't do 300W if they did have 20A DC in.

Have you tried this test with the car running? It may be that the battery is unable to keep up and so the inverters are falling on their faces. If you have a constant input of power from the car, the results will likely be better.

As others said, not getting that current out of a Cigg lighter socket. Hook it direct to battery. Do you jump start your car through the cigg socket? Lucky to get 10 amps , let alone 15, through the cigg socket.

The rule of the thumb is, if it comes with a permanently attached cigarette plug, it's not gonna put out more than 100W. It's just basic math and liability consideration with the cigarette plug. The big numbers on the sticker is nothing but marketing, if the number is small it won't sell, so they make it big. Of course, if it's UL tested, the numbers are going to be more usable, but hey, people don't like to pay for small numbers.

With switch mode power supplies the frequency and wave shape doesn’t matter at all. Switch modes have a rectifier as the first thing after the power button more or less. You can run them off 170V DC - that’s what they internally run on when you plug them into 120V mains. The most efficient way to run a 120VAC-rated switcher supply off a car battery is to connect 14 of said batteries in series, for about 170VDC nominal ;) No inverter needed. 😅 The simplest and most efficient inverter is a 100Hz mosfet-based half-bridge driver plugged into the secondary of a 120V to 24V center tapped transformer. You’ll get something square-waveish without load out of the primary, and that’s perfect for switchers and for transformer-based supplies alike. You can put one together from junk parts for next to nothing and it will beat the pants off any “modern” inverter usually. It’ll be unkillable, require no forced air cooling, and will outlast you. Of course a few more parts are needed to make it practical, but that’s the basic idea. The old switch mode supplies that have a voltage doubler, ie. switchable between 120V and 240V, will require to be set to 240V and supplied with 330V DC. The voltages can be +3% to -10% around those mentioned above. So, whatever problems you had with switching supplies had nothing to do with waveshape etc. They weren’t getting a high enough peak DC voltage to get their input capacitor high enough to keep the supply going. That’s all. Supplying DC to a supply with a direct mains connected transformer will saturate it and blow the input fuse as expected. For a 200W inverter you’ll want 6AWG wires going from inverter to battery terminals. Forget about the stupid lighter socket. It’s a fire hazard and was never rated for as much current as a 200W application will use, never mind 300W.

Confused as to why you say that switchmode stuff cares about frequency/sinewave shape a lot. My understanding was that since line voltage is immediately rectified (after a bit of filtering maybe) you could pretty much supply them with DC and they wouldn't care less

your 12v socket is only rated for 150 Watts you will need much better wiring and connection to be able to get 300+ watts out of any of them units 12v at 300 watts = 25 Amps and that's if these were all 100% efficient and they are prob all more like 50-70% so you would need even more then 25 amps to get 300 watts out like 30-35Amps this is 100% user error

Light bulb filaments are pretty unique loads because they are effectively total short circuits until they warm up. For example if you tried to use a light bulb to replace a speaker load, you will almost immediately trip the amplifier's short circuit protection. I believe that's likely why these are cutting off. Nobody is designing inverters to run AC light bulbs because it would be kind of stupid to do that in the first place. You need some pure resistive loads to test these properly.

You really should be using a scope meter (even a cheap one) if you want to have any idea on the output of these inverters. as you can see a kill-a-watt can struggle with finding the zero crossing transition of a modified sine inverters (and leading to the insane frequency readings that change over load) you also have a low DC voltage problem, you could help this some by bonding the negative to chassis ground at your outlet, running your engine alternator, taking out the secondary fuse from your circuit, etc

Here is the problem as I see it.....your car battery might not be able to supply the current......or also you are putting the inverter to an extreme resistive load.....in other words you are over working it......300 watts is mainly to split it up to many resistve loads........not just one........I may be wrong but the inverter potentially is drawing a lot of current from your battery to produce 300 watts......more than 25 amps??......simply put if your car battery is weak.....just connecting an inverter to it might overload the battery.....how much current is the inverter drawing from the battery....also if you are on a max 15 amp line......I can only see the inverter output being able to supply 180 watts max...that is why no issues with 100 watt bulb...

Dude, you are running a 386 from a car?

Insufficient current on your 12 volt side probably due to cable resistance. Regarding the kill a watt, I dont have one because they're not available in the uk but have a couple of different brands. In both cases they use a capacitive dropper to power their circuitry in series with a safety resistor. I've found that if you plug them into a modified sine wave inverter the safety resistor burns up after a few seconds. I've had to repair them twice. I'm assuming the capacitive dropper is passing a high AC current due to the sharp edges of the square wave.

Dude charge your battery or change your junk battery

I think they are all crap. I would rather bodge one from an old UPS.

Take it from a guy that does low voltage by trade and has messed with Solar panels, had a full desktop in a car in the early 2000's and use to do automotive audio and hear me out. The wiring is too small, 12AWG would be alright for a small audio amp but for that distance you should use 8AWG. Also it wouldn't hurt to run a ground, granted the body is the ground but it's to ensure a proper ground. Since this is a car I would also have to factor in the Alternator output and what the car is already consuming current wise. Also ditch the cig lighter and breaker and just use a fuse. Put in a fuse that suits the inverter. Remember Watts divide Volts equals Amps. Your best bet is to get a proper inverter and toss in a time delayed relay for the ignition so the car can start up and a couple of seconds later the inverter powers on. This way the car battery doesn't have to deal with so much inrush current at once. The cig lighter adapter is used for temporary use for low wattage devices such as phone chargers, laptops and stuff like that. Anything else that consumes a constant 100watts should not use a cig lighter. I see trade guys ignore this and wonder why their cig lighter adapter feels hot to the touch and sometimes is melting. A floating AC neutral for a home is where the neutral isn't balanced and is a very bad thing. However for this case the AC neutral and DC ground are not shared so the AC neutral and DC ground are isolated. In all your best bet with your current setup is I would put the inverter closer to the battery to get rid of that voltage drop. Put the inverter on the passenger seat or floorboard. maybe double up on the 12AWG and be sure to have the inverter off until the car is running and don't forget to switch it off before turning off the ignition. Lots of those cheap inverters don't have any protection on sudden power off states and can spike the device that it's plugged into. When it comes to cheap inverters just double the output rating to get the output you want.