Power Inverter
Power inverter wattage
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Posted on Thursday Oct 10, 2013 at 8:39 PM  
Thinking about buying a power inverter to run 120v ac air pumps on the boat. Is 400 watt continuous power with 800 peak output adequate for most applications?
Posted on Thursday Oct 10, 2013 at 10:14 PM  
It depends allot on what you want to run with it, and how it's going to be connected to the 12vdc source. If you're going to run a low wattage air pump, a few low wattage lights, or run an 150W electric knife, then a 400W converter will be fine - especially if you connect direct to the battery. I'm an electronics engineer and I tested a number converters this year, putting them through the paces to see whats really happening. Some models have lower standby current drain. Some have higher efficiency when they are running a heavy load. Again, it comes down to how big your load is and the quality of the 12vdc wiring. If you're going to connect it directly to a 12v battery posts, then a 400W converter will probably power up to 250W continuously, just fine. If you connect that same converter to a 12vdc cigarette lighter socket, then expect maybe half of that capacity, at best. Or, expect the converter to start out carrying the load, and then a while later activate it's overload buzzer and to shut down.  They say that a cigarette lighter is good to a max of 10 amps. But most cigarette lighters don't have heavy gauge wires going to them, and the wire run from the battery to the socket is a long distance - and that 10 amps spec is pure fantasy.

Here's some notes I made, using bulbs as a load, using different converters:
     Bulb load                 Amps pulled at the 12vdc source
     ------------                  ------------------------------------
     52W bulb - - - - - - - - -  4 to 5 amps
     100W bulb  - - - - - - - -  8 to 10 amps
     250W Floodlamp - - - - - 25 to 29 amps

If you run the converter off the cigarette lighter socket, your load better be very light. 
Which 12vdc/120vac converter did I buy? Cobra CPI 480
It's not the very best - but it's good enough. They are all pretty comparable.
I thought the Stanley converters Lowe's sells to be pretty good quality, and have some neat bells and whistles. 
In my configuration, I didn't want the converter to ever be able to discharge the main boat battery.
Something about the thought of being on the lake (in the dark) with an engine that won't turn over, is unpleasant. 
So, I power my converter using the 24vdc trolling motor batteries.
I bought a dc to dc converter (Pyle PSWNV480) which converts 24vdc to 12vdc. 
The more you practice, the luckier you get.
Posted on Friday Oct 11, 2013 at 3:21 PM  
My lighter plugs are installed with 14 gauge wire but they have 5 amp fuses at the batteries so probably not the way to connect. As you mentioned if connected directly to the battery they have their own safety disconnect so does this mean fuses are not needed to protect from electrical short. Is this a better use of battery power or is using 12 v accessories the best option.

Harbor Freight will have Chicago Electric inverters on sale next week for 21 dollars and my thoughts were to try one for odd appliances such as lights, aerators and small things. I am sure these are not the best products but having no experience with them did I not want invest much in something I may not use much.

Thank you Dan for your feed back.

Posted on Sunday Oct 13, 2013 at 7:25 PM  
I don't think you can hurt any model you buy. One of the cheapest converters I tested, worked really good. All the models I tested had built in overload protection and in addition, half had overload fuses. If the converters don't have a strong 12volts at the input, or if they are asked to do more than they can, they will beep and shut off. It's my guess that you'd pop that 5A fuse if you did any significant load. 14 gauge wire is a really big wire! I'd think the fuse for that would be more like 15 or 20 Amp!

This spring I bought an 120vac air pump bubbler at Academy and I couldn't find a wattage listed on it or it's literature so I put a current meter on it. The current was so low it was hard to even measure! So I would guess the wattage at 10 or 20 Watts. 

Buy the cheap converter and run it off the cigarette lighter. If that's all you put on it, it should work just fine.  Let me know how it turns out.
The more you practice, the luckier you get.
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