Thread: help with 380w sinewave inverter

Discovery

Some *rough* numbers to consider:


*600W at 240V is 2.5Amps (assuming Power Factor of 1)
*600W at 12V is 50Amps
*The inverter would be at best 70% efficient, so that 50 Amps becomes at least 70A

Now, I assume that the CPAP is only drawing 600W at the instant of startup, and it would then drop back to 300W based on the F&P rating.

The cable size you would need for that current draw is huge - in the order of 16-20mm CSA (cross sectional area).

I'll bet that the cable on the 12V side of the inverter is nowhere near that large.
The cable from the socket to the battery terminals also needs to be that size.
Smaller cable will result in a huge Voltage drop and the inverter will have a low-Voltage cutout somewhere around 10.8V to protect the battery from over-discharge.
It should also be terminated with lugs directly onto the battery terminals - definitely not a 10Amp cigarette lighter plug.

Don't forget to fuse the 12V supply either, you don't want to burn your truck to the ground if there's a fault.

Regards
Scott

I have an 800W invertor and the supply cables that it came with are nearly as thick as the battery leads which are connected to the battery for starting.

Dave

Is the inverter a pure sine wave?
Do you have the model number from Jaycar?
Does the CPAP need pure sine wave or is it happy with modified square wave?
I have a 300W pure sine wave and it was about $400 to purchase, the modified square wave inverters were much cheaper.
cheers

I think we can assume by the title of his post it is a pure sinewave.
One simple way to test if the inverter is big enough is to just try wiring the inverter straight up to a 12V battery via a short set of thickish leads (by short I mean no more than 30cm ) then try to run your machine off it.

If it works like that you will be OK if you run thick wires in to the cab. Ditch the cig lighter socket and use Anderson connectors or similar. If it doesn't work you need a bigger inverter, but If F&P medical specified that size inverter you can only trust they have the experience to know.

Bearing in mind that if it really is ~ 25 amp constant drain your battery isn't going to last that long if you don't keep charging it.

Do you have the actual specs of the cpap machine ? do these things run constantly or just on demand ?

FWIW I run a RESMED from a (Jaycar) pure sine wave 150W BUT NO Humidifier... that's where the juice is consumed... to get over this I fill the tank with warm/tepid water and turn off the humidifier and wrap the tank with Alfoil...shiny side facing inwards. Stays at an acceptable temp for a couple of hours and provides enough humidity to get me well into la la land .

Electric motors often require a greater current to start than to run, so an inverter that could easily handle the running load of a device may overload and trip out on startup. You need to find out if the max startup current of the machine is greater than the max capacity of the inverter.

Stephen.

if that size inverter has been recommended by the manufacturer of the cpap (whatever medical device that is) we can assume that the inverter is big enough to handle the start up current.

too much voltage drop on the 12 volt side at start up. ditch the ciggy socket. use your jump leads to see if the inverter will handle the star up current, then wire in an anderson plug if that works.

I've got the same pure sine wave from jaycar 380W..

I can't even run one laptop from it. Its 'extremely disappointing'. When I spoke to the guy in Jaycar I said I wanted to run two laptops + a display and he said that the 380 will be plenty.

..but as soon as I plug anything larger than a phone charger in it its goes into protection mode...

looking at one of the laptop power supplies and doing a little maths I realised 240x1.5 is 360... so how the hell did this nob in jaycar think I was going to be able to run 2 of them!

or am I missing something. I used to have a 300W inverter that we used to run 3 laptops from.. but that was a few years ago.. do laptops really draw that much more power now?



(BTW, sorry if this is stealing a thread... but its pretty related)