Do I need a 24 volt converter for a 24 volt device on my 24 volt system?

[Vanholio]

I'm building a 24 volt system in my van. My CPAP machine runs on 120vac, but there's a DC converter available (for $90!) with a cigarette lighter plug "65 watt DC DC converter." It says the plug can be used in 12 or 24 volt systems. Reading the specs, the adapter has a 12A/250V fuse, input is 12|24V at 8|4A, output is 24V at 2.71A. Must be used in negative grounded systems only.

Why do I need this thing? Can't I just make a wire with a 12 amp fuse to plug it directly into my 24 volt system? Or is the adapter also protecting the device from pulling too much current?

Note that I don't want to fry my $1000 cpap so if I need the cord I need it, I just want to understand why.

 

[John Frum]

I'm building a 24 volt system in my van. My CPAP machine runs on 120vac, but there's a DC converter available (for $90!) with a cigarette lighter plug "65 watt DC DC converter." It says the plug can be used in 12 or 24 volt systems. Reading the specs, the adapter has a 12A/250V fuse, input is 12|24V at 8|4A, output is 24V at 2.71A. Must be used in negative grounded systems only.

Why do I need this thing? Can't I just make a wire with a 12 amp fuse to plug it directly into my 24 volt system? Or is the adapter also protecting the device from pulling too much current?

Note that I don't want to fry my $1000 cpap so if I need the cord I need it, I just want to understand why.

Since it indicates 24 volts I bet its a resmed xpap machine.
The resmed units need a 3 volt supply as well just to be difficult.
It can be bodged but its a bit of a pain.

Is the xpap machine your only ac load.
If yes then it will likely make sense to get the dc apapter.
If no then you will have an inverter anyways so not such a big deal.
Typically inverters are about 85% efficient so that is a consideration as well.
My wild guess is the dc adapter is ~95% efficient.

 

[John Frum]

Why are you building a 24 volt mobile system?
If you have 12 volt loads or alternator charging its probably a bad idea.

 

[Vanholio]

Why are you building a 24 volt mobile system?
If you have 12 volt loads or alternator charging its probably a bad idea.

I've got plenty of AC loads including lightweight lapidary equipment, some kitchen stuff, etc. I'm setting up 600watts solar panels to about 500ah DIY battery, a 12|24 volt B to B, 2000 watt 24 volt inverter. I want to go 24 volt to allow a smaller charge control and lighter wires and a cooler system. No shore charging. Most of my DC loads are 12 volt so I'll have a 24|12 converter for that. I just wondered if I can safely avoid spending $90 on a cable.

 

[John Frum]

I just wondered if I can safely avoid spending $90 on a cable.

Sure just run it off the ac wall wart.

 

[Vanholio]

Since it indicates 24 volts I bet its a resmed xpap machine.
The resmed units need a 3 volt supply as well just to be difficult.
It can be bodged but its a bit of a pain.

Is the xpap machine your only ac load.
If yes then it will likely make sense to get the dc apapter.
If no then you will have an inverter anyways so not such a big deal.
Typically inverters are about 85% efficient so that is a consideration as well.
My wild guess is the dc adapter is ~95% efficient.

Nah, I've got other AC loads in addition to this thing. It's a Resmed Airmini CPAP.
Thing is I normally power the inverter off when I'm not using it. So running a 2000 watt inverter all night when I'd otherwise have it switched off means I'm using power to have the inverter on in addition to the efficiency loss. I mean, I'll have enough battery power for it, I just hate to waste energy.
But it sounds like if I want to run it on DC I'm safer coughing up for the $90 cable to be safe.

 

[John Frum]

But it sounds like if I want to run it on DC I'm safer coughing up for the $90 cable to be safe.

I found this

AirMini CPAP Machine Power Cable (5.5 x 2.5mm male to AirMini)

AirMini power cord

www.bixpower.com

Special CPAP Machine Power Output Cable 5.5 x 2.5mm Male to AirMini

Says there is no conversion.
I assume but you should confirm your ac2dc brick has a standard 5.5 x 2.5mm male plug on the end that is simple 24 volt positive and 24 volt negative.
If that is the case you may be able to make something easily.
You may need a buck/boost converter to regulate the voltage though.

 

[MrNatural22]

Here’s another source for power cords. A little cheaper. I have a respronics machine that uses 12v so my cord was only $30.
looks like resmed stuff is a little more costly.