Fuse Size for Battery Backup

I'm working on a 12V battery backup with a Resmed converter for a ResMed AirSense 10. What would be the ideal auto type fuse size for this purpose coming off of the battery? I'm thinking 10 amp, however I would like suggestions.

Put down the tools.
Step away from the bench.
Go back to school.
You are simply not ready to do this stuff.

TennesseeCattleman wrote: ↑

Tue Jul 13, 2021 9:47 am

with a Resmed converter

The specs for the converter are here:
https://document.resmed.com/documents/p ... lo_mul.pdf

Technical specifications
Fuse type Fast blow, low breaking capacity
Input 12V/24V 13/6.5A

The ResMed converter has a cigarette male plug.
You just need one of these alligators to female socket with at least a 15 amp fuse.

Resmed has a 20 amp in line fuse. Seems a little on the heavy side for 14 ga wire and a 90 watt machine, just by doing the math a 10 amp should work well, 15 at the most coming off the battery. Has anyone had problems with a 10 amp fuse blowing when just when running this machine under normal conditions off of a 12 volt battery?

10 amp should work. According to the National Electric Code, 15 amps should be the maximum for 14 AWG wire. But the NEC does not apply here.

TennesseeCattleman wrote: ↑

Wed Jul 14, 2021 6:18 am

90 watt

You need the fuse on the input not the output.

GrumpyHere wrote: ↑

Tue Jul 13, 2021 10:47 am

The specs for the converter are here:
https://document.resmed.com/documents/p ... lo_mul.pdf

How do you get to the fuse in the AS10 converter? I guess I need to look at the fuse and get a spare like it. I thought my setup was complete, but now - a backup fuse. If it's not one thing, it's another. CPAP life.

GrumpyHere wrote: ↑

Wed Jul 14, 2021 9:36 am

TennesseeCattleman wrote: ↑

Wed Jul 14, 2021 6:18 am

90 watt

You need the fuse on the input not the output.

The fuse will be between the battery output and the converter input. lol

Air Jordan wrote: ↑

Wed Jul 14, 2021 10:43 am

GrumpyHere wrote: ↑

Tue Jul 13, 2021 10:47 am

The specs for the converter are here:
https://document.resmed.com/documents/p ... lo_mul.pdf

How do you get to the fuse in the AS10 converter? I guess I need to look at the fuse and get a spare like it. I thought my setup was complete, but now - a backup fuse. If it's not one thing, it's another. CPAP life.

On the cigarette lighter plug, at the tip where the metal plunger comes out of the plastic housing, that plastic piece that holds the metal plunger unscrews from the main plug body (BTW - for future reference this is the location of the fuse on any cigarette lighter plug I've ever used). It's a little difficult to get off - I used pliers with a cloth to avoid gouging the plastic. When unscrewing this hold it all upright else the fuse and *the tiny spring under it* will drop on the floor. Fuse holder lists this as the type to use: F20AL,250V. So it would be something like below - Fast Blow - 6x30mm:

https://www.amazon.com/Acting-Ceramic-F ... 21&sr=8-11

TennesseeCattleman wrote: ↑

Wed Jul 14, 2021 11:10 am

GrumpyHere wrote: ↑

Wed Jul 14, 2021 9:36 am

TennesseeCattleman wrote: ↑

Wed Jul 14, 2021 6:18 am

90 watt

You need the fuse on the input not the output.

The fuse will be between the battery output and the converter input. lol

The converter outputs 90 watts while drawing 13 amps on the input side.

But if you enjoy replacing the fuse frequently, it's your build.

I'm not a electrician, wouldn't likely need to ask this question about fuse size if I was a electrician. I have read many posts on this forum and others calling for anywhere between 7.5 and 15 amp fuses for a Resmed cpap battery backup.

I think the CPAP machine can load up 90w, or the 12v DC converter out put up 90W. So the 12V battery at full charged is 12.6V, with 90W it would draw less than 10A. (W=V x A). you can get 15A fast burn also ok. I bought KFD DC car charger travel adapter, it rate 12A input. So I used 15A regular fuse. It's never broken on me. Hope this help.
Tony.

Tony415 wrote: ↑

Thu Jul 29, 2021 2:46 pm

I think the CPAP machine can load up 90w, or the 12v DC converter out put up 90W. So the 12V battery at full charged is 12.6V, with 90W it would draw less than 10A. (W=V x A). you can get 15A fast burn also ok.

The CPAP machine itself may draw less than 10A.
But you and OP keep ignoring the ResMed converter between the battery and the CPAP.
The fuse is for ALL the equipment drawing current from the battery.

15A fast burn is ok.
It protects a 20A fuse within the DC converter according to the specs.

The DC converter is specified by ResMed docs as drawing up to 13 amps at 12V.
The fuse used should support at least 13A plus some margin.
A rule of thumb is 30% margin.

So 7.5A and 10A fuses may work some of the time but be prepared to replace them more than necessary.

Thanks GrumpyHere for explaining the power draw of the converter, will try a 15 amp fuse.