Hi everyone,
I am a cpap user, this means I use a machine at night to blow air into my lungs as I stop breathing at night without it.
The machine runs on 240 AC mains, which is unusual as normally they run into a laptop changer like thing.
However mone runs straight from the mains.
Its a lowenstein prisima vent 40 p, and according to the manual uses 0.43 amps at 240volts at 100watts.
When we get a power cut I want to be able to run it off a power bank.
These store DC power so I will need an inverter. The one they recommend is 12v DC 200watt max, 550w surge, 180w continuous.
Or I can buy a power bank with a inbuilt 3 pin plug.


So now to my question…
what size power bank would I need to run this for 8 hours?
If it uses 100watts, do I need a 800watt power bank? (Ideal world conditions)
Or because I am converting DC to AC is it different?

how long will something like this run my machine for? https://offyourgrid.ie/product/bluetti-ac180p-portable-power-station/


Hope this makes sense to somebody!!

thanks so much.

 

The power rating of the inverter simply tells you what power it can output, not how long it can do so.

It is the battery capacity that tells you how long it can provide power.

Battery capacity is usually given in Amp hours. It is more useful to know the capacity in Watt hours. To convert Amp hours to Watt hours, you multiply the Amp hour rating by the battery voltage. So, if you have a battery that is 12V at 10 Amp hours, that is 1209 Watt hours.

To get run time from Watt hours, you divide by the power needed. For example, the 1200 Watt hour battery can put out 100 Watts for 12 hours.

Actually, you must de-rate it to about 80% because the conversion from DC to AC is not 100% efficient

So, to answer your question, to run your 100W machine for 8 hours, I would look for a capacity of 1000 Watt hours. For a 12V battery, that would be about 80 Amp hours.

 

The device you suggested with a 1440Wh spec should, in theory, run your CPAP for 14h from a fresh 100% charge. I wouldn't go smaller as you might not always be able to guarantee a 100% charge and the capability of such devices are often 'exaggerated'.

 

Something else to consider is that most inverters produce a pseudo sine wave output (rather than a pure sine wave of more expensive models). The problem is that some motors do not run well (or at all) when supplied with the inverter’s pseudo sine wave.

However I believe that most cpap units use a dc motor, so this should not be a problem for you.

There are cpap units that come with the option of both ac and dc supply operation (given the potential criticality of operation) – maybe if you upgrade your unit with this option, you will only need a battery rather than battery and inverter, but you’d still need a battery charger.

 

Thanks everyone.
I thought I was on the right lines in calculating what I needed.
It was just the DC powerbank to the AC of the cpap machine was making me question myself.
I didn’t know if DC watts, amps and voltage, transfer over the same when converting from DC to AC.
That made sense in my head but not when I wrote it down!
Really appreciate your help.

 

One thing not mentioned so far is the power factor of the CPAP. For a resistive load, the power factor = 1, and the power that must be supplied in this case is volts × amps = watts.

For a switching power supply (inside the CPAP), the power factor will be less than 1 – think of this as efficiency. There are many here who can better explain this, but cutting to the point:

Ampsapparent = Ampsreal /PF

Apparent Power = Ampsapparent × Volts

The power factor of switching power supplies may be as low as 0.65 but designs do exist that bring it close to 1.0.

Learned this the hard way. Driving a ¼ HP furnance blower motor with a UPS on the face of it should require 746 watts/HP × ¼ HP = 186 watts. Nope. It was drawing nearly the full capacity of a 1000 VA (apparent power) UPS. 50 amps from the 24 volt batteries!

And everybody knows that watts/HP = half the year Columbus "discovered" America, right?

 

Just added some photos of the cpap power usage per manufacture booklet, and of a sample powerbank I would buy.

hope it helps