CPAP Battery Life?

[cpappy]

Hi everyone,

I'm considering getting a battery pack for my CPAP system for when I go travelling later in the year. My concern is about the length of time that the battery pack will actually last for. I'm using a System One REMstar Pro.

I'm interested in this Super CPAP Battery, but it says that it has a capacity of 266Wh (approx 22Ah).

The manual of my system says that it uses 5.0A when powered by 12V. Now 22Ah / 5 is about 4.5 hours of life (nowhere near enough). That page says that it will power my machine for at least 7 hours. Am I working something out wrongly?

In fact the Philips Respironics battery only has 14.4Ah and claims to power my machine for 20 hours. 14.4 divided by 5 definitely doesn't make 20!!

Can anyone tell me how battery life is calculated, and what I'm doing wrong to get the numbers I'm getting?

Many thanks!

[Tigers Fan]

Maximum

Hi onemoresolo

Welcome to the forum.

I think you'll find that 5A is the maximum draw of your machine running at maximum pressure continuously. If you are running at 5A, might be an idea to abandon camping trips and check into hospital!!

Your calculations are correct for Maximum power draw but for typical power draw the makers numbers are correct. What constitutes 'typical' is known to them alone, probably.

By direct ratio, your 22Ah battery would give 30.56 hours - and sounds even less likely than their figures.

If you 'Search' this forum - uppermot green bar on this page - there are several threads on camping, batteries, night fishing, boating, camper vans and the battery-using-CPAP-like.

TF

[cpappy]

Hi Tigers Fan,

Thanks for your reply. I know that in reality my machine is never going to draw 5A, but my concern was how far away that was from how long they say the battery life will last.

They state 5A as a maximum, but for my machine to run as long as they say on their own battery it would have to draw an average of 0.72A - little over 10% of the maximum.

It just seems strange that to go from standard use to absolute maximum, the increase in power usage is almost 10-fold.

Something just doesn't seem right about it, I'm sure the battery does last as long as they say (otherwise there would be plenty of bad reviews, but there are none!) so I'm just wondering why what's written on paper is so far from reality.

I wonder if there's some sort of power meter that I can see exactly how much energy my machine is using when running off of 12v....?

[Tigers Fan]

Ammeter

You can buy a multi-meter for not a lot of money at Maplins and the like.

I agree, their figures seem optimistic.

But you don't need to reinvent the wheel! other forum members have solved the problem and posted.

TF

[g3az66]

Battery Life

Hi there,

I have the exact CPAP machine and the very battery you are wanting to buy. I used the combo on a recent camping trip. I used it for 4 nights with about 7hrs usage per night.

The battery itself has 4 led's that display the remaining charge. After 4 nights it was still displaying 3 led's. Very impressive if you ask me! So I reckon it would do a week easily. I did not use a humidifier with it....

Hope this helps and if you need to know anything else then fire away

Gary

[cpappy]

Thanks Gary, that's reassuring to know!

Clearly the maximum power usage figure is way above what is used in reality, which is why it can run for so long on a battery that by their figures would only run for a few hours.

It would be nice if manufacturers could put more detailed power consumptions figures in their manuals rather than their very conservative maximum figures (although I'd love to know under what circumstances it would draw 5A when on average it's about .5A!). At least that way you'd be able to work out more accurately what sort of battery you'd need to run it, and how long it will last.

Thanks again for the info, I can now order the battery safe in the knowledge that my travels will apnoea-free

[Tom @ Intus]

Thanks Gary, that's reassuring to know!

Clearly the maximum power usage figure is way above what is used in reality, which is why it can run for so long on a battery that by their figures would only run for a few hours.

It would be nice if manufacturers could put more detailed power consumptions figures in their manuals rather than their very conservative maximum figures (although I'd love to know under what circumstances it would draw 5A when on average it's about .5A!). At least that way you'd be able to work out more accurately what sort of battery you'd need to run it, and how long it will last.

Thanks again for the info, I can now order the battery safe in the knowledge that my travels will apnoea-free

Hi onemoresolo,

Welcome to the forum!

As you've found out, manufacturer "maximum draw" values are often very conservative, and quite far from regular use. The System One rating is particularly high though. The M Series was listed as 3A, and despite the System One using an almost identical amount of power during regular use that has been given a maximum of 5A. For all intents and purposes, in reality they use the same amount of power.

Most CPAP machines are now very efficient and don't need huge amounts of power. Both of those batteries you mentioned will power almost any CPAP machines comfortably for at least one night, and many for longer than that.

The battery run times are always calculated through regular usage, rather than the maximum provided by the manufacturers, simply because they do not provide accurate representations of battery life. We do often get asked "how can the maximum be so far from regular use?" and it's not easy to give an answer in truth. I don't know how these maximums are calculated, but the important thing to remember is that the System One, and most CPAP machines, will use considerably less power than the manual would suggest!

Also, thanks to both TF and Gary for your help in this thread

Tom @ Intus