Measuring current draw on CPAP for battery calcuilations

[OMyMyOHellYes]

Well, in the spirit of folksuses that have wondered how much current their CPAP draws in some threads over the last couple of months...,  It has been a long time and I didn't document well the last time I did any measurement, so I thought I would redo.  (I also found a steal for a new multimeter on Craigslist and I thought this would be a good project in which to play with it....)  

I will measure on three or four non-consecutive nights (all night long) and record the average current draw (amps) for each night, both with and without the humidificator attached and running on a setting of "2", which is my normal setting as of late.  Then I'll average the three or four and call it close to done.

Now some of you may remember that I have a simple home-made UPS setup for my CPAP which includes a 35AH 12V battery, a 10 or 12 amp Astron 12 volt power supply, and a West Mountain Super PWRgate that serves dual purposes as both an automatic switch from mains (the Astron 12 V power supply) to the backup battery (and back when mains come back online) as well as providing a smart charger to keep the battery always charged/maintained. all this is hooked together using 12 ga zip wire and Anderson PowerPole connectors.

So, if anyone is still reading, I built a rig to hook the circuit up through the multimeter for measurement.   (Remember, when you test current (amperage), you have to break the circuit and run it in series through the meter, so all I'm really doing is providing a way to insert the meter into the positive side of the circuit powering the CPAP).  I started with a three foot long stereo speaker cable with banana plugs on both ends that I ordered from Amazon.  I cut that in half and added a PowerPole connector to the cut end.  That wire assembly will go into the multimeter.  Then I made another assembly with zip wire to insert into the positive side of the circuit.  It will go between the output from the Super PWRgate and the CPAP, plugging into the exixting PowerPole connecotrs.  It has a standard PowerPole connector on one end.  On the other end, I put two PowerPole connectors, both of these wires go into the positive (red) side of those two connectors.  Then I cut a short piece of wire and bridged the two empty black connectors.  It will also show how much excessive drain the humidificator adds.

I have attached pictures of this set-up if anybody cares to see the test apparatus.

I will start my testing tonight and run it for six-ish nights sometime over the next couple of weeks and report back with results.  Note, these will be specific to me, on the PR System One 560 at my pressure range.  And I guess, average the average pressures from Sleepyhead as a point of reference.

I'll end my rambling.   Y'all sleep good.

OMMOHY

   

   

[OMyMyOHellYes]

Oops.  My bad.  Somebody can please delete this reply? if they care to...

OMMOHY

[OMyMyOHellYes]

Night one - 

.778 amp average draw with humidificator set to "2", so the 35 AH battery I use as a UPS backup would yield 30 hours (4.3 seven hour nights) of effective supply if I include a 50% safety buffer in the calculation. That was lower than I expected.  

It will be interesting to see how much the humidificator draws (I can get 3-4 nights out of a single tank filling if I don't pull it out and refill.)  I don't know how much the thing is cycling at that setting, but it may not be a lot.  Also, I don't use a heated hose.

OMMOHY

[aquanaut20]

Would it not have been easier and safer to use an AmpClamp?

...  Philip

[silversnore]

there is hardly any amperage to deal with and the additional cost of the clamp could be 50 bucks easily. Plus it is not nearly as much fun as OMMOHY had doing it his way!!!!!

[OMyMyOHellYes]

Would it not have been easier and safer to use an AmpClamp?

...  Philip

I don't see any way that it is safer than using a multimeter.  In this case you are talking 12 volts DC at 750-800 milliamps.  

Most of the clamp meters on the market are AC (yeah, there are AC/DC variants).  But I see them as WAY specialized tools aimed at safely working far higher amperages and voltages.  And primarily AC current (is "AC current" redundant?).   I see them as a way to see IF there is AC flowing in a line and IF it is deadly before you go to working on it.  I personally think they're "ballpark" meters.  I don't think most of them really even go into milliamp ranges.  And they are way expensive.  

And a good multimeter provides lots more utility to me and most other folks.  I know lots of hams that use multimeters all the time, but I can't think of anybody I know that uses a clamp.

My meter setup, without the end wiring assembly, makes it way easy to check open voltage on my other standby batteries or radio power supplies:  just plug in the meter to the open end of the PowerPole-terminated wire that comes from the battery.  Works on my radio power supplies nicely too.  And it can test all the AA batteries I have lying around to see if they are new or dead.

Just a few thoughts.

OMMOHY

[OMyMyOHellYes]

Oh - other thought - are there commonly available DC clamp meters that will record average current as current swings up and down with each breath ??

OMMOHY

[OMyMyOHellYes]

Three nights in - predictable but interesting results for the use of the humogrifricator.  The rest of this test I intend to "flesh out" my two primary settings:  Heat setting "2" and setting "0".  What I have to this point for those settings is data that I consider interesting, but as each night it different, I want a little bit deeper data on the settings I use.  I want to see how "tight" the numbers are for each setting over time. 

For useful battery capacity, we use the stated battery capacity (AH) of the battery and divide that by 150% of the observed/expected draw.  That additional 50% is intended to allow for for sub-optimal charge state of the battery and/or degraded capacity over time (they lose capacity over time regardless of care and maintenance):

• Current draw for "0" heating setting was 0.438 amps.  That would give me 53+ hours/>7.5 nights use (w/ 50% margin)
  
• Current draw for "2" heating setting was 0.778 amps.  That would give me 30 hours/ >4 nights use (w/ 50% margin) 
  
• Current draw for "4" heating setting was 2.027 amps.  That would give me 11 hours/barely > 1.5 night use (w/ 50% margin)
  

As expected, these validate the blower itself is a pretty thrifty user of power.  The humogrificator is an electricity guzzling hog.

Side note:  last night I bumped humidity setting to 4, a setting a consider ridiculously swampy for purposes of this test only.  I didn't find it at all comfortable compared to lower settings/no humogrificaiton.  So yeah, now I can say I tried it.

OMMOHY

[silversnore]

I have a clamp for measuring amperage. I have large electric dc motors that draw in excess of 100 amps, and my multi meter will not accommodate those loads. I would not be without a multi meter. It is the best diagnostic tool period. Everyone should have one and know how to use it.

[OMyMyOHellYes]

Yeah, clamps are good for big heavy applications.  Not so much so for more precise measurement unless you invest a spitload of bux in some very specialized kit.

Multimeters usually won't handle more than 10 or 20 amps, and those loads are usually time limited to very short periods.

I have found having a good quality multimeter around to be a good thing.  (Now it is good, quality multimeterS since I found the NIB Agilent (was HP Test & Measurement Division) meter on CL for a screamin' steal).  There are some reasonably good ones out there for reasonable prices (~ $50-75 - I would avoid the el cheapo varieties from places like Horrible Freight)

An Aussie EE named Dave Jones has a blog with several meter evaluations, teardowns and comparisons.  Googleate "EEVblog #91 $50 Multimeter Shootout" if you want a peek; For slightly more upscale meters Googleate "EEVblog #99 $100 Multimeter Shootout".  Warning.  Guy makes the Energizer bunny look like a sloth.

OMMOHY

PS
For Dave's video on $25 multimeter Googleate "EEVblog #1007 Is a $25 Multimeter Any Good?"