amp cpap

[tim.johnson40@gmail.com]

what brand and model number of cpap uses .4 amp per hour, staying 4 nights without elect. will
be using 36 amp battery and solar charger. thanks, tim

[yardbird]

Depends on pressure. The harder they blow, the more power they suck. What's your pressure?

[Guest]

press-8 cmh20

[nomoore]

what brand and model number of cpap uses .4 amp per hour, staying 4 nights without elect. will
be using 36 amp battery and solar charger. thanks, tim

I just tried my Intellipap Autoadjust for one 8 hour night on my 16ah battery. I measured the voltage when done after sitting for a few hours and it was 12.54 volts. That means it had used about 22% of the charge. It still had 78% left. If it consistently does that I should be able to get 3 nights from this battery and still be above the 20% mark that you're not supposed to go under. I am going camping for 2 nights this weekend so I'll get to see if it is consistent.

My pressure setting is 11.5cm min and 20cm max. My 90% pressure for that night was 13.5cm.

If my one night wasn't a fluke then a 36 amp hour battery and the Intellipap Autoadjust at 8cm should give you 4 nights and not even go below 50% charge. Your battery should last a long time if it doesn't ever go below 50%.

[timbalionguy]

what brand and model number of cpap uses .4 amp per hour, staying 4 nights without elect. will
be using 36 amp battery and solar charger. thanks, tim

I just tried my Intellipap Autoadjust for one 8 hour night on my 16ah battery. I measured the voltage when done after sitting for a few hours and it was 12.54 volts. That means it had used about 22% of the charge. It still had 78% left. If it consistently does that I should be able to get 3 nights from this battery and still be above the 20% mark that you're not supposed to go under. I am going camping for 2 nights this weekend so I'll get to see if it is consistent.

My pressure setting is 11.5cm min and 20cm max. My 90% pressure for that night was 13.5cm.

If my one night wasn't a fluke then a 36 amp hour battery and the Intellipap Autoadjust at 8cm should give you 4 nights and not even go below 50% charge. Your battery should last a long time if it doesn't ever go below 50%.

The way you are ascertaining battery charge remaining is not accurate. Most batteries, including gel cells have a fairly 'flat' discharge, especially during the first third or so of their capacity. 'Flat discharge' means the battery's voltage does not vary with state of charge. Nicads, for instance, are known for having a fiercely flat discharge, and will suddenly fall off of nominal voltage only when nearly exhausted. Most modern rechargeable batteries, like NiMh and LiIon are also quite flat.

The only way to know for sure what your state of charge is, is to measure both voltage and current, and from them compute ampre-hours. There are devices on the market for doing this, used by video camera people and ham radio operators to keep tabs on their (expensive!) batteries. If you know the load is constant, you can calculate ampere-hours as well. It is interesting to note that a lot of modern electronic equipment, including most devices that are using some sort of computer (like a CPAP machine) are 'constant power loads'. That means that as the battery voltage decreases, the load current increases as the power supply compensates to keep the operating voltages constant inside the machine.

If you do get an ampere-hour or DC watthour meter, you might use this on charge as well, to see what your charging efficiency is like.

I would use your machine on the battery you intend to use before the trip, and run it until the battery is discharged. On a good 12 volt gel cell, this is typically 10.4 volts under load (check the manufacturer's data sheet if you can find it, they have a wealth of information about proper charge and discharge of their batteries). You might actually try this a couple of times, as new batteries often increase in capacity as they are used, and older batteries decrease capacity.

I have an IntelliPAP Autoadjust machine as well. I should try this with my batteries to see what kind of life I get. I have some camping trips possible this summer, and I would sure like to be able to use CPAP on them. I also want to know how much operating time I have if we should have an extended power failure.

[nomoore]

The way you are ascertaining battery charge remaining is not accurate. Most batteries, including gel cells have a fairly 'flat' discharge, especially during the first third or so of their capacity. 'Flat discharge' means the battery's voltage does not vary with state of charge. Nicads, for instance, are known for having a fiercely flat discharge, and will suddenly fall off of nominal voltage only when nearly exhausted. Most modern rechargeable batteries, like NiMh and LiIon are also quite flat.

The only way to know for sure what your state of charge is, is to measure both voltage and current, and from them compute ampre-hours. There are devices on the market for doing this, used by video camera people and ham radio operators to keep tabs on their (expensive!) batteries. If you know the load is constant, you can calculate ampere-hours as well. It is interesting to note that a lot of modern electronic equipment, including most devices that are using some sort of computer (like a CPAP machine) are 'constant power loads'. That means that as the battery voltage decreases, the load current increases as the power supply compensates to keep the operating voltages constant inside the machine.

If you do get an ampere-hour or DC watthour meter, you might use this on charge as well, to see what your charging efficiency is like.

I would use your machine on the battery you intend to use before the trip, and run it until the battery is discharged. On a good 12 volt gel cell, this is typically 10.4 volts under load (check the manufacturer's data sheet if you can find it, they have a wealth of information about proper charge and discharge of their batteries). You might actually try this a couple of times, as new batteries often increase in capacity as they are used, and older batteries decrease capacity.

I have an IntelliPAP Autoadjust machine as well. I should try this with my batteries to see what kind of life I get. I have some camping trips possible this summer, and I would sure like to be able to use CPAP on them. I also want to know how much operating time I have if we should have an extended power failure.

My battery is an AGM lead acid battery. Everything I have read says that voltage is a valid way to determine the state of charge. They have a pretty linear drop in voltage. I am basing my measurement on the manufacturers recommendations for determining SOC. I measured the open circuit voltage after the battery sat unhooked for 6 hours.

Here's the chart given by the manufacturer:
12.84v = 100%
12.5v = 75%
12.18v = 50%
11.88v = 25%

[timbalionguy]

My battery is an AGM lead acid battery. Everything I have read says that voltage is a valid way to determine the state of charge. They have a pretty linear drop in voltage. I am basing my measurement on the manufacturers recommendations for determining SOC. I measured the open circuit voltage after the battery sat unhooked for 6 hours.

Here's the chart given by the manufacturer:
12.84v = 100%
12.5v = 75%
12.18v = 50%
11.88v = 25%

You can get away with doing this, to a certain extent with lead-acid batteries because they don't have as flat a discharge as do more modern battery types. However, it is not as accurate as you have been led to believe.

Try putting a load on your battery and measuring the under-load voltage every 15 minutes or so. You will see a quick initial fall-off, and then a fairly steady voltage that will decrease slowly. As you approach end of discharge, it will decrease more rapidly. The bottom line is when you reach minimum discharge voltage, your battery is discharged.

You will have a somewhat more accurate idea of state of charge if you use the data you collected from the aforementioned test. Temperature, rate of discharge, and the tendency of the load to be 'constant power' all have a definite effect on the numbers you collect.

I have maintained lead-acid battery plants for emergency communications purposes for many years, as well as for broadcast camera batteries (They have historically been NiCad, but are now more often LiIon).

Where did you get the figure of .4A for the IntelliPAP? That seems a bit low, based on the numbers in the manual. Perhaps they are including the load resulting from the heated humidifier.

You can make your own current shunt by getting a .1 ohm, 10 watt resistor from an electronics store (get the metal-cased style. They are more accurate in value). Put it in series with the machine, and you can then measure current by measuring the voltage drop across the resistor.

[nomoore]

You can get away with doing this, to a certain extent with lead-acid batteries because they don't have as flat a discharge as do more modern battery types. However, it is not as accurate as you have been led to believe.

Try putting a load on your battery and measuring the under-load voltage every 15 minutes or so. You will see a quick initial fall-off, and then a fairly steady voltage that will decrease slowly. As you approach end of discharge, it will decrease more rapidly. The bottom line is when you reach minimum discharge voltage, your battery is discharged.

You will have a somewhat more accurate idea of state of charge if you use the data you collected from the aforementioned test. Temperature, rate of discharge, and the tendency of the load to be 'constant power' all have a definite effect on the numbers you collect.

I have maintained lead-acid battery plants for emergency communications purposes for many years, as well as for broadcast camera batteries (They have historically been NiCad, but are now more often LiIon).

Where did you get the figure of .4A for the IntelliPAP? That seems a bit low, based on the numbers in the manual. Perhaps they are including the load resulting from the heated humidifier.

You can make your own current shunt by getting a .1 ohm, 10 watt resistor from an electronics store (get the metal-cased style. They are more accurate in value). Put it in series with the machine, and you can then measure current by measuring the voltage drop across the resistor.

I didn't determine a .4amp power consumption. That was requested by the OP but I wasn't calculating anything by that. I was trying to calculate whether his 36 amp hour battery would last 4 nights with the Intellipap machine.

It's interesting that you say to measure the voltage under load. I do realize that the voltage of the battery would be lower under load. I also know the voltage will rise again after sitting for several hours. Everything I have read says to check the battery voltage after the voltage has had a chance to recover because that's when the voltage is linear. I'm not arguing with your experience, just saying that now I don't know what to believe.

I never thought about computing current using a current shunt. I think I would like to do this though so let me get it straight.

If I remember right from my high school electronics class, current is the same through every device in a series circuit right?

That means if I put a 0.1 ohm resistor (r1) in series and then measure the voltage drop across that resistor I can calculate the current running through the whole circuit with the formula Current = Voltage(r1) / Resistance(r1) right?

So for example if I have a 0.1ohm resistor and measure a 0.04v drop across it then 0.04 divided by 0.1 = 0.4 amps through the whole circuit right? And because the resistor is so small it has only a miniscule effect on limiting that current right?

[timbalionguy]

I didn't determine a .4amp power consumption. That was requested by the OP but I wasn't calculating anything by that. I was trying to calculate whether his 36 amp hour battery would last 4 nights with the Intellipap machine.

It's interesting that you say to measure the voltage under load. I do realize that the voltage of the battery would be lower under load. I also know the voltage will rise again after sitting for several hours. Everything I have read says to check the battery voltage after the voltage has had a chance to recover because that's when the voltage is linear. I'm not arguing with your experience, just saying that now I don't know what to believe.

That's an interesting piece of information you have about measuring the at-rest voltage of the battery. I have a portable gel-cell power source that does something similar. I have found it to be nonlinear, and furthermore, read inaccurately as the battery aged. Measuring voltage under load is what I have been taught and what I have experienced (after all, the purpose of a battery is to power something!). But it logically follows that there has to be an open-circuit voltage that indicates full discharge. And as I think about it, I recall seeing it somewhere. In any case, you can ruin sealed electrolyte batteries by overdischarging them. An open cell battery is a little more tolerant of overdischarge. In any case, I would consider using a circuit to cut off power to the machine if the battery voltage got too low. That protects both battery and machine.

I never thought about computing current using a current shunt. I think I would like to do this though so let me get it straight.

If I remember right from my high school electronics class, current is the same through every device in a series circuit right?

That means if I put a 0.1 ohm resistor (r1) in series and then measure the voltage drop across that resistor I can calculate the current running through the whole circuit with the formula Current = Voltage(r1) / Resistance(r1) right?

So for example if I have a 0.1ohm resistor and measure a 0.04v drop across it then 0.04 divided by 0.1 = 0.4 amps through the whole circuit right? And because the resistor is so small it has only a miniscule effect on limiting that current right?

You are quite correct in your math. Isn't Ohm's law wonderful??

You could also improvise a shunt using a suitable length of copper wire of a known gauge. You just use a length that gives you .1 ohm (or whatever other value you want to use for a shunt), according to a copper wire table.

[sam1234]

I tested my battery (24lb, 12v, 35Amp deep cycle) by using it each night connected to my APAP machine until the battery check in the morning using a tester (from Harbor freight) said that the battery was in a "fair" state of charge. I use a ResMed converter, which supposedly prevents exceeding the discharge limits of my battery.

So I got approximately 3 nights (@ ~8 hours). When I went camping this weekend, on a motorcycle. The temperature was between 30 - 40 degrees farenheit at night. I did get two whole nights, using it for about 9 hours or so each night, before the battery tester said that it was "fair".

I did not check to see if my pressures were consistent from the test to the actual usage. However, the margin observed during the test assured me that I should be able to depend on the battery providing two nights of reliable power even with all of the variables including pressure and ambient temperature.

I will be using this battery for a series two night stints over a three week camping trip. I hope to be able to find an electrical outlet every third day for charging.

[nomoore]

A coworker let me borrow his nice (Fluke) multimeter. It measures DC amperage. I measured the Intellipap Autoadjust on 2 settings. I measured while all masked up. The amperage would swing one way and the other significantly when I breathed so I measured when holding my breath. I figured you do as much breathing out as you do breathing in so a good average would be when holding your breath. This was with a well sealing Activa mask if you are wondering about vent rate affecting it also.

8cm H2O = 0.49 amps (pressure requested by original poster)
11.5cm H2O = 0.63 amps (my minimum pressure)

So with my 16ah battery at 11.5cm, taking into account not going under 20% state of charge, the battery should last 20.3 hours. That should last me two 8hr nights and leave me plenty of room also for a higher pressure during the night since it is an Auto.

And for the original poster, 36ah at 8cm, also taking into account not going below 20% SOC, the battery should last 58 hours. That should give him 7 nights before it drops under 20%. Depending on the battery type he might not want to let it drop that low ever though. So it should give him 36 hours before dropping under 50%. That's 4 nights and some change.

Edit: These numbers seem to be pretty close to the numbers CPAP.com gives for battery life on their batteries for this machine. It does appear though that they calculate battery life to 100% discharged.

[sam1234]

Resmed has a nice piece about using batteries and the draw rate for their products using an inverter or one of their converters http://www.resmed.com/assets/documents/ ... -guide.pdf

My Current draw at 12 V DC (amps) is probably close to 1.22 (based on the chart). So a 15 to 18 Amp battery for an 8 hour period (including 50% safety margin) is not unreasonable. That is in line with my actual experience.