Battery operation of CPAP devices for sleep apnea treatment
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Battery Operation of CPAP Devices for Sleep Apnea Treatment, Part 3--Very Technical Notes

by Jerry Halberstadt with Gary Collins, Charles Woodson; and Shane Finn and others at ResMed.

Very technical notes

Questions, answers, and issues

How much electricity will the CPAP draw?

You should ask your manufacturer how many amps your unit draws at your prescribed pressure level at 115 VAC. You then need to figure out the draw in amperes at 12 volts (the actual voltage may vary from 10 to 12.7 volts). To figure the current required by your unit at 12 Volts, see the Technical Notes below.

At a 10 cm water pressure setting a CPAP which draws 2.5 amps would need 20 amp-hours in 8 hours. Therefore, the battery should be at least 35-40 ampere hours to support one night of sleep.

At a 20 cm water pressure setting a CPAP which draws 4.5 amps would need 36 amp-hours in 8 hours. Therefore, the battery should be at least 54-72 ampere hours to support one night of sleep.

Respironics units would require a 30-40 amp-hour battery for two nights; such a battery weighs about 20 pounds.

The true current draw of your CPAP can be measured using an ammeter. Or estimate it based on Shane Finn's note below, that is, take half the power figure stated in VA and you have approximate watts. Divide the wattage by 12 volts to get the approximate ampere draw of your inverter. Multiply the ampere draw of your inverter by the number of hours you plan to use it, then multiply again by 1.5 or 2 for a reserve factor.

Example: ResMed Sullivan III device rated at 80VA. 80*0.5=40 watts. 40 watts divided by 12 volts=3.33 amperes drawn by the inverter. 8 hours * 3.33 = 27 amp-hour rating. For a reserve factor, use 1.5 * 27= 40 amp-hour rating of the battery. To be even more conservative, compute the inverter current assuming 10 volts: 40 watts divided by 10 volts = 4 amperes drawn by the inverter. For 8 hours, we need a 32 amp-hour battery and for a reserve, a factor of 2 gives a 64 amp-hour battery. A 95 ampere-hour battery should give ample electricity for two nights without recharging, perhaps if one sleeps six hours the battery could last for three nights. 3*6*4=72 amp/hrs--within the capacity of the battery.

1/ To calculate battery current:

where Einv is the efficiency of the inverter power conversion; usually around 90% = 0.9

where Vbatt is the battery voltage plateau; often 11V but if you don't know use 10V and be conservative

where Vcpap is the mains voltage at which the Icpap current is measured; 115V usually may be substituted

where Icpap is the current drawn by the CPAP device at Vcpap

where PFcpap is the power factor converion from VA to watts; most probably 0.5

where Ibatt is the current that will be drawn from your battery by the CPAP through the inverter

Ibatt = (Vcpap / Icpap * PFcpap) / Vbatt / Einv

If you have the CPAP demand in watts, substitute that number for the bit in brackets which is calculating watts from the other information. The formula then becomes:

where Pcpap is the CPAP device power demand

Ibatt = Pcpap / Vbatt / Einv

Note: the equations are read left to right as if they were being typed on a calculator.

Example B: Sullivan III rated at 80VA at 115 VAC How many watts does it need?

more likely, 80VA * 0.5 = 40 watts

What is the CPAP device power demand?

Ibatt = 40/ Vbatt / Einv


2/ What kind of voltmeter do I need for checking the output from the inverter? My Fluke Multimeter only gives a reading of 91VAC from the inverter.

A true Root Mean Square (RMS) reading voltmeter is needed to check the output voltage of 115 volts. Other meters may read low because they are calibrated for a sine wave output. The inverter uses average sensing for voltage control and so you will see some change in the RMS readings during use. You should see the RMS voltage rise later during the battery discharge, even though the average voltage won't have changed. However, the most important parameter of the inverter output voltage is not the average or RMS value but the peak voltage (which can only be measured using a CRO). Comments on a 91V inverter output reading...Most Fluke multimeters are true RMS so the reading is probably correct.

3/ Will I have enough power to run a heated humidifier?

Heated humidifiers are not compatible with most types of inverters, and such use is likely to result in burning out the humidifier heating element.

WARNING: Do not use an inverter power source to run a heated humidifier.

Technical explanation, adapted from information supplied by Shane Finn at ResMed: Humidfiers use a triac or thyristor to regulate the power by turning on partway through each mains cycle. The controller assumes the mains voltage is sinusoidal. However, inverters generally do not provide a true sinusoidal output, thus the controllers make wrong decisions and cause the humidfier to deliver full power continuously, and may burn out the heating element. Another, more intelligent type of controller is termed an integral cycle controller or burst controller, and would provide full power in short bursts. However, we know of no heated humidifier using this type of controller.

4/ I have a ResMed Sullivan III CPAP and will only use the battery backup system a couple of times a year, either during a power failure or to go camping. Can I use a unit which is less expensive than the Radio Shack inverter approved by ResMed?

For your occasional usage the inverter should be OK if you want to use it, or have problems powering your humidifier. I have never heard of that particular inverter and cannot endorse it but your usage is short so it shouldn't matter. [This may not be true for another type of CPAP since their designs may differ.]

5/ How much electricity does my CPAP really use? Use Ohm's law

Short answers

Short answer 1: Ask your CPAP manufacturer. Short answer 2: if there is a VA rating, divide by two to get the value in watts at the rated voltage. Short answer 3: Probably 4 amps or less. Then see short answer 1! For the long answer, see below.

Long answers for electricians: Ohm's law

The VA rating on the CPAPs means Volt-Amperes. It implies that the machine is not a resistive load and the current is out of phase with the voltage. That is not an ideal load. It is some magnitude of a complex capacitive/ inductive load. That is why it is complex to predict how much it will draw from the battery. The inverter is also a complex load, but it really appears as a DC load to the current meter and the meter averages the load. The inverter actually draws current in very high current pulses but the averaging of the meter makes it appear that current is less because the current is averaged with respect to time. In other words if the actual draw pulse is 16 amps at a 50% duty cycle the meter will read 8 amps. If you increase that to a 75% duty cycle the meter will read 12 amps.

The formulas you are working with are essentially Ohm's law and the power formulas. The basic ones are: I=E/R transposed that would be R=E/I or E=IR. This is true for DC circuits. The symbols being: I=current in Amperes, E=voltage in volts, and R for resistance in Ohms.

Added to this you have the power formulas with P being power

in Watts. They are: I=sqrt(P/R) or I=P/E, R=E/I or R=E^2, (the ^ symbol is used for raised to the second power or squared It's hard to show an exponent notation on the internet).

The third formula is

P=(I^2)(R) or P=EI or P=E^2/R.

AC gets considerably more complicated. Essentially we are concerned with the DC consumption from the battery. The rating on your CPAP is in Volt Amperes that includes a factor called impedance which roughly corresponds with resistance in DC. I know this sounds complicated and it is. The reason in part, that your current draw from the battery is somewhat out of line with the measured power being drawn by the CPAP is that the current in a circuit with large capacitors leads the voltage by 90 degrees. In large inductance circuits the current lags the voltage by 90 degrees. Don't let this confuse you. You are really concerned only with the current draw from the battery. When the CPAP is drawing current from the inverter it never sees DC, obviously...

So if you got all that what I am saying is... Once you have your battery and inverter set up, the best way to calculate your backup time is to take the total current draw measured with an Amp meter in series between one lead of the inverter and the terminal of the battery. That will give you the current in Amperes. This value divided into the Amp-hour rating of the battery will give you the total hours you can expect from the battery. The value will of course be approximate and depend on the charge condition of the battery the temperature the lower the temperature the lower the voltage will be and the battery will last somewhat longer. If the temperature is higher the voltage will be higher and the battery may discharge sooner. Higher temps also mean the internal leakage of the battery will be higher. The danger with cold is that as the battery discharges there will be danger of it freezing if the temp is low enough. This is because the sulfuric acid gets more and more dilute as the battery discharges and more water is relesed by the chemical reaction. You are correct in saying as the CPAP pressure increases the current consumption increases. That is one reason I stressed, measure the current.

6/ How do I select an inverter?

Get this information directly from the manufacturer of your CPAP device, or a home care company representing them. Protect your equipment and your warranty!

An inverter for CPAP needs a peak power rating of around 200W to provide the ability to start the power supply and motor; the running consumption of the CPAP is 40 watts or less. Bi-level and smart, automatic pressure setting devices may require more. For example, ResMed VPAP products require a peak power rating of around 400 watts, about double their CPAP products. ResMed reports that probably any inverter will be acceptable for occasional use with ResMed products--less than a week during one year. Longer use risks causing damage to the CPAP device. However, of 30 inverter products tested, only two were approved for long term use (more than one week/year) with their CPAP products.

ResMed has approved the Radio Shack Portable Power Inverter, Cat. #22-132A $99.99 for use with ResMed Sullivan CPAP devices. It accepts input from 10-15 VDC and produces 115VAC/60Hz; delivers 200 W for 5 minutes, 140 W continuously, with about a 90% efficiency. Protects against output short circuit, overheating, over power, low battery shut down @ 10 V; output on/off switch. Weight: 15 oz (425 g). The battery input is through a cigarette lighter plug (provided); you will also need to purchase an adapter that accept the lighter plug on one end and the other end has battery clamps (red is marked + to match the positive pole of the battery).

WARNING: If you have a ResMed CPAP, don't rely on this information until you confirm it yourself with ResMed. If you have a CPAP device made by another manufacturer, get their specific recommendations.

Other inverters, if approved by your CPAP manufacturer, may offer additional convenience and safety features.

Technical note on commercial UPS and standby sytems:

The low cost systems do not produce a true sinewave waveshape, therefore caution should be exercised in matching the UPS system to the CPAP, and usage kept to a minimum. Extra filtering in the UPS system is not needed since the CPAP has built-in filtering. UPS products may produce interference with radio and TV reception, since they are not required to meet the same standards as medical products.

Gel-filled batteries

A gel filled, sealed battery would cost much more than a lead-acid battery, however it would simplify maintenance and safety procedures. If you can afford the luxury of a gel battery, a 38AH battery weighing 31 pounds costs about $140. You wouldn't have to worry about adding water or spills. The big advantage of gel batterys is that they don't leak because they are sealed. However, their discharge current is lower than liquid acid batterys because they can't get rid of the hydrogen and oxygen they generate. They must also be charged slower. They may require a different setting for the charger and a manufacturer or competing marine batteries suggests that gelled cells may not be suitable for deep cycle use.


This document provides information only. The reader is warned that he/she is solely responsible for any use made of this information. Making, installing, and using a battery backup system can damage CPAP equipment, or may cause personal injury, fire, or explosion. Therefore, the services of a qualified electrician should be used. The Safety Warnings are part of this document and should be read together with it. Safety Warnings


The technical information which helped me create my battery back up system and this newsletter is based on correspondence with CPAP users: Gary Collins, Charles Woodson, and Lawton Mullins; and Shane Finn and others at ResMed ResMed Home Page . Their materials are used with permission. Gary lives on a mountainside and is plagued by frequent power outages. His electrical knowledge and creativity have been important inputs for this article. Charles enjoys camping and travel and is able to use his car battery while he sleeps. Lawton is determined to enjoy his canoe camping trips. I appreciate the advice and information provided to me but warn the reader not to rely solely on this information. Reference has been made to manuals of manufacturers including Schumacher, Power-to-Go, RadioShack, and EverStart, and to Phantom of the Night.

Appropriate use

The reader is responsible for any uses or interpretations made of the information provided, and is urged to obtain the services of a qualified electrician before purchasing, assembling, or installing a battery backup system.

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