Camper Trailers Tech Tips

CPAP 12volt battery supply





12volt battery supply

In this article we will look at the power requirements needed to run a CPAP from a 12 volt battery system. Most camper trailers have a 12 volt battery system or an auxiliary battery in the vehicle to power accessories such as a 12volt compressor fridge, a tyre pump or to charge a phone, camera or laptop etc. With careful power management a CPAP can easily run alongside other 12volt accessories on an existing 12volt battery system.

at home

Cpap's have different power supplies depending on make & model. Some CPAPís are 240 volt & plug straight into the 240 volt outlet at home while others are 12 volt or 24 volt. These run at home from a 240 volt socket via a supplied AC adapter which steps the 240 volt down to the suitable DC operating voltage.

12 volt adaptor

CPAPís that use a DC adaptor at home are perfect for camping. A DC converter for the specific make & model of the CPAP can be purchased & simply plugs into a 12 volt cigarette socket to run the CPAP on 12 volt or stepping the 12 volt supply up to 24 volt.


An inverter is needed to run a 240 volt CPAP in your camper trailer. An inverter converts the 12 volt battery power to 240 volt. Remember 240 volt from an inverter is just as dangerous as the power from the socket in your house. Electronic equipment needs a Pure Sine Wave inverter so it will not damage sensitive computer equipment inside the CPAP.

Modern inverters are far more efficient than those in the past. A rough calculation when working out the power draw of an inverter is for every 100 watt at 240 volt, you can expect the inverter to draw about 10 amps from your 12 volt battery. Also when sizing an inverter keep in mind that the motor usually doubles the draw at start up. It is a good idea to disconnect the inverter after use as it can flatten the 12volt battery if left in standby mode.

power draw

You will find the watt & amp draw stamped on the CPAP power supply. The power draw will depend on the CPAP settings you are running. It is essential you know what the CPAP power usage is overnight so you can calculate the battery size needed. A volt meter is useless as you need to count the amps going in & out. If you do not have a 12volt battery monitor that does, it can easily be done depending on the operating power source of your CPAP.

12 volt adapter For an accurate 12volt power draw you can connect a Watts Up meter between the CPAPís cigarette plug & the camper trailers cigarette outlet socket. The Watts Up meter will record the amp hours used overnight

240 volt inverter You can use a mains power meter to check your power draw  & use the calculation of 100 watt at 240 volt equal 10 amps draw.

12 volt battery type

An AGM (Absorbed Glass Matt) deep cycle 12 volt battery is the most common battery type suited for camping as it can be discharged more deeply & accepts a charge more readily than a cranking or starter battery which is used to start your vehicle. The capacity of an AGM deep cycle battery is stated in amp-hours (Ah). Other suitable camping battery types that are now on the market include Lithium Iron Phosphate batteries (LiFePO4) & Lead Crystal batteries.

battery sizing

The CPAP draw will depend on the CPAP make & model as well as the settings used. As an example a 240 volt CPAP in auto mode may draw around 50 watts via an inverter which means an approx. 5 amp draw on the 12 volt battery. That is for every hour it runs, it uses 5 amps from the battery, the term is 5 amp hours. Therefore over 8 hours it will use 40 amp hours from your battery.

A good rule of thumb for best battery life is not to discharge an AGM battery below 50% capacity. With the above 40 amp hours used to run the CPAP on auto mode overnight via an inverter, a 100 amp hour battery would be suitable, however you will need to charge it every day.

alternator charging

When travelling from day to day it only takes roughly one & a half hours to recoup the power used overnight through say a 25 amp DC to DC charger, that is 40 amp hours divided by 25 amp = 1.6 hours.

solar charging

To fully charge an AGM deep cycle battery from a solar panel with an average 5 hours of sun, you would need roughly a 150 watt solar panel. 40 amp hours divided by 5 hours sun = 8 amps input per hour. 8 amps times 18 volts panel output = 144 watts.

camping for two days

If it was raining or heavy overcast & you were camping for two days using the CPAP on auto mode via an inverter you would use 80 amp hours from the battery. Keeping in mind not to discharge an AGM battery below 50% capacity, you would therefore need two 100 amp hour batteries. With two 120 amp hour AGM batteries you would have three nights use of the CPAP without recharging, that is 40 amp hours used per night x 3 nights = 120 amp hours.

reducing the cpap battery consumption

running in auto mode

On auto mode with the humidifier & heated air hose on, the CPAP will draw approx. 5 amps, therefore over 8 hours it would use 40 amp hours from your battery.

reducing battery draw

The humidifier draws around half the power to run the CPAP. By turning the humidifier off as well as the hose heater you can reduce the power draw to one-third. Some CPAPís can be run in 'aeroplane mode' which turns the humidifier off.

real life battery draw

Carol uses a ResMed AirSense10 CPAP. It is a 24 volt machine & at home plugs into the 240 volt outlet via a 90 watt AC adapter which reduces the output to 24volt. When camping there is a similar 12 volt DC converter lead with a cigarette plug.

When used in the camper trailer from the 12volt battery system, the ResMed AirSense10 draws 3.75 amps on in auto mode as easured through the battery monitor over 8 hours, that is 31 amp hours.

By turning the humidifier & the hose heater off we can reduce the power consumed to 11 amp hours over night or 1.4 amps per hour over 8 hours. That is one third of the power needed to run the CPAP in auto mode.

The humidifier itself uses 21 amp hours overnight.

our setup

We enjoy our remote bush camping & seldom visit a caravan park during a trip. We have a Redarc BCDC1225 DC to DC charger to charge the batteries when driving & a 150 watt solar panel via the Redarc's inbuilt MPPT solar regulator when camped.

By running the CPAP out of auto mode we can easily fully charge our two 120 amp hour AGM batteries by after lunch while also running our 40 litre Engel fridge as a fridge. We reduce the power consumption of the fridge by cryovacing our meat & not running it as a freezer.

15 amp hours seasonal average fridge power consumption over 24 hours +
11 amp hours CPAP per night with hose heater & humidifier off
= 26 amp hours over 24 hours.

We have two 120 amp hour AGM batteries. If we have 50% of our battery system available for use, we would be able to camp for around 4 to 5 days without recharging our batteries running the CPAP & fridge.
120 amp hours divided by 26 amp hours = 4.6 days.

We have a 150 watt solar panel & when sunny are usually fully recharged by lunch. By theory we need at a minimum 100 watt solar panel to fully charge our 12volt battery set up by just after lunch.
26 amp hours divided by 5 hours sun = 5.2 amps
5.2 amps multiplied by 18 volt solar panel output = 93.5 watts

further reading

Setting up a battery system for a 12volt compressor fridge 
Richardís Solar Spread Sheet 
Watts Up power meter
Enerdrive battery monitor 
Redarc BCDC1225


info by Rob



november 2019