Camper Trailers Tech Tips

CPAP 12volt battery supply

 

 

 

cpap
12volt battery supply

In this article we will look at the power requirements needed to run a CPAP using a 12volt battery system when bush camping with no external 240volt. Most camper trailers these days have a 12volt battery system or an auxiliary battery in the vehicle to power accessories such as a 12volt compressor fridge, a tyre pump, 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 the make & model. Some CPAP’s are 240volt & plug straight into the 240volt outlet at home while others are 12volt or 24volt. These run at home from a 240volt socket via a supplied AC adapter which steps the 240volt down to the suitable DC operating voltage.

12volt adaptor

CPAP’s 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 12volt cigarette socket to run the CPAP on 12volt or stepping the 12volt supply up to 24volt.

inverters

An inverter is needed to run a 240volt CPAP in your camper trailer. An inverter converts the 12volt battery power to 240volt. Remember 240volt 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 100watt at 240volt, you can expect the inverter to draw about 10amps from your 12volt 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 or 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 required. A voltmeter is useless as you need to count the amps going in & out of the 12volt battery system. 

12volt adapter If the CPAP is run direct from the battery adaptor via a 12volt cigarette socket, the power draw & amp-hours used overnight can be found with a Watts Up Meter http://www.campertrailers.org/watts_up_meter.htm or alternatively a 12volt battery monitor.

240volt inverter You can use a mains power meter to check your power draw of the CPAP from the inverter. As a rule of thumb 100watt at 240volt equals 10amps 12volt draw from the battery.

12volt battery type

An AGM (Absorbed Glass Matt) or Lithium Iron Phosphate (LiFePO4) 12volt batteries are the most common battery type suited for camping as they can be discharged more deeply than a 12volt cranking battery which is used to start your vehicle. The capacity of AGM & Lithium batteries are stated in amp-hours (Ah).

There are also a number of lithium 12volt battery portable power stations that are suitable for the job, as well as the CPAP manufacturers power banks.    

battery sizing

The CPAP draw will depend on the CPAP make & model as well as the settings used. As an example a 240volt CPAP in auto mode may draw around 50watts via an inverter which means an approx. 5amp 12volt draw from the battery. For every hour the CPAP runs, it uses 5amps from the battery or 5amp-hours. Therefore over 8 hours it will use 40amp-hours. A 100amp-hour 12volt battery would be suitable.

 alternator charging

When driving between camps on a touring holiday & charging by a 25amp DC to DC charger from the vehicles alternator, it will take approx. one & a half hours to recharge the battery of the 40amp-hours used by the CPAP overnight. That is 40amp-hours divided by 25amp = 1.6 hours.

solar charging

To fully charge a 12volt battery using a solar panel with an average 5 hours of sun input, you would need roughly a 150watt solar panel. 40amp-hours divided by 5 hours sun = 8amps input per hour. 8amps x 18 volts panel output = 144 watts.

camping for two days

Unfortunately the sun may not shine every day so it is a good idea to install a larger battery to cover your needs & also play catch up using larger solar panels to fully recharge the battery. Richard’s Solar Spread Sheet is a great tool to help calculate the battery & solar panel size that will suit your needs  http://www.campertrailers.org/solar_spreadsheet.htm   

reducing the cpap battery consumption

running in auto mode

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

reducing battery draw

By turning the humidifier off, as well as the hose heater, you can reduce the power draw considerably. Some CPAP’s can be run in 'aeroplane mode' which turns the humidifier & heater off, reducing the power draw.

real life battery draw

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

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

By turning the humidifier & the hose heater off using 'aeroplane mode' the power consumed is reduce to only 11amp-hours overnight or 1.4amps per hour, (1.4amp-hours) over 8 hours or around one third of the power needed to run the CPAP in auto mode. The humidifier itself uses 21amp-hours overnight.

our setup

We enjoy our remote bush camping & seldom visit a caravan park during a trip. We have a Redarc BCDC1225D DC to DC charger to charge the batteries when driving & 250watt solar panels via the Redarc's inbuilt MPPT solar regulator when camped.

By running the CPAP in 'aeroplane mode' we can easily fully charge our two 100amp-hour Lithium Iron Phosphate (LiFePO4) 12volt batteries by lunch while also running our 40 litre Engel fridge as a fridge. We also reduce the power consumption of the fridge by vacuum packing our meat & not running it as a freezer.

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

We have 250watt of solar panels & when sunny the batteries are fully recharged by lunch.
26amp-hours daily use divided by 5 hours solar sun input = 5.2amps
5.2amps multiplied by 18volt solar panel output = 93.5watts
So in theory we only need at a minimum 100watt solar panel to fully charge our 12volt battery by just after lunch.

We have 200amp-hour of Lithium Iron Phosphate (LiFePO4) battery capacity that can be safely discharged to 20% capacity if needed, giving us 180amp-hours battery usage. Therefore depending on the season we have over 6 days camping with no solar input. Of course this is the worse case scenario. 

further reading

Setting up a battery system for a 12volt compressor fridge http://www.campertrailers.org/12volt_compressor_fridge_draw.htm 
Richard’s Solar Spread Sheet http://www.campertrailers.org/solar_spreadsheet.htm 
Watts Up power meter http://www.campertrailers.org/watts_up_meter.htm
Redarc BCDC1225D https://www.campertrailers.org/redarc_bcdc1225d_install.htm 
Redarc Smart Battery Monitor https://www.campertrailers.org/redarc_smart_battery_monitor_install.htm
Enerdrive battery monitor http://www.campertrailers.org/enerdrive_elite.htm 
 

info by Rob

 

 

november 2019
update october 2025