cpap
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
CPAP’s 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
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 12 volt cigarette socket to run the CPAP on 12
volt or stepping the 12 volt supply up to 24 volt.
inverters
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 CPAP’s
cigarette plug & the camper trailers cigarette outlet socket. The
Watts Up meter will record the amp hours used overnight
http://www.campertrailers.org/watts_up_meter.htm
240 volt inverter You can use a mains
power meter to check your power draw
https://www.jaycar.com.au/mainspowermeter/p/MS6115 & 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 amphours (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 onethird. Some CPAP’s 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
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
Enerdrive battery monitor
http://www.campertrailers.org/enerdrive_elite.htm
Redarc BCDC1225
http://www.campertrailers.org/bcdc1225_install.htm
info by Rob
november
2019
