wiki

Dave, a ham radio operator and a long haul truck driver came to us with a question. On some trips, he stays overnight in his sleeper cab. During the night, he needs to keep power going to his little fridge (Peltier based cooler). As well, like me he has sleep apnea, and needs to run his CPAP machine (24v). This is no problem when he can keep the truck running all night to provide the power, but in some places that are near residential areas, the noise of the running truck is unacceptable. His question to us was: Is it possible to find / build a battery pack that would run these two devices for 10 hours?

1. Safety is a top priority. can not risk damage or a fire in a very expensive truck
2. Can't risk damage to CPAP equipment
3. Cost needs to be considered and minimized
4. Driver may move to another truck - no modifications to truck
5. Truck consumes 250 milliamps from battery when it is shut off
6. Truck has a load shedding safety system to ensure it can be restarted. If battery voltage hits 12.3, things shut down
7. Battery pack must fit in a space that is 8 x 20 x 10 inches. This probably eliminates the two battery approach shown in the diagram below.
8. Dave's CPAP pressure is 6.5, less than the average of 10 used by manufacturers to estimate battery endurance
9. Dave uses a heated humidifier, which is a heavy load on battery capacity.

References:

1. CPAP Manual
2. Fridge manual
3. Official Resmed 12 Volt adapter
4. New Commercial CPAP battery
5. next

• fridge and CPAP are connected to new battery pack
• fan and bunk warmer are connected directly to truck power

details: http://www.amazon.ca/Pilot-24-CPAP-Battery-Backup-System/dp/B00V52WUXM

Further thoughts on the Pilot-24:

• need to think about the 3.75 Amp rated output if we're adding fridge
• this thing is light, tiny, and very portable
• doesn't need any other adapters or converters to run CPAP
• would need an adapter to run fridge - probably an available product. will look.
• CSA approved, and much safer than anything we could build
• no dangers of battery gas production, or spillage
• it has lots of information displayed
• it has lots of safety features, including thermal protection, etc.
• it uses the CPAP's power adapter to recharge. Would need to use truck's inverter.
• need to do power consumption measurements to be able to tell if we can do fridge off same device

Canadian Tire models have details here:
http://www.canadiantire.ca/en/sports-rec/boating-water-sports/boat-parts-accessories/marine-batteries.html

Notes on above approach:

• batteries in series have double the voltage, but same Amp Hour capacity
• using a 24 V battery instead of two 12 V ones would be better, if we can find one
• need to confirm CPAP has voltage regulator & filtering on DC input
• switch must be rated for 24+ volts, and 6+ Amps
• need to use appropriate wire gauge
• The limited space available (8 x 20 x 10 inches) likely won't hold 2 batteries, so for this approach to work, we'd need to find a 24 V battery
• The inverter's only use is to power the charger while the truck is running.

Things to do:

• measure actual current for CPAP and fridge - may be able to use programmable power supply to graph current over time.
• figure out what DC connector is used by CPAP, and build a current measurement cable
• decide if a sealed battery is a requirement for safety or not.
• consider how to charge the battery, and what the associated costs are
• figure out how we get 24V out of a 12V battery
• take apart Doug's CPAP to see if it has voltage regulator & filter on DC input.