Friday, October 21, 2022

CPAP Battery Solutions

Not actually Erin.
& is used with permission.
In my last post I talked about wanting to be able to power my CPAP during a sustained power loss of several days. I don't yet have a solution, but enough readers expressed interest in the topic that I thought it might be useful to show the progress of my research. 

Power Requirements
I bought the electricity usage monitor plug I talked about and found the information very useful. For example, even though my CPAP (a Philips Dreamstation 2) is rated at a draw of 80 watts, the actual wattage it draws depends on how much pressure and humidity it provides. In fact, it's the humidifier which is the greatest power hog: with it running at the highest level my CPAP draws about 76W, but with then humidifier turned off that number drops to 25W! 

This information is exactly what I needed to know, because it tells me three things:
  1. How many nights of performance I can get per battery bank (which are typically rated in Watts of storage);
  2. I can triple that number by turning off the humidifier;
  3. I need to research non-powered humidity alternatives, because dry air up my nose really irritates me. 
For those who are wondering "Why not just turn the humidifier down to level 1?", the answer is that doesn't make enough of a difference energy-wise. My CPAP's water reservoir has a metal plate at the bottom which is heated electrically until the water in the reservoir evaporates at the chosen rate. Regardless of whether it's humidity level 1 or level 5, I'm still running what is basically a hot plate, and that is what eats the power. 

The difference between levels is at most 5 watts; that's 40 watts over the span of an 8-hour sleep. Assuming a 600W battery, level 1 will drain about 560 watts over the course of a night, leaving me with insufficient battery for the next night unless I can recharge it in some way. With the humidifier turned off, though, that drops to a draw of 200W per night, which gives me about three full nights of sleep before the battery is drained. This is a much better use rate of battery power. 

DC vs. AC
Speaking of use rate, my research tells me that I will increase my battery's endurance if it comes with a 12V DC port into which I can plug a 12V CPAP power cord

Put simply, the electrical theory goes like this:
  • Electricity comes out of the wall as 120V Alternating Current. 
  • This device runs on 12V Direct Current. 
  • When I plug my CPAP into the wall, there's an adapter between the outlet and appliance which converts 120V AC into 12V DC. 
  • This conversion reduces efficiency because of the Second Law of Thermodynamics.
  • Normally this isn't a problem because houses have a constant supply of power, but when you're working from a fixed battery, this becomes a problem. 
  • Batteries store and provide power as Direct Current. 
  • It would be dumb to waste DC battery power by converting it to AC via inbuilt inverter, only to then plug an AC to DC adapter into it to convert it right back. 
  • I can skip that wasteful step and power my DC device by plugging a DC plug into a DC port. 

Heat Moisture Exchanger (HME)
Even though I plan to use DC power, I still need a humidity source that isn't an electricity hog. This is where Heat Moisture Exchangers, or HMEs, come in.

HMEs capture the heat and moisture in your exhaled breath, and  then return it to you as you inhale, giving you the benefits of a humidifier without actually using one. They do not draw any power and last for one week of use, which ought to suffice for all but the worst hurricane-caused power losses.

What's more, you can buy them in bulk; Amazon sells them in packs of 10 for $30, which at $3 per HME is quite reasonable in my opinion. 

I am looking forward to testing these, because pre-COVID I used to travel a fair bit giving interviews and presentations for my charity work, and hauling a container of distilled water for the humidifier was an inconvenience that often caused spills. Being able to toss an HME into my bag and skip the water use for a week will make my life that much easier. 

I am still shopping for a battery bank and solar panels, but I have my selection narrowed down and I should be pulling the trigger on it soon. My next post on this topic may not be for a while, but when it comes I will give you my recommendation for batteries and, hopefully, a solar panel to charge them. 

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