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High capacity 12V supply for CPAP

Thu 20 Aug, 2020 1:25 pm

I use the Ryobi 1+ cordless system for my portable power tools. The batteries are perhaps the cheapest way to get a DC supply at a sufficient ampacity to allow 5 to 7 hours of run time.
However I haven't yet seen a 12V adaptor for these batteries.
Anybody here have any experience in modifying stuff to change 18V to 12V?
Ryobi make a USB adaptor and I am buying one anyway to use for recharging my phones next winter. ... y_p6210800
I was thinking that somebody with a 3-D printer could make a shell easily enough but also willing to hack the adaptor itself by buying a second one.

Re: High capacity 12V supply for CPAP

Thu 20 Aug, 2020 2:35 pm

What you are looking for is a buck/boost converter that will convert your power source to 12v. The buck/boost converter is the easy part but you need to know the maximum power draw - usually momentary on startup of the cpap. As to battery pack you could work with anything from 18v down to 3.6v (Li cells in parallel). As the voltage of the power bank drops so the capacity has to increase. The basic formula is P=V*I where power is the the amps required by the cpap unit multiplied by 12 volts. Example assuming the cpap draws 1 amp the P = 12 (1 x 12). This power requirement can be met by 18v x .67 amps or 7.2v * 1.67 amps or 3.6v * 3.34 amps or many other combinations. You also need to allow for the converter loss 5%? for decent one. Li battery packs that output USB 5 volts are just 3.6v or 7.2v packs with the appropriate buck/boost module to deliver 5v.
The single LI cell voltage is nominally 3.6v but ranges from around 4.2v to 3v as the power is drawn out. The buck/boost converter handles this variability and outputs a constant 12 volts in your case.

Re: High capacity 12V supply for CPAP

Thu 20 Aug, 2020 2:47 pm

How do you design a buck/boost circuit and how much room would something like that take up?
Unit uses 99 Watt Hrs in 8 hours usage

Re: High capacity 12V supply for CPAP

Thu 20 Aug, 2020 3:12 pm

Places like Jaycar sell a range of converters. Here is an example: ... e/p/AA0236. Might be worth having a chat to someone in one of their stores. Would probably be a good idea to take the CPAP device with you.
Last edited by michael_p on Thu 20 Aug, 2020 3:14 pm, edited 1 time in total.

Re: High capacity 12V supply for CPAP

Thu 20 Aug, 2020 3:13 pm

The unit you reffed in the cpap thread is 14.4v (nominal) so 4 Li cells in series (3.6 x 4) and delivers 99 Watt hours (Wh). The resmed pack will deliver the required 12v without the need for a buck/boost converter. This converts to Amp hours(Ah) - Wh/V = 99/14.4 = 6.875 Ah or 6875mAh. So a 5v USB (output) 10,000 mAh Li battery pack with a buck boost converter will deliver the same or more run time. and cost you about 1/10th of the $599 of the resmed unit - aren't medical mark ups amazing! A 20000mAh pack would double the run time.

For simplicity I would look for a buck boost converter that takes 5v input and delivers 12v, I would expect the power requirement would be 2A to cover the startup surge. The reason for this is that the battery pack does not need to be modified but results in a slightly lower efficiency.

PS - No need to design anything - just a cheap little circuit board - some cheap converters

Re: High capacity 12V supply for CPAP

Sun 23 Aug, 2020 11:08 am

Thanx Mark I will investigate

Re: High capacity 12V supply for CPAP

Mon 24 Aug, 2020 12:59 pm

Milwaukee Tools have some amazing hi-drain batteries ... son/46033/

And for those guests who are amazed at Mark F's electrical calculations
Ohms Law picture attached

Thanks Mark i think you may have revealed your day job correctly pointing power usage over time is in amp hours not watts.

Re: High capacity 12V supply for CPAP

Tue 24 Nov, 2020 4:30 pm

Having done some minor research I've reached a decision to buy a small 12V Lithium technology deep cycle battery.
Jaycar have a small one that while more expensive and heavier than a powerpack can be recharged easily with a small solar panel and can be used with a cigarette lighter socket. ... y/p/SB2210
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