WELCOME! to the forum,! and best of luck to you with your CPAP therapy.
You could put a Schottky diode in one lead. The purpose is to prevent damage if the power is connected backwards. The Schottky diode has a low forward Voltage drop. Even better, if you can find one, would be a Schottky diode, 4 diode bridge. I can be connected to always steer the current the correct way no matter if you connect to the battery backwards.
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I hope that this is not off-topic for this chain, but I am considering the HDM Z1, and wonder: What is the operating voltage of the unit? (Or, what is the voltage of the optional battery?). It seems that one could buy something like the Allpowers 23000mAh solar device sold on line, and power the Z1 for a lot less money than their $400 battery. (It occurs to me that 23000 mAH may be at 5V, in which case if operating voltage is say 15V, I only get 7.66 hours to discharge).