You need some sort of battery pack to charge from solar or your electric charger. You then plug this battery pack into your cpap and switch to 12v-24v Dc. (You cannot "charge the device")
Very similar to a battery that you can buy for cell phones - You charge it before hand and keep it in case you need to use it to charge your phone.
I know that Sleepnet in Cape town had them at one stage


Or I suppose you could run a cable directly from your cigarette lighter to your 12v input on the machine