I am someone who is into RC models and use lipoly batteries in series and parallel for different applications. I connected two 12 volt 18ah SLA batteries in parallel to double the capacity (36 ah) because I had them. I know I could connect the same two batteries in series, and eliminate using the converter, but the capacity would be 18 ah. I imagine I might get one or possibly two nights CPAP use. I may try two 12 volt 2Oah LIfe POR batteries, now relatively cheap, in parallel with converter or in series without converter and test for actual results. Will have to buy a Lithium charger though. I may convert to Lithium batteries for my mobility scooter also.