Adding More Batteries to My Class A - What gauge Wire?

Hope you have a pure sine wave inverter for that CPAP. I've had two different model Respironic units, the first was a CPAP the second a BiPAP. On MSW they would cycle on an off the LCD screen looked like elfish script out od a J.R.R. Tolkien novel. I called Respironics and they screeched "No Warranty! No Warranty!" Luckily neither emitted magic smoke. But the MSW scrambled the data and rebooted the unit with original settings.