Post
by Scott_G » Mon Jan 24, 2022 1:21 pm
I'll update this to say that the DC/DC converter is still working well in my setup, having been used every night since early November 2021. My main objective was as a long-running UPS to cover power outages, and indeed I have slept right through several power outages with no interruption at all. It's been too cold here for camping, but I look forward to using it there as well.
My main disappointment is the lack of efficiency. It isn't just a straight power cord, as was used with my previous 12V Respironics PR60. Rather, the Resmed DC/DC converter is a "boost" device that takes in 12V in and puts out the 24V needed by the AS11. That boost process incurs some efficiency loss, and that means the DC/DC converter is only slightly more efficient than the combination of an Inverter with the standard AS11 AC/DC converter.
I did some short efficiency tests at my pressure and flow, not comprehensive tests with multiple pressure/flows or heat settings, so consider these numbers APPROXIMATE: In my tests, the standard AC/DC converter in combination with a sine-wave inverter was about 64-70% efficient, while the DC/DC converter was about 72-78% efficient. Actual efficiency in a given setup will depend on several factors, including the quality of the inverter and the actual power draw at given pressure, flow, and humidity settings. But both methods waste a lot of power, mostly dissipated as heat from the DC/DC converter, or from the Inverter and AC/DC converter.
That does give the DC/DC converter an edge of perhaps 8%, and (depending on the situation) that might mean the difference between a partial and full night's sleep. But in terms of runtime from a given battery source, there isn't really a HUGE difference between using the DC/DC converter versus using an Inverter and standard AC/DC converter combination.