Resmed DC/DC Converter 24V/90W for S9

I've got 2 PowerPaks similar in nature to the Duracell one shown here:
http://www.amazon.com/dp/B000TKHMWK/ref
except mine have removable jumper cables and I don't have a radio....

Both use float chargers and are plugged in all the time -- have had one of them for going on 10 years and it still holds a charge and is still going strong!
I'm seriously considering just grabbing the DC/DC cord and plugging it into one of them and letting it go at that.
So long as mains are up it'll hold the charge on the battery and as soon as mains drop off I'll be running directly off battery.
The reason for going to the DC/DC is a) the DC port is always on on my PAKS and b) its more efficient since there are no inverter losses so battery will last longer.

Anybody see any issues with this?

I've seen references here where other folks have used it. My only concern would be how long they would last if you actively use the H5i and/or ClimateLine tubing.

It's got AGM going for it, but the 28AH capacity could be a problem if you use ClimateControl and/or EPR. Go back a few posts and take a look at the ResMed battery guide. 28AH may only last 4 hours, depending on your settings.