Thanks for starting a separate discussion!
I am almost positive that the output (into the Dexcom) will be less than two watts. That value is already about double the maximum power which “smart” charges try to push into AA NiMH batteries, and AA batteries are much, much larger; almost certainly capable of handling a lot more power than Dexcom’s tiny battery.
So start from there, and then add plenty of extra “headroom” for loss in your AC inverter. (Maybe 6 watts, == .5 Amps, of “wasted energy” from a 12v auto battery. Turns on the indicator light, fires up the electronics, and so on.). The Dexcom charger itself has more power loss, while it’s doing the exact opposite job (AC back into the DC you started from, but at different voltage and with “smart” electronics tresting the battery from time to time.) So expect right around 10-20 watts of total power from your original 12V battery source. That’s a really tiny load, shouldn’t be a problem. If you’re away for more than 10 days, you should run the car (radio and other accessories OFF) at moderate RPM’s to stoke the battery back up.
As I mentioned earlier, the important !!! is not the load; it’s whether than Dexcom charger can accept cruddy AC from a cheap inverter and still convert it into a nice, stable DC current into the Dexcom. In AC power from the wall, the voltage goes up and down smoothly, it’s an exact sine wave. But inverters which to try create AC from DC power sources don’t have actual generators with moving parts; they just modify the DC power electronically. They usually send a ragged signal, with choppiness. These distortions in the pattern happen with each cycle- 60 times per second, and because they’re not random “noise”, they can be described mathematically. It’s called “total harmonic distortion”.
If Dexcom can’t say whether the charger and battery can handle “modified sine wave” as input to the charger (without shortening battery life or hurting the electronics inside the Dexc), then I’d pay extra to get a “pure sine wave” inverter. And it’s a lot of extra money, you will not find a good one for less than about $100. (“Modified Sine Wave”, for comparison, can be purchased for only $20.)
DO NOT BUY a “pure sine wave inverter” which doesn’t state it’s maximum “total harmonic distortion” among it’s specifications. (That figure should be there, right along with the “peak watts” and “continuous watts” numbers.) If they don’t give you the number then it’s probably garbage, pushing out tons of distortion, but still claiming to be a “pure sine wave” inverter for marketing purposes.
(Unless you’ve got some fancy equipment, it’s really hard to measure harmonic distortion- except in high-end audio, where you can hear it with your ears. So these liars think they’ll get away with false advertising, and they nearly always do so successfully.)
The most widely respected Internet-Selling Inverter among Solar Power people who need medical-quality, ultra-clean Inverters is probably “Don Rowe”. If I were shopping today, I’d get this one:
$90 with free shipping. They’ve got another one (with less maximum power) for $20 less, and you don’t need anywhere near 180 watts – but the cheaper one doesn’t state it’s maximum total harmonic distortion in the specifications. This one states “less than 3%”, and that’s an excellent figure.
Radio Shack? Absolutely not! They don’t sell an inverter of this type (not even ONE). Everything they’ve got in on the Internet is cheap “modified sine wave” garbage, and the prices are way too high for the stuff they are selling. Yikes! I wonder, WTF was that Dexcom person “thinking”?