The other day, I concluded that the Medis 24-7 Power Pack fuel-cell gadget-charger wasn’t a very interesting product, based on its specifications. The spec sheet didn’t make it look as if the fuel cell could do anything you couldn’t do with much cheaper conventional batteries.
I’m now indebted to blogger Techskeptic, a man after my own heart except less lazy. He, as he mentions in the comments for the original Medis post, actually bought some Medis power packs and tested them thoroughly.
Techskeptic tested three Medis Power Packs, and found that they actually managed to deliver only about nine to 13 watt-hours into real loads. Medis claim twenty watt-hours in their literature, and it’s that figure on which I based my own unimpressed response.
So these things actually appear to be even worse than they seemed.
The lousy real-world performance could be due in part to Medis optimistically listing the amount of energy the fuel cell actually (sorta-kinda) delivers on the sticker, rather than the amount of energy that makes it out of the Power Pack, down the cable and into the device you’re charging. There’s a DC-to-DC converter, you see, that takes the very low output voltage of the fuel cell (less than one volt) and boosts it to a gadget-charging level. And that converter turned out to be only about 70% efficient at best. Into a one-watt load, it dropped to about 60%.
So Techskeptic concluded that the Medis device didn’t even beat a pack of six alkaline AAs. Actually, you’d probably get better results than the fuel cell if you hooked a similar voltage-booster up to a single humble D battery.
(Little kits to make that sort of converter, usually to allow you to replace low-capacity 9V batteries with beefier but lower-voltage cells, have been around for ages. Here’s one that’ll boost the output of two cells to 9V; I’m sure I’ve seen single-cell versions as well, but can’t find one right now.)
So Medis’ numbers would appear to be, at best, sort of like the old gross horsepower measurements that told you how much power a nude engine - no transmission, no air filter, no exhaust system, no alternator, no nothin’ - on a test-stand once managed to deliver. This did not have very much to do with the amount of power that would make it to the rear wheels of a car powered by the same model of engine.