Who understands GMag magnesium power cells?

They advertise a 4-5 oz. "puck" that contains a magnesium "power cell" which is activated by adding salt water.
Claim it'll charge six 1,000mAh AA batteries in several hours and continue to do so 20 times, charging 120 batteries in total before the cell is depleted... so they're claiming around 144 watt hours in total for say 5 ounces of puck...

Anyone with knowledgeable opinions, or specific knowledge?

from the little bit of reading I did the physics are sound - it is a fuel cell. The economics are not so sound, but I suppose it depends on your environmental philosophy and how you would plan on using 120 1000mAhr AA cells. You could purchase 40 AA 30000mAhr cells for about $10 at the dollar store (+ tax). This would also give the same amount of W-hrs. Would last 5-10 years and cost 1/6 or less of the fuel cell. You might be able to get higher peak currents from the rechargables. I say it is an interesting science project, but the economics don't really make sense. Just thought of the weight factor, if this is a significant driver for you then maybe it tips the balance in favor of the fuel cell. 40 AAs weigh a few pounds, less than a pound for the Gmag I suspect.

This sounds to me like a development of the "water activated batteries" that are commonly fitted to life rafts. When the raft is deployed the battery is activated without human interaction and powers internal lights, and external strobe light and sometimes a radio beacon.
The main merits of the devices are light weight, long shelf life, and for life rafts carried in aircraft the absence of lithium .

The drawback is that SALT water is required, in case of accident on a freshwater lake the raft itself will still inflate and work as intended but the lights wont work. Lakes are generally small enough that red flares can be seen from shore, and rescue arranged quickly.

After looking into this more I can see where it could have a niche. As broadgage said, infinite shelf life, sort of. To use the product as advertised, you need rechargeable batteries, which do have a shelf life. But in the life boat application, everything is just ready to use the power provided as is.

I think this is a very interesting concept, but really is only applicable in a SHTF scenario, and you still need to have rechargables that are in a good state. This technology was first demonstrated back in the 60's and is still being researched today.

As far as price goes, the mfg sells them for $70. You can get them on ebay for $30, that is starting to make sense in terms of price.