The NiZn are awesome, but they have a couple draw backs. Their voltage at peak can be as high as 1.8 volts. When 4 are used at once that's 7.2v, when the recommended is 6v. I have heard that the extra voltage can kill a flash that runs on 4 batteries (even though the flash should be able to handle it).

I hear they work great in anything that uses 2 AA batteries though.

Thanks, Willis. My favorite line (of course) was "So don't think it is the necessarily NiZn batts' fault. It's physics."

It seems like they're saying it's not actually the resistance of the batteries that is causing the problem because they say the same thing happens with an external power supply as well. That means even the lithium iron phosphate batteries I mentioned before wouldn't help, either, because it's the power delivered to the flash tube itself that isn't getting completely converted to light (and it never will with a xenon flash tube). What someone needs to do is mod their SB-900 with a fan blowing across the tube. Or, better yet, a water cooling system that you could wear on your belt and be the nerdiest photographer around but be able to shoot at these kinds of recycle times for extended periods.

The folks at Strobist got around to posting some info on these guys for anybody that's interested:

http://strobist.blogspot.com/2010/01/new-nizn-batteries-offer-lightning-fast.html

Wow, nice find jonny. I hope that does come to camera tech soom.

They might be worth a try. To clarify their capacity rating, on the picture of the batteries you can see they say 2500 mWh (milliWatt*hours), but the standard way of reporting it for NiMH is in mAh (milliAmp*hours). If you want to know (in joules) the energy storage capacity, you can't compare the two straight up. For example, I've got some 2600 mAh NiMH's in my flash right now.
2.5 W*hrs = 9000 J
2.6 A*hrs * 1.2 V = ~11000 J
so the NiMH actually edges out the NiZn batteries. It's a pity engadget didn't test a set of high-capacity NiMH's, too.

As far as using them for flash, to me it seems like the capacity isn't probably as important as the internal resistance of the batteries (the quicker you pull charge out of them, the more they resist and heat up). If the NiZn have better flash recycle times and don't heat the unit up as much while doing it, then that could make it worth it to switch. I wish they'd been more thorough.

Also, someday soon I'll wager we'll see these batteries being used in flashes and not just in autos.

I'll be listening also. That is the one thing I don't like about my SB-900. As much as I appreciate the camera not killing itself, it is a bit annoying at the same time.

Soap?

I'd go with Eneloop batteries.

I saw this out on engadget today and was curious to know your thoughts. I've read that low voltage (1.2 volt) NiMh batteries are more taxing on flashes than higher voltage Alkaline cells (1.5 volt). These are 1.6 and rechargeable, so I thought it might work better in a SB-900 (which is prone to overheating).

I don't know a whole lot about this stuff, so I leave it to the forum to educate me.

http://www.engadget.com/2009/11/09/powergenix-nizn-rechargeable-aa-batteries-finally-some-cells-w/#continued