I thought I'd add this info here, too. I just picked up a Powerrunner PR2310 li-ion battery, and it works great out of the box for powering the JB3. Original message is posted below. In it, I mention run-times. These were calculated based on power consumption draw of the JB3 at 300mA for recording, which was posted in a more recent thread than the above posting which assumes a 600mA draw. I have no idea which is right, but if the JB3 draw is more like 600ma, then the runtimes I listed below should be cut in half. Either way, the Powerrunner appears to be a good method of providing external power to the JB3. If it were me, I would remove the internal batteries when using the external powerrunner (or any external battery). That way, you do not use the external battery to charge the internal batteries.
Well, not as nice as using internal batteries, but I just got a powerrunner li-ion external battery, at the doorstep when I returned from DC yesterday. I got it here:http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=15055&item=5773905259
Switchable output voltage of 3, 4.5, 5, 6, 7.5, and 12. Works like a charm with the JB3 at the 5v setting, right out of the box. No mods necessary, no buying extra parts or adapters or adaptaplugs. Really a very easy power solution to the JB3. At the 5v setting, it has a 7.2A-hr capacity, which given the power draw of the JB3, should be enough to run it for 24 hours. Or stated otherwise, the equivalent of 8 internal JB3 batteries, which would cost apparently about $400 if you could find them. The model I got, PR2310, was $70 from the above link. The same vendor has the PR2200 model which is the same physical size, but has half the capacity, for $50. This should run the JB3 for about 12 hours, or the equivalent of 4 JB3 batteries ($200).
I'm testing my battery out right now, running my JB3 without its internal batteries. Seems to be working great, booted up fine and I'm recording at 44.1k wav seemingly fine. I'll check the recording file in a bit, but I imagine it is all fine. I doubt I'll have the motivation to test exactly how long this will run, since it will probably be at least 20hours and more like 24.
I need to dig up the other thread, but I thought someone else tried this product and said it didn't work. I'm guessing he didn't have access to a voltmeter. The one issue I did find is the cable that comes with it is labelled wrong, so according to the actual labeling on the cable you'd have to run it with the adaptaplug in the "tip-negative" position in order to get it to have +5v at the tip (which is really "tip-positive"). I discovered this right away with my voltmeter before I ran it into the JB3. So besides this mis-labeling on the cable (no idea if this extends to the other ones they sell, but I imagine), it works fine, you just need to insure you've got it set up to provide +5v at the center tip of the cable (hence access to a voltmeter is handy).
Anyway, nothing against Marc's VR box, but to me it is easier just to have one battery, not a battery and a voltage regulator box (unless you're trying to run two pieces of equipment off the same battery setup). Bottom line, the Powerrunner is an excellent option for out-of-the-box JB3 powering, allowing long runtimes at little cost.
Note: The powerrunner is also available from the vendor off ebay at their actual website, though for a bit more:http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=919