Mike, I spent some time digging around for info on this Battpak gizmo today and can at least answer your second question. A few sites that sell this product have a FAQ associated with it (here's one). From that FAQ:
Q7 Can I power my appliance with the BattPak" while I charge the batteries?
No, you must first remove the power input (iSun®, AC or DC input) before using the BattPak" as a power source for your devices.
As for whether the device will work off of a 24V source, that's unclear...but I'd guess "no". The device *does* have some capability to handle variable input voltage, as it's designed to work with the same company's solar power gizmos. According to other parts of that FAQ, the device can accept 6V input when charging 1-3 batteries, so it's smart enough to handle that. It is also obviously capable of stepping down the input voltage appropriately to match the individual battery cells inserted for charging. However, without more infomration, it's hard to know if it is sophisticated enough to handle input voltages much greater than 12V. I would not assume that it could without some assurance form the manufacturer, and so far I can't find that in my web searches.
The corporate website for the company that makes (made? it's possible the device has been discontinued) the Battpak is here: http://www.icpsolar.com . Unfortunately, I can find no product information via that site. Some of the sales outlets linked back to this host for FAQ and other product details, but those lnks no longer work.
I have a handful of unused Eneloop batteries and a decent charger lying around (mostly) unused, so for the price of another handful of batteries and this gizmo, it's a fairly cheap experiment for me to see if it will provide a reasonable amount of runtime for the iFly. If it doesn't work out, it might still make a handy emergency cell phone charger, or I'll just eat it as a failed experiment and be a little wiser for it...
BTW, if anyone else thinks about buying this thing, the same company Walter linked to also sells it through Amazon.com for ~ $10 cheaper.