Usually when Li-Ion batteries go dead they can’t be revived, but perhaps others have a different experience?
As for using the Hirose connector for external batteries, I do that all the time. I built a cable with the Hirose on one end and banana plugs on the other. Then I connect the bananas to various battery packs. I have used 12 V SLA packs, 14.4V power tool batteries, a 12.8 V LiFePO4 battery, and a battery pack consisting of two Sony M-type batteries connected in series.
The 12.8 V LiFePO4 battery is the power for a bicycle light, and it's also perfect for a 722.
http://www.batteryspace.com/waterbottlelifepo4battery128v6800mah8704whforhidhalogenbikelighttrial-techfemaleplug.aspxThis battery will run my 722 for over 8 hours. Just don’t try to take it on to an airplane; it looks way too much like a bomb.
Here are some tips:
1. DON’T EXCEED 18 V INTO THE EXTERNAL (Hirose) CONNECTOR! Sound Devices says > 18 V will cost you an expensive repair. Note that “18 V” power tool batteries will be much greater than 18 V when charged, so DON’T CONNECT “18 V” POWER TOOL BATTERIES.
2. In the 722, the switchover from external battery to internal battery can sometimes cause the unit to shut down. This is because the 722 will toggle from internal to external as the external battery recovers when its load is removed. So you can’t just assume you can change the external battery after the power has automatically switched to internal. The switchover occurs at about 9 V, which is lower than you want to discharge a 12 to 15 V battery anyway. In particular, Li-Ion doesn’t like deep discharges.
3. Soldering to the Hirose is difficult because the pins are so small and so close together. Make sure you don’t short pins due to excess solder.