I've had my 696 mounted in the panel powered by the avionics bus for 7 years now, and it has worked flawlessly. I pulled it out to update the software and maps, and the battery can't keep the thing on for longer than a few seconds. I think it came with an AC adapter, which solves the immediate problem. But this unit serves as emergency backup instruments in the event of power loss during flight, so 5 seconds of battery usage probably isn't going to be enough to get me on the ground safely.
Garmin wants $200 for a new battery. I don't know what the life expectancy of these batteries are, but I wanted to see if anybody else has experienced similar problems.
I bought one with a "bad" battery that would last a minute or less. I found that it had two problems, a dead battery pack and a bad battery connector. The previous owner had, I think, always used it connected to external power. The battery pack never got charged, so the NiCD cells were deeply discharged. The battery pack has an internal charge control circuit, which refused to even attempt to charge the cells.
So... step 1 was to split the pack and test the cells. All were very low, well under half a volt, but they were all identical. None were reversed or internally shorted. I un-soldered the cells from the charge control board. Fifteen minutes on a NiCD charger had enough of a charge on them that the pack would charge. Perfect! The pack was good. I re-soldered the cells in. If they had been bad, those cells are available... they're an uncommon size, but not unobtainable.
The next issue was the bad connector. The 696 battery connector uses spring loaded pins to contact bare pads on the battery pack. The spring on one of the pogo pins was weak, and one was completely broken. The pack would charge, maybe, until you bumped or squeezed it. I ended up disassembling the 696, finding a suitable replacement and replacing the connector. It's a surface mount part, and the replacement I found is functionally equivalent but mounts slightly differently. Anyway, it worked. It's a twitchy repair in a very tight area of a pretty expensive board, so I definitely would not recommend it if you're not comfortable with some "interesting" SMT repair work.
After the repairs the battery would charge just fine, and would last over 4 hours consistently, with XM playing.