I bought a 396 last year at Airventure.  Before I got around to wiring it to
ship's power we were flying 3 to 4 hour legs plus automobile use always with
weather operating.  Recharging was usually within an hour or so after use on
either 110 or 12 volt auto plug.  The battery died in January.  Garmin was
very helpful with software updates (the battery has a chip) but it took a
warranty battery to fix it.  My guess is it does not like deep discharges or
the heat generated by recharging.

The 396 is a giant leap forward in features and function from the 196 or
296.  Aside from the benefits of XM Weather, the screen is much brighter and
easier to read in sunlight.  My plan is to make it my backup nav in the
Legacy since it will default to its own battery if ships power is lost.

Steve Colwell