Sternone
No experience of the 496/596. Probably a similar situation. Battery performance is very individual and does of course change with age, usage, temperature, the backlight settings, tracking on and if EGNOS is enabled. My post was intended to draw attention to the possibility of the forecast battery life remaining being over-stated. In a previous post an airtraffic controller sited two occasions when navigation assistance had to be given to ‘flat battery’ pilots. (of course the Portable GPS should not be the primary nav - there have been enough warnings about this)
I would suggest ground testing your unit to avoid unpleasant surprises. Also to know the remaining life when the 'low battery' warning appears so that if in-flight one has the option to reduce battery loads.