Have a little crank on the outside of the time capsule that turns a generator and charges a battery. Put a sign that reads "Please turn crank".
Someone ought to turn it within 100 years. 
On a serious note, if you can sleep your processor and it uses 0.35 uA, it would require a 306.6 mAH battery. This assumes a zero internal discharge rate, and some form of battery chemistry that can last 100 years.