After film, batteries are the most important consumable in photography.  I occasionally use an old Nikon F and guess the proper exposure. Very traditional, lots of fun, but not always the most appropriate method under more complicated exposure conditions.  Batteries provide capability; matrix metering, auto focus vibration reduction, etc.  On most of the more modern cameras, run out of power and you won't be able to expose film under any condition. 

Always use the type of batteries recommend by the camera manufacturer and below are a few tips to insure continue camera operation:

• If your camera can use them, always carry a backup set of lithium batteries.

• Lithium batteries operate in very cold situations and have extremely long shelf lives.

• Always mark and use rechargeable batteries in sets; mixing will reduce battery life.

• Some rechargeable batteries can lose 1% of their battery life every day.

• Some rechargeable batteries need to be fully exhausted before recharging regardless of what the manufacturer recommends.