If you are using a digital camera you are definitely going to need a battery, but many film cameras also operate on batteries. In film cameras, the batteries operate the flash and they also operate the automatic mechanisms inside. These can include an auto focus lens and an automatic rewind. You may also need batteries for other film accessories such as meters. It's always a good idea to have at least one spare battery as you never know when your power is going to run out.
It's also a good idea to use rechargeable batteries so get yourself a charger. All you have to do is place the battery in the charger and plug it into an electrical socket. If you do this, you will always have a charged battery in your bag. Batteries come in various sizes, shapes and power. Make sure you have the right ones for your camera or accessories. They range in price from a few dollars to almost $100.
