The Canon AE1 Program battery is a 4LR44 6V battery. You can check out prices and pick them up from here. They usually cost under $10 each.
One of the most important things to be aware of is knowing how long your battery is going to last. To avoid getting caught up shit creek without a paddle you should always ensure to carry a spare battery around with you. These are so small and cheap enough that you can afford to keep a few of them stored in your camera bag or pouch.
What Is The Best Canon AE1 Program Battery?
The best batteries on the market are said to be either Duracell or Energizer, one article claims the best batteries for your camera are the Energizer ultimate lithium, but these can come in at almost 5 times the price of other batteries.
If you are wondering how long does the Canon AE1 Program battery last, there is no clear cut answer as it all depends so much on usage. However, with regular use and not taking into account too many long exposure at night time, the battery is expected to last over a year. This has also been my experience with the camera.
If you are into long exposures during night time, there is a hack you can do to ensure the battery doesn’t die and the shutter closing prematurely on your shot. It involves wiring up a circuit outside of the camera battery housing with four 1.5V AA batteries wired in series. Ensure to connect these with the proper polarity with blue tac to the camera’s positive and negative. For more information on this refer to the following article.