I recently purchased a 5d Mark III via the Beach Camera flash sell. I also bought a spare battery (third party) as a back up. During a soccer match I forgot to recharge my Canon battery and opted to use the spare battery. Around 50% charge remaining the FPS in burst mode seemed to slow down - it was a noticeable change in FPS. I went home charged the Canon battery and tested my FPS again, it was back to normal. So is this phenomenon a battery issue or a camera body issue? Please note that on the Canon battery I never shot burst while the battery was lower than 50%. When the Canon battery finally does reach 50% I intended to test burst mode, but that might take a whole week to reach 50%!
Thanks for any help!