When i had my EOS Rebel T1i, I had 5 batteries. I number them and make sure they all get roughly equal use (allowing a battery to sit for ages without any use isn't good for them.)
Two of the batteries were genuine Canon batteries. Three of them were 3rd party batteries. I don't recall the brands by name, but there were 2 different 3rd party brands.
My experience with them is that all three third party batteries were noticeable degraded or failed after a little over a year. The genuine Canon batteries were still working extremely well three years later.
No battery will last forever -- think of them like the tires on your car. One trip to the store doesn't mean you need a new set of tires, but they do wear at least a tiny (if imperceptible to us) amount on each use and after enough use they will eventually need to be replaced.
The third party batteries were noticeably cheaper than the Canon batteries -- but apparently not just in price, but also in quality. If I have to buy batteries 3 to 4 times more often then am I really saving anything? More importantly, if the next time I need to use my camera I discover my batteries aren't working well enough to get me through the day then what I saved on batteries just cost me missed photo opportunities.
After that experience, I concluded that it was safer to stick with the genuine batteries and I wasn't so desperate to save a few bucks to risk the bad experience. I no longer buy 3rd party batteries.
As the saying goes "your mileage may vary" -- but those were my results.
Tim Campbell
5D III, 5D IV, 60Da
