Upload & Sell: On
There is no magic in making batteries. In point of fact, Canon is not a battery manufacturer either and that's the primary reason why their batteries are so expensive, it's not because they are any better.
If you buy a third party battery from a reputable manufacturer, you won't have any problems and you will save a ton of money.
I used to believe almost precisely what you wrote here. That's why I originally bought several third party batteries for my (original) 5D, and bought them from SterlingTek, who generally have a, uh, "sterling" reputation. (By the way, I'm pretty certain that companies like SterlingTek are not the "manufacturers" of the batteries, but rather the importers and vendors.)
A couple of summers ago I needed additional batteries for my 5D2 since I was going to spend 8 days on the trail in the Sierra Nevada doing almost nothing but photography. Knowing the excellent reputation of Sterlingtek and have a very positive experience with the 5D batteries I had purchased from them - in fact, I had often recommended them to others - I decided to order a pair of the Sterlingtek replacement batteries for the 5D2. Not only did I trust the company, but their price for the pair of batteries was excellent.
When the two batteries arrived, I put them into my Canon charger. One appeared to charge, though the indicator lights did not work in the usual way. I put it (battery #1) in the camera and the camera powered up.
When I put battery #2 in the charger indicator lights, as I recall, simply came on and stayed on. I tried the battery in the camera, but it held no charge. A second attempt to charge it was no different - that battery could not be charged. I contacted SterlingTek and they quickly shipped a replacement at no additional charge and I returned battery #2.
When battery #3 arrived, it had essentially the same problem as battery #2, the one I had returned. It could not be charged and it did not work at all.
At this point, the pattern of unreliability in these batteries was clear to me, and I had no more interest in continuing the cycle of battery testing, shipping returns, and corresponding with the vendor. I decided to be philosophical and figured that I had gotten one functional battery (battery #1) at a price that was still a bit less than buying a Canon battery.
A short time later, I left on my lengthy pack trip. Since the actual capacity of the Sterlingtek replacement battery was unknown, I decided to use it first, thinking that it turned out to have a bit less capacity, I still had my set of Canon batteries to work with and I knew how many hundreds of shots each of them could take.
The remaining Sterlingtek battery #1 died after something like 40-50 shots.
Fortunately, I was able to stretch my use of the Canon batteries, metering it out so that I had just enough power to complete the trip.
I learned my lesson. My attempt to save a few bucks on a battery endangered my ability to make photographs, and that is not worth any small savings that might be measured in a few tens of dollars. While being cheap is not my highest goal in life, I'm willing to use an excellent third-party product when I find one - remember, I used several of the 5D SterlingTek replacement batteries and recommended them to others. However, I'm not willing to use unreliable or defective products just because they cost less.
In the end, my final tally with the SterlingTek batteries was:
#1 - held perhaps 15% of a charge
#2 - dead on arrival
#3 - dead on arrival
I have no reason to think that SterlingTek doesn't sell other fine and functional products, though my confidence is now considerably lower. But when it comes to the 5D2 replacement battery? Sorry. No thanks.