I have a Canon 1DX mk2 that i bought without a charger the battery that came with it was clone and didn't want to charge with a Canon LC-E19 I borrowed from a friend but it did with a Canon LC-E4 charger. The catch being the camera was wise, with it being a clone and the screen did not work with it so i made the mistake of thinking it was charged and it wasn't. Canon Batteries are rather expensive so I set about buying an old LP4 that was useless and tried to change the batteries, i used 18650 cells as per the original but getting them to fit was a nightmare so much so I gave up with that plan .I even tried sanding the sides down a bit to make it fit .
Plan B to repeat the process with a new old battery ,was to undo the plates at the front 8 screws in total then prise the plastic open like an oyster shell to reveal the inners .I took the old ones out, 2 cells were good and one was dead. I replaced them with 16650 cells as this would be easier to get them in the case. I used a powerful soldering iron to remake the strap connections. This means not to much heating of the surrounding cell, I also made a jig to hold the cells in the same shape as the battery box. It worked first when i tested the set up and has continued to to hold charge that i put in via Canon LC-E4. It can be done
