[davidmanze]

Most  Li-Ion batteries have cells inside them that looks like AA or AAA batteries, Logically Camera manufacturers make the shell then put those batteries inside.
Those batteries are dirt cheap, they look like this
So instead of buying new BP511 batteries to replace the aging ones I have, bought a pack of those and put 2 inside  each battery, it went to function as new !!!

My experience is they are not the same cells apart from their size,voltage and rated capacity which they rarely achieve, they are not the same quality and usually do not have the same life expectancy as OEM versions. 
 Canon, Nikon,Sony etc have very stringent specifications about the cells have manufactured for them  ..... I've proved this many times where I've bought TPM Nikon equivalent batteries to save money, they just didn't last, whereas all my Nikon ones are still going with no failures to date. 
 Just to add to that my ebike came with a 48V Li battery and still works after 6 years, I bought a spare "no make" version which lasted for 15 months ..... I now only buy ebike batteries from Germany which use the Samsung 35E cells, the batteries are €450 each ..... they use same cells that Tesla used to use in their earlier electric cars.
 Btw, cheap cells often produce gas which trips a pressure switch under the positive terminal and they go open circuit ...... you can depressurize them using a watch screwdriver but it doesn't last long before it gasses again .......