MacBook Pro replacement battery capacity issues

I bought a replacement battery for my MBP Mid 2012 (non-retina). I already received two replacement batteries and both have the same issue. They suddenly loose half of their capacity when reaching 50% status! So I charged them both up to 100% and made an SMC reset. Then I disconnected the Power Supply and used my MBP as usual. As soon as the remaining capacity of the battery reached about 50%, it dropped immediately down to 7%! Even after three recharges the problem still exists. ifixit already sent me a second battery because I believed the first one to be faulty. What could cause this strange behavior? The MBP is running macOS 10.4.3. Are both batteries faulty?