Make sure the fans are spinning freely and not dislodged, since replacing the battery requires the whole motherboard to come out, I would start by retracing steps checking whether everything is back in place correctly, other than that I cannot really think of anything else. If you did not remove the heatsink from the motherboard then I do not suspect thermal paste to be the issue, so that leaves either an issue with the fans and airflow into the heatsinks, or possibly an aftermarket battery causing something to misbehave which is more unlikely. You can use a tool such as TG Pro to monitor different temperature sensors on the Mac which might help in narrowing down where the issue is