I’ll try my best. I have the OWC Nupower and it does not last any longer than an original new battery. I wouldn’t put too much stock into that claim. Related it the 65 watt spec, it’s definitely higher watt than OEM, but probably makes a small unnoticeable improvement.
I don’t think there is any correlation to the fans running and the battery other than the fans killing the battery sooner. If your CPU heats up because of intensive processes, then your fans kick in to cool the machine. Battery has nothing to do with it to the best of my knowledge. Batteries don’t give off heat while discharging.
Thanks for your reply. What you say does make a lot of sense, I don't know either why a poor battery would impact on the fans kicking in, but I did read a few times that it makes the machine work harder when battery is old. Just think it's an 8 year old MBP so isn't going to be tip top. Can't see what the original watt spec is on here. Just that it says 'replace soon' and cycle count 1871...
Re the 'OEM', it's £39GB versus the OWC which is over £100. Hence I think it's probably safer to buy the OWC.