I've got BMW 520d my startstop button got slow so I thought battery no key fob still don't no as soon as I detached the battery to put new one on I lost all power now I got nothing at all elec thanks 

Morning Alan

Welcome to the Forum

If I am understanding your issue 1. You say your Stop Start button got "slow" do you mean you had to push it several times? Or was the car turning over slowly when starting?

2. You say "no key fob" do you mean the Remote Locking isn't working ?

3. You say "as soon as you detached the battery to put a new one on you lost all power" did you fit a new battery?

Starting at the end with the Battery (3) If you have replaced the battery is it the correct one for your car ? Does your car have "Stop/Start" built in or just the Stop/Start button ? A new battery should be minimum 90ah but probably 110ah and AGM, when taking a battery out disconnect Neg first the Positive. Take care as the positive has a fused link that is designed to cut power in an accident when fitting the new battery the reverse Pos connected first then Neg. Before everything will work you need the battery registered to the car by someone with a BMW Diagnostic program ISTA, BMW 1.4.0, Autologic, Carsoft or similar without registration the battery may not charge correctly and some functions may not be available. Most decent garages will know this and be able to do it.

Item (1) The Start/Stop button can be changed but if the car was turning over slowly with a fully charged battery get the starter checked they can fail and draw massive current killing batteries.

Item (2) If the cars battery has failed or is very low the remote locking will not work and once a new battery has been fitted and registered the key may need registering with the car again. It should open OK using the key and with the key in the ignition start.

Hope this Helps

Happy New Year

Dave