RPM increase when positive cable disconnected

[B-Reynolds]

Any reason why the RPMs would increase when I disconnect the positive battery cable? I noticed this by mistake on a 1999 Mercruiser 3.0 while trying to track down why several gauges are not working.
this is a recent purchase so I have no history on the boat. I have noticed the boat is slow to get on plane but otherwise runs fine.

 

[Bt Doctur]

Well, there is no load on the alt so that would increase the rpm. but disconnecting a battery cable with the engine running is
not a good idea either. It can cause the alt to go wild, kill electronics ignitions, most electronics, panel bulbs and running lights