*AL* said:
I was told never disconnect the battery when the engine is running as damage might be done to the electronics. Disconnecting the battery was a test years ago for the old dynamo system, if it cut out it wasn't working. Don't disconnect an alternator on a modern car whilst running.
you can do it. have done it plenty of times to test without any issues (i used to be a mechanic). there really is no difference because when the car is on, the battery is not in use, all it does is recharge so there would be nothing different happening if you take off the battery leads.
pop starting a manual car with a dead battery is the same exact thing.