disconnect battery while running

you said it's never good to have the battery disconnected while running the car as it can cause an electrical spike but as a diagnostic we disconnect the battery to see if the alternator keeps the vehicle running as a form of eliminating the alternator from the equation would you suggest not to perform this or is it acceptable for a quick analysis?

Back in the day it was regarded as a big no-no, although I know of people who did it without apparent problems.

Nowadays electronics are much more resilient and regulators faster acting, but I'd still be wary of doing so - maybe at idle and with lights or some-such to act as a drain?

Exactly what are you checking for with the disconnect?

I would never recommend this practice nor see a situation where this would ever be necessary. How long does it take to get a DMM and test that way?

Some cars can cope with this happening, others cannot, and it can be quite damaging to components in the vehicles. I know of a race series that smoked a number of ECU's and other components on the engine when this test was done (kill switch testing) and the wiring jobs done where not up to scratch. The current found that the path of least resistance was through either the ECU or Driver box, blowing either/both of them up.