I use a small 12V battery (specifically a motorcycle battery) to power my Snap-on digital timing light with the RPM function and a delay flash for checking timing advance, and use the inductive probe on the spark plug wire to trigger the flash. You don't necessarily need to use vehicle power to run the timing light electronics. If the output is erratic, use a jumper wire from the vehicle frame to the auxiliary battery negative terminal to provide a common ground. My digital volts-ampere tester works the same way.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!