You can run a meter with a battery charger
Your response to Jim Murdoch (May Mechanic on Duty) is partially incorrect. He would not be able to use a battery charger to run a tach/dwell meter. Smart chargers need to see at least 9 volts to turn on. In addition, the output isn’t fully regulated; there is a significant AC ripple on the output. That will drive the meter crazy.
Using a 12-volt battery would be the best way to get an accurate reading from the meter. He would also need to make the negative connection of the 12-volt battery common to the negative post of the 6-volt battery. The meter won’t see the pulse without a common ground.
Your points are well taken but they don’t apply to all battery chargers. I’m not an expert on battery chargers but can say with certainty that the large Snap-On charger I’ve been using for a little over 20 years does not need to see anything to turn on and provide current. I’ve used it to independently power a variety of different things ranging from electric fuel pumps to tach/dwell meters.
But in light of your experience with a different charger perhaps my answer should be modified to say that a 12-volt tach/dwell meter will accurately measure the engine rpm and dwell angle with a 6volt vehicle as long as the meter is powered by 12 volts so all one has to do is connect the meter to a separate 12-volt battery or to a 12-volt battery charger that can deliver adequately regulated voltage to the meter.