You can run a meter with a battery charger

Question:

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.

Answer:

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.