As I mentioned.....I have never tried testing one using a battery charger so I can't really speak with authority to that.

I'm not sure if this is a known working tach that you want to check for accuracy or if you are simply wanting to know it will function.

To check for functionality, connect the tach to a car that has a coil.
I have done this with Mopar tach's (and aftermarket) many times.

Lift the hood and place a towel or similar on the inner fender area and put your cluster/tach on it. Connect an alligator lead from the battery (+) terminal to the tach's long stud (blue wire terminal). Connect battery (-) terminal to the tach's metal case, ensuring a good ground connection.
Connect an alligator lead from the coil (-) terminal to the tach short stud (gray wire). Start the engine and rev it, look for tach deflection.



This absolutely does work so providing all leads are good and connected properly and you get no tach response.....then the tach is bad.

This may not work if the car has MSD ignition.
It DOES work if the car has Mopar electronic ignition.

EDIT: After er-reading the thread I realize you just want to see if the tach will function.