In article Bephh.212$pr5.108@trnddc01, "west"
wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.

Cordially,
west

yep. the amp meter will definitely need a shunt. that value is dependent
on the actual coil current for full scale.

the voltmeter can just wire directly across the output voltage
terminals, but the ammeter will require a shunt.

if the full scale is 50 amp, and the meter is a 1 ma movement, then
49.999 amps must be taken care of in the shunt. the value of the shunt
can be found like this:
you need to apply enough current to the ammeter to get full scale. this
will require a very careful setup.
then measure the voltage across the meter terminals.
now you know V and I.
then you can calculate R for the shunt sine it will be V on the meter,
and 50.000 - the current required to get a full scale reading on the
ammeter.