A simple, and very effective shunt can be made from a piece of steel sheet metal about 3/8" wide, 1/32" thick, and long enough to bolt to both ammeter posts. Enough current will bypass the ammeter to keep it from pegging out on a heavy charge, and there will still be enough current flowing through the ammeter to make the needle deflect a little, indicating charge or discharge. There's no need for a terribly sophisticated electronic device when the ammeter is going to be turned into an inaccurate charge/discharge indicator by whatever shunt is installed.
Jerry
