The ammeter does have a positive and negative post, but if it's connected backwards, it just reads "discharge" instead of "charge" as the generator/alternator output goes to the battery. The direction the current flows will depend on whether the vehicle is positive or negative ground. If the reading is backwards, just reverse the leads at the back of the ammeter.

Be sure all the "load" connections such as the supply wires to the ignition switch, headlight switch, etc., and the charging device output are all connected to the downstream side, and the only wire connected to the other side goes to the battery cable attached to the starter. If accessory wires are connected to the BAT side, the meter will not read charge/discharge accurately.
Jerry

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