The alternator output wire and the battery positive should always be the same voltage.
Engine off, system voltage is battery voltage
Engine running, system voltage should be alternator output voltage. But if the alternator can not produce enough power to run the electrics, then voltage will drop to battery voltage. Its easy to see on the ammeter if the power is comming from the battery as it will show discharge.
When the engine is running and the alternator system is working properly, power comes from the alternator. The ammeter will be centered, or if the battery was low, it will show battery charging.
'All power goes through the ammeter' is false, and misleading when diagnosing a power feed problem.
