A common technique for adding electrical equipment is to wire it in to the battery feed.
That can be OK for short time, high draw items, like a dump bed or a plow.
But if the engine is running, the ammeter will show power going though the charge line to the battery and then to the added equipment. So the ammeter is no longer reading just battery charging. Further, whenever the battery is running these items (alternator can't keep up or engine is off) the ammeter doesn't show the battery discharge.
I suspect this is one of the reasons ammeters disappeared from cars about the time TBI and electric fans became common.
