I tried replacing the Ø2" ammeter on a 20-year-old adjustable-amperage Chinese battery charger. After trying it in different situations, I found that the ammeter is special for charging batteries. It will give a reading when charging a battery. It will not give readings when a DC motor (pulling 4.5 amperes) is connected to the charger (while a DC ampere panel meter will give a reading). Can someone please explain this or give a link to a good explanation? And, what are the actual specifications for the type of meter I need for measuring the DC motor's current draw? How can I make sure I don't get a battery charger ammeter?