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There are two main wires that normally come off the battery, one extremely large one that feeds the starter, and one reasonably thick wire (usually around 10 ga) that feeds everything else in the car.

A classic style ammeter runs in series with the battery wire that feeds everything else in the car. It samples which way the current is flowing and how much current is flowing. If the alternator is pushing current into the battery (charging the battery) the gauge reads positive. If current is flowing out of the battery, it should read negative, and usually means that your charging system is not keeping up with the electrical demands.

Since the classical ammeter is hooked up in series, it involves very thick wire (like 10 ga) to be routed all the way to the ammeter in the cockpit of your car. With all that current flowing through the ammeter, a short circuit on the ammeter in the cockpit can be fairly dramatic, and some classic car guys avoid ammeters for the possibiilty of causing a fire.

There is also an ammeters out there that have an external shunt. An external shunt is basically a very low value resistor that is in series just as the ammeter was in the previous example, and then the ammeter is in parallel to the shunt. Basically MOST of the current goes through the shunt and only a small percentage goes through the ammeter. By knowing the resistance of the shunt and the meter, the meter is able to calculate the total current by measuring only the small current that goes through the meter. This kind of setup allows small wires to be run to the ammeter in the cockpit, and the high current to remain in the engine compartment. The danger in these setups on some old cars is there isn't a fuse on that small ammeter wire, So if the shunt is damaged or a connector breaks, it would try to flow all the current through the remaining path which is the small ammeter wire and that would melt that wire (along with whatever else it is next to).

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โˆ™ 2011-09-12 21:29:37
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Q: How does an electric ammeter work?
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