Quote:
Originally Posted by Grandma’s 68
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Yes, that is a good description of how ammeters use a shunt to measure current without having it all flow through the meter itself.
The system used in these trucks is very similar, but not as refined or precisely calibrated. It's designed to let the driver know if current is flowing to or from the battery (charging or discharging) and roughly how much (more needle deflection = more current) but not marked with any numbers to measure the current in amps. So instead of using a calibrated shunt resistance, they simply rely on the inherent resistance of the charging system wire. And the meter movement itself has a bit heavier gauge wire than what you would typically find in an ammeter used with a shunt resistor. The principle of operation is the same though.
The 4 amp fuses (mentioned earlier in this thread) serve two purposes. (1) They protect the wiring leading to the meter if there is a short to ground. And (2) they also protect the meter movement and it's wiring if there is an open circuit in the shunt wire. An open circuit in the shunt wire blocks current flow (like completely closing off the restriction in the water flow analogy). That leaves the meter (or paddle wheel in the bypass) as the only other path for current (or water) to flow. But it's not designed to safely carry that much current so the fuses provide protection in that situation too.