Ammeters have internal shunts usually. These can be supplemented by an additional wire. These things are done to "recalibrate" an old meter to read/handle an upgraded charging system. It's all about the size of the shunt/resistance (big copper buss bar or big wire 'jumper"/bypass/whatever) to get the right voltage drop/small (millivolts) sample current for the little old meter movement. In our case with older trucks we only need to see charge/discharge, not some exact value. As you start the motor, battery gets lowish, so you see plus on the meter for awhile. Showing alternator is replenishing/charging battery and running other things. Then goes to "zero" (needle biased to plus side for battery holdup) showing battery is filled up and alternator is running all. If you see negative after driving awhile, either you have too much load and battery and/or alternator can't keep up or something is wrong with battery or charging system....or meter. Battery voltage will drop and won't start motor. Negative reading on ammeter can also indicate the wires are on the wrong terminals.
The voltage regulator is to handle it all for you.
The battery was only to start the motor. Now-a-days it runs clocks, security, radio memory, etc. when motor is off.
