The Stovebolt Site Forums The Shop Area The Electrical Bay Wiring in the circuit breaker and shunt?

I teach electrical systems every day, and I see it every time I do a high-load battery test followed by a max output alternator test. For the first 5 to 10 seconds of an alternator output test, whatever max capacity the stator can produce will go right into the battery. It's not uncommon to see 120 amps or more at less than 14 volts after doing a loads test.

To do an alternator max test, load-test the battery to half its cold cranking amp rating for 15 seconds at least twice. This will break down the surface charge of the battery and let it receive a high charge rate for a few seconds. Turn on the high beam headlights, the wipers, the heater blower, and any other accessory available for a couple of minutes. Start the engine, and immediately rev it up to 2,500 RPM or so. Observe the ammeter on the charging system tester. The full capacity of the alternator stator will be applied to the battery. On one we installed on an '06 Chevy Tahoe just before Christmas, the output was 140 amps for about 15 seconds. The voltage did not rise above 14 volts during that time. After about 30 seconds, the amperage dropped to 45 amps, and the voltage went up to 14.2. 10 minutes of running time later, the amperage was just under 15, and the voltage was 14.8.
Jerry

Edit: After a heavy cranking session on a cold morning, the battery will take the full alternator output- - - -whether it's 55 amps, or 150 amps. As the terminal voltage rises, the current will drop dramatically, but for a few seconds, the only restriction to current flow will be the resistance of the alternator stator, because the battery is acting almost like a dead short. If it can deliver 300 amps to the starter, it can also accept a 300-amp charge rate, at least for a few seconds. DC current will flow either way at the same rate.
Jerry