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Bad grounds cause this.
Indeed.

It can be confirmed fairly easily with a digital multi-meter that can read down to 0.01 Volt. Start the engine, turn on load such as lights and heater fan, then measure the Voltage between the battery negative post (not the cable clamp) and a) the engine block, b) the alternator case, and c) the car body. (Be sure to scrape the surfaces to ensure a good contact.) Do this with the engine idling, with it running at around1200 rpm, and again at around 2000. With good grounds the Voltages will be just a few hundredths of a Volt. If any of the measurements is much higher and especially more than one or two tenths of a Volt, there's too much resistance in the ground circuits.

When there's resistance in the ground, the alternator cannot charge the battery properly, and then when the engine speed drops, and the alternator output drops along with it (this is shown in the specs), the system Voltage goes down quickly.

This is but one of the tests that could be carried out when diagnosing a dimming symptom, or to check the "health" of the charging system.

The addition of the large gauge cable bypasses the existing grounds, thereby ensuring a good path.
 
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