Recently obtained digital voltmeter.Battery newish 3 yr old,12 yr old 80A alternator with new voltage regulator(Monarch aftermarket)
Found voltage across batt terminals 13.99 with engine idling and with engine off,immediately 13.1 V,cooled down 12.55V .
With volt gauge,the voltage drops down to 11V if the car is very hot like idling in traffic with air con and fan on.Once moving,the volt gauge moves up to 13-14V range.
is the heat sensitivity or idling sensitivity normal?Can someone post a treatise on what can be done with a digital elctrical meter which are now so cheap to buy?

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Tom 2010 C250 4-matic Sport (Canada)

Voltage is related to current and current is related to power. Power is the end result. Current over time is power.

Current is the most usefull criteria in judging alternator output but considerable can be told from what you have seen.

The most important piece of info is that at idle with the AC on you are using more current than you are producing.

Every car is slightly different but usually you will be using about 50 amps with the AC on and the aux fan running.

With an 80amp alternator it is very likely that you can only get 30-40 amps at slow idle. Whenever the voltage of the runing system is less than the voltage of the battery with everything shut-off then current is flowing from the battery. Whenever the voltage is greater than battery voltage the alternator is making more curent than is being used and the battery is being charged.

A charged battery with very little load will have voltage and current regulated. Voltage will be between 13 and 15v and the current if known would be just a few amps. If one turned on the lights the alternator output would rise to include the lights and maintain the small charging current. The alternator is thus controlled till it is doing everything it can. Once at peak for the speed the voltage drops.

The aux fan proably takes close to 20 amps. The inside fan close to 15 amps, the ignition and fuel pump take another 10 amps. With head lights and wipers one would just about take everything that 80A alternator can make at speed. At idle it only takes the AC and outside fan to overcome the alternator.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician