qldit

Good Morning lt2duffer, nominally a lead acid battery is threoretically 2.2 volts per cell, but generally a fully charged battery will only show slightly over 12 volts in static state.

So a static theoretical voltage on a properly charged 12 volt battery is the amount of cells times the voltage or 6 X 2.2 = 13.2 volts, but as a general rule this is not seen.
Usually it will show in the order of 12.5 volts or something like that.

To charge a lead acid battery properly, an increase over the static cell voltage of .2 volts is needed, so this is where the previous 13.2 volts is added to, by the 6 X .2 for that charge voltage, so you appreciate this will equal an extra 1.2 volts and when added to that 13.2 volts then equals 14.4 volts.

This is why you will see 14.4 volts mentioned commonly in 12 volt battery charging circuits and how it's importance applies.

Now to make it slightly more interesting, the battery elecrolyte in most vehicles is dependent on where they are used and the atmospheric conditions etc.

So in the tropics a battery acid SG in a fully charged battery will be in the order of 1220, whereas in a colder climate the SG will commonly be found as high as 1300.
(Checked using a hydrometer)

This SG electrolyte selection has bearing on the voltage regulators used in alternators and the selected voltage that they regulate at, normally you can expect to see regulators using alternator voltage outputs in the order of 13.8 to 14.4 volts.

Most modern vehicles use alternators with amperage output capacity greater than 35 AMPs. Typical current family sedans have 75 or 100 amp alternators fitted.

When the vehicle is first started the alternator output is maximum until the battery voltage increases and then virtually "float" charges at a much lower current but maintains a relatively steady voltage on it.

If extra power is needed to support accessories, that voltage should remain in the higher state and the current from the alternator should simply increase to support the extra demand.

You can expect a slight voltage drop in the cable from the alternator to the battery under heavy load conditions but you will appreciate even with all accessories running and reasonable engine revs you should not see a discharge situation such as you are mentioning.

If we consider that charge cable and it's terminations has good integrity with it's connections between the alternator and the battery terminal, and your drive belt is in good connection, then the problem you describe should not happen.

It sounds like your alternator is defective, check the voltage both on the battery terminals, and at the alternator output terminal under running and loaded conditions.
It is not uncommon for one or more of the diodes in the alternator to fail and cause a similar symptom. (no guts!)

Under loaded operating conditions you should see no voltage apparent between the engine and the chassis.

A high resistance connection in the negative battery line to the engine or chassis can also give this symptom.
I prefer to have the negative battery cable directly connected to the engine and this ensures better reliability, but a lot of vehicles use the negative lead to the chassis and rely on some pithy cable to go from the chassis to the engine.
This is usually a source of problems in those designs.

Cheers, qldit.