Keeping it simple...start by monitoring the voltage of the electrical system. You can take readings at the battery, or you could keep track of things by tapping the cig. lighter socket, etc. from inside the cabin.
A fully charged flooded lead-acid battery is usually held at 13.8 volts, but considering the cycling in a car and variences in ambient temperatures, it tends to be slightly higher. In my Volvo, with 30 degree F temps, the voltage initially goes as high as 14.8-15v after starting, but tappers down to around 14.2-14.4 over a 15 mile trip.
For a flooded lead-acid battery that's sitting idle, a voltage of 12.6-12.8 represents a full S.O.C. (state-of-charge). A voltage of 12 represents a near zero S.O.C..
If the voltage regulating circuitry, which I assume to be housed within the altenator itself, malfunctions in that it maintains a high voltage level independent of the batteries S.O.C., than as it approaches full S.O.C., excess gasing will result, causing loss of water in the elctrolyte. With the likelihood of having exposed plates, thermal warping and risk of gas ignition from open-air short potential, go against the life of the battery. So too does not having the battery reach a full S.O.C., for sulfation that develops on the plates as the battery is discharged can in time crystallize, effectively insulating that area of the plate from any charge contributions. The result is a battery with decreasing charge/reserve capacity.
The role of the battery is to serve as a power source for starting and emergency back-up, should the altenator fail. Once the engine is started, the altenator takes over the electrical loads, as well as for recharging the battery. Not only does the altenator's circuitry control the charge cycle for the battery by sensing it's load, but it may also adapt the "program" through temperature compensation, as a higher initial voltage is needed to overcome the increased internal resistance that the battery exibits with decreases in temperature.
Another test is to see how much current draw exists when the engine and all accessories are off. To do this, you will need a amp meter (there are many multi-meters that enable many test funtions such as voltage, amperage, resistance (ohms), etc.), and for safety, wear eye/face protection such as googles or a clear face shield.
**Before disconnecting any cable from the battery, make sure you have any security codes that are required for any equipment in the car, as they will have to be re-entered upon reconnection of the battery in order for them to operate.**
With the ignition and all the usual accessories off, disconnect the negative power cable at the battery terminal (a small spark may be noted as the ECU and any security equipment have a small current draw). Connect the amp meter between the negative cable and the battery post. again, a small spark may be noted when making the finial connection. Take a reading. Remove the amp meter and reconnect the battery cable.
I'm not sure as for what may or may not be considered as far as unusual with today's equiped vehicle, though I imagine it's in the relm of tens of milliamps (0.010+), especially if the car has an alarm system, remote car starter, etc, which are armed/on stand-by when the engine isn't operating. Much higher numbers could represent an electrical short of some resistance, or possible impending component failure with the said systems that remain "on" (make sure the car doors are closed and the dome/hood lights are off.
Anyone have more specific figures?
