I'm wondering if anyone has anyone knowledge as to the logic/behavior behind the battery voltage shown on the DIC.

At times the voltage is static at 14+ v.
Other times it hovers around low-13 to hi-13.

I'm just wondering how it represents the state of the battery - does either give you an indication of charging or not? That sort of thing.

thanks

 

It's primarily the voltage regulator, regulating the charge into the battery.. I wrote this for another forum:

A fresh battery will have a resting voltage of around 12.7V. (measured at 65 degrees after sitting with no draw for 12 hours). The only way to measure this accurrately is with a digital voltmeter AT the battery terminals. Just switching on your car will skew the numbers as you're throwing a bunch of power draws into the equation which will lower the V. readout. When an alternator is running it will put out between 13.5 and 14.1 volts into the battery depending on the batteries state of charge. This voltage is regulated by (duh!) a regulator. A batteries ability to hold and accept this charge has nothing really to do with how much you're trying to stuff into it. A fully charged batter will generally show an alternator supplied voltage of about 13.7 V or less depending on the voltage draw of the cars electrical systems. A battery with a low charge will typicallly show 14V + or so as the battery capacity is replenished by the alternator output. A totally bad battery may not accept a charge at all. A higher charge rate will show at the first start of the day as the battery recovers lost voltage from sitting and parasitic draws as well as needing to recover from a high load engine start. Batteries naturally age and over a period of time their ability to hold a charge and full voltage diminishes. At 4 years or so this usually drops to about 50% of the original capacity. A bad or shorted cell can also cause failure as all cells need to be fairly consistant...

Test a battery by either:
1)Taking a resting voltage reading
2)doing a load test

Test an alternator by:
1)measuring voltage while running. Anything over the batteries resting voltage means it's putting out. Voltages less than 13 or so means it's not putting out enough..Sometimes a bad or shorted battery will not accept any voltage even when the alternator is good.

Precise Resting Voltage measurements (% of charge) are a good indicator of a batteries health:

 

Yes, it gives you some idea what's going on. Lead-acid battery charge rates are usually controlled by voltage, with the following nominal values:

The "fast charge" voltage is about 14.4 V.
The "slow charge" voltage is about 13.8 V.
The no-load battery voltage is about 12.5 V.
The full-load battery voltage is less, often about 8.2 V.

So, typically, you'll see the voltage at about 12.5 until you start the car, where it'll drop down a bit (probably too fast to see) while starting, then the charging system will run it up around 14.4 for a while before tapering down to 13.8 or so.

Of course it might not be able to do this while the RPM is very low, especially if there's a lot of load on the electrical system (like headlights and fan motors). So seeing the voltage drop off a bit while sitting at a stop light isn't unusual.