Tom Manning

To be sure, I need to retest the fuse behind the subwoofer on the passenger footwell

Yes, the 100A fuse.

trshot

Sorry for the delay. I was finally able to test the 100A fuse and it is good, is it possibly a bad BCM? is there a way to test it? I scanned the car and there are no codes related to a battery issue.

matk

I’ve just read the whole thread and I was wondering what evidence you have that the Starter Battery isn’t charging, as I can’t see any so far?

Putting a voltmeter across it is not an accurate way of establishing if it’s charging. How many starts have you done since you thought it wasn’t charging? If it actually wasn’t charging at all, after a few starts you’d start seeing a lower voltage, I can’t see you posting any evidence of that?

If it’s in fact actually fully charged, all it does is start the car and then nothing else, so there’s no other load. If in good condition it would take minutes/seconds to restore any lost charge after a successful start, and then once fully charged, why would you expect to see an increased voltage across it?

I can’t help but feel you don’t actually have an issue here at all

Tom Manning

Putting a voltmeter across it is not an accurate way of establishing if it'’s charging.

That is the only way to check if a battery is charging. The voltage should come up pretty quickly - but not instantly - to about 14 volts IIRC after starting

matk

Agreed on a one battery car but why would the the Battery Control Module be putting charge onto a fully charged Starter Battery, when it’s the Consumer Battery that’s under load when you’re actually driving?

Tom Manning

Does the charging system for the front battery stop functioning when the front is fully charged? I haven't tested that but I doubt it. As far as I know both batteries charge constantly with the engine running: the rear from the alternator and the front via the BCM.

g0rsq

From WIS it stops charging when the battery is fully charged.

There is a charging system in the BCU which charges up the starter battery. This charge rate is determined by the battery voltage and the battery temperature (it calculates the battery temperature by measuring the voltage and charge current, and uses the interior temperature sensor as a reference)!

Once the BCM has detected the Starter battery is fully charged, it "disconnects" the charging system and switches to a "float" charging mode to maintain the voltage on the battery until it is next required.

So my take on that is after starting the engine, you may see a charging voltage on the starter battery (depending on how discharged it became and the charge rate from BCM), but only until it is fully re-charged, then you will only see the battery standing voltage,

I agree with matk. No information has been given as to whether there is an issue with the starter battery, or if the OP is just concerned he can not see an expected higher charging voltage on it when the engine is running.

Dual battery systems are unusual, and can be confusing if unaware of how the system works.

trshot

thank you, both are new Diehard AGM batteries.

matk

I’ve just reread everything again to try and make sense of it. Am I right in thinking that the diagnosis flow from the OP was……

1. Car giving dash warnings related to battery.
2. Voltage tested both batteries while car is running.
3. Discovered Starter Battery voltage showing lower than was initially expected.
4. Actually the Starter Battery is behaving exactly as it is designed too.

Have I got that right?

If so I’d suggest that in my experience dash battery warnings always refer to the Consumer Battery and not the Starter Battery. Have I got that one right also, as I’m not sure?

I can see that the OP has jumped down the rabbit hole that it must be a Starter Battery issue when I’m feeling it’s nothing to do with the Starter Battery.

I’m feeling this is a Consumer Battery issue and if in fact that battery is new, then it’s feeling to me that the error messages are coming from a parasitic draw on the Consumer Battery while the car is supposed to be in system sleep.

Thoughts?