I've heard of this famous Y cable. My indie eyeballed it and said there's a little surface rust but they think it is ok to leave for a while. I drove the car today for an hour (very warm day today here in Sydney) and there was no issues with re-starting the car after letting it sit three times heat soaking.

My volts were reading as per below. This is pulled over idling after 50 minutes of start/stop traffic.

Volts ok?

 

Volts are low, so a Y cable is in your future when it drops much lower. Voltmeter has to show pretty much where you are simply to charge the battery. Here is what my voltmeter shows when the car is at final temp and it has a year-old battery, Y cable and alternator. You can see the difference.

 

Doesn’t the voltage eventually drop below 13 once the battery has been fully charged?

 

No, the alternator outputs a constant voltage that is adjusted for ambient temperature.
When cold out the alternator outputs a higher voltage.
At 25C- ish the alternator outputs 13.8V
When warm out the alternator outputs a lower voltage.

 

I’m pretty sure many owners have reported voltage below 13.8v, some at 13, maybe some in the high 12’s.

When you say “when warm,” are you talking about the alternator, engine, or battery?

Doesn't the regulator control the voltage, not the alternator?

 

When warm, I mean the engine is at operating temperature and may be warm enough for the radiator fans to turn on. The purpose of this exercise is to point out that the Y cable is the more likely source of voltage drops to the 12v range. Mine dropped into that range and when all the radiator fans turned on, the voltage would drop to 12 or below. At that point the battery is not being charged. When the battery is not being charged, it fails prematurely. Owners who see voltage dropping ~12v range as the car fans turn on but is over 13 at startup are 99% experiencing a corroded Y cable. I posted my picture of the voltmeter reading to show what it looks like if the alternator, battery and Y cable are all new. With the engine warm and the radiator fans running, I'm just under 14V. My car had just completed a100-mile coffee run.
One thing worth noting. There are no other car electrical systems that I know of that have an electrical connection from the alternator to the battery that can fail by corrosion. Because of this, there are no commercial battery/alternator testing systems that can diagnose it. My very very knowledgeable Indy, who only does Porsche, pulled out his Snap-On battery/alternator testing system and said both the battery (newly installed by the dealer a year ago) and the alternator were bad. Neither were true, but I replaced both and the Y cable and now I'm running 14 V. The proof that it was only the Y cable was the voltage drop when warm and a physical examination of the Y cable, which was indeed toast. I wasted $1000 on that misdiagnosis and I'm hoping to help others avoid the same when they see the voltmeter drop like that.

 

thePRF is correct,,,

Modern 'smart' regulators have a temperature sensing element. When cold, voltage output is higher. When everything heat up,the 'heavy charging is is mostly finished and volts reduce.

It could be confusing for a starting to fail Y cable slightly decreasing voltage to be confused with regular alternator function....

jmo

 

I don't want to confuse this any more than I have, but when it's all new and it all warm and you have driven endless miles, the voltmeter reads 13.8V like mine and you are right about the alternator having some voltage control because my cold start is about 14.1V. If it drops below 12.5v when everything is warm, your battery isn't being charged and you most likely have a Y cable issue as the cause. The important point about the cable is that the sign of a problem is the change in voltage between cold start and fully warmed up with is a drop of more than 1 volt, it's likely the cable.

This below is from multiple sources searching for battery charging data on 12V lead acid batteries:

The optimal voltage to charge a 12V lead acid battery is typically around 14.4 volts during the bulk charging stage, which is considered a safe and efficient level for most battery types; however, the exact voltage may vary slightly depending on the specific battery and its application, with a "float" voltage (for maintenance charging) usually around 13.5 - 13.8 volts once fully charged.

Bulk charging voltage: 14.2 - 14.4 volts
Float charging voltage: 13.2 - 13.8 volts

• Important factors to consider:
  • Battery type: AGM, gel, or flooded lead-acid batteries may have slightly different optimal charging voltages.
  • Temperature: Charging voltage should be adjusted slightly based on ambient temperature.

 

Agree with all of this.

I just want to say that 12.4-12.7V is about steady-state of a healthy battery that’s fully charged. Anything below that, and energy is leaving the battery. Anything above that, and energy is entering the battery.

So if folks see 12.8-13.2V being displayed, that doesn’t mean there is a problem any more than seeing 14.4V being displayed means their charging systems are in better shape than if less.

During engine startup, it’s fine if voltage drops well below 12V as the current demands are high and temporary (expected behavior).