Redd

I had a voltmeter plugged into the lighter socket during a recent road trip, just to get an idea of what "normal" should be in the Q5. And yes, I know the voltage at the lighter plug is not "full" battery voltage.

I know on some cars that will be 14.3-14.4 exactly, all the time, while on others it might be 12.6 at idle and 13.8 at cruise. Each manufacturer is doing something different with different logic, even when all loads an conditions are accounted for.

But I was surprised to see that Q5 sometimes putting out 14.6 volts (way high) and sometimes 12.9 volts (rather low) on the highway, with no obvious indication of why it was cycling around the voltage through that large a range.

Does anyone know how over-sophisticated Audi's charging logic is? Or what it is based on? Or am I really looking at a new car with a bad (sloppy) regulator in it? I have no idea what Audi is up to, just that such a wide voltage range would mean "bad regulator" on most or many other makes.

rdA4WtQ5

The logic these days is the voltage is higher when decelerating and lower when accelerating. Was that happening?

Redd

No, these were voltage changes while on the highway in cruising "steady state" conditions. Could be related to the initial charge the battery had on it (high after the car had been sitting for a week) or outside temperature (not so much, fairly steady) or the alternator being smart enough to change from bulk charge to float charge--IF it is real high technology. And it might have even read high at the front cig lighter because that's close to the alternator, there must be some further drop on the way back to the battery. Of course I have no idea how Fritz ran the wiring in this thing, I can't begin to guess. (Like no fuse being labeled for the lighters, and "dashboard" equipment like the radio being in the back of the car...)

So, easier to ask.

tenspeed

My theory is that if the battery or the vehicle is calling for more power (amps), the voltage and amperage raises to meet the need. Electrical power is measured in watts. The formula is amps X volts = watts (P=IE).

I see the highest voltage (around 14.6vdc) soon after starting the engine. The battery is being recharged. After a while the voltage will drop down to the low 14's in my GM product.

Some of the Audi products have a regenerative braking system that packs power into the battery while traveling downhill and uses that power to reduce the load on the engine. If your Q5 has that system, it might explain why you saw 12.6vdc.

Redd

I've read that we have conventional wet lead batteries, and that we have sealed valve regulated (AGM) batteries as well. Anyone know for sure which is in the 2014 Q5? The charging protocols differ for the two types. Offhand 14.6v sounds higher than I've heard for either one, even for the initial bulk charge.

woodepic

I just ran into this... I was testing a 360 Watt light bar, trying to make sure that my 2015 sq5 had the power output to support it. Car was at idle the whole time.

Before I turned on the light bar, voltage was 14.4v, and vcds said that alternator load was about 500w.

Then I turned on the light bar, voltage was about 14.3v and alternator load was about 900w.

It stayed that way for 30 mins (at idle) until suddenly the voltage dropped to 12.9v with the light bar still on, and 900w of alternator load according to vcds.

I assumed the alternator failed, so I immediately turned off the light bar and drive home. On the drive it bounced around from 12.9v - 14.0v, but when I got back and started idling, the voltage was still 12.9. With the car off the voltage was 12.7v, so the alternator was still doing something.


We'll see if my car provides 14.5v when I start it up tomorrow. Maybe the alternator was over heating? Although maybe these Audi's use a fancy charging logic that periodically switches from 14.6v (providing all the power you could ever need) and 12.9v (to make sure your battery isn't dropping charge?). Now I'm curious.

A6Gary

If you see the battery voltage drop below ~14V while the engine is running, your alternator is most likely "dead", i.e., not sending out a charge to the battery. Recently, I had this experience on my old 1998 Dodge Ram 1500 when the alternator died while driving back from a short trip. All of the electrical functions started to stop working (speedometer, fuel gauge, temperature gauge, etc.) when the voltage gauge approached the 12V mark. Had to ensure I didn't stop because I would be able to restart the engine! Barely made it back to our house.

woodepic

Okay update, I redid my test, and it looks like the 12.9V is normal, just part of the charging logic.


Before the test, my battery was at 12.3V.

I ran my light bar, with the car at idle for 1 hour.

Initially the voltage was 14.4v, but after ~20 mins, the voltage dropped down to 12.9V (still well above the battery voltage). I assume this is because it is done charging the battery.

After a while, I turned on heated seats and rear defrost. Voltage immediately jumped to 14.3V to accommodate the larger load.

After the 1h was up, I turned off the car and checked the battery. It was now 12.4V, aka higher than before. So it looks like the fancy charging logic works 🤷‍♂️


So I assume it's completely normal behaviour for our Q5's to do this. I know they have battery management computers, and batteries are complicated so this doesn't surprise me.

woodepic

As long as your voltage is above your battery voltage, your alternator is definitely doing something. On our Q5's the alternator has a sensor that let's us log alternator load in vcds. I'm not certain how this is calculated, but I assume that's how much power the alternator is providing (not the battery).

I know you can also see in measuring blocks the current going in/out of your battery (could be helpful for diagnosing a bad alternator)

SMac770

Will depend on wet flood vs AGM, and what the J533 perceives as the battery state of charge and the present electrical system load. At some point, it might shift from charging (14s) to floating (13s) depending on who knows what, to prevent overcharging. The J533 normally targets 85% SoC.