ericbergan

What is the normal voltage for the battery?

I have an A7 with the stop/start feature, which has quit working most of the time over the last couple of months. I've taken it in a couple of times and the shop keeps insisting it's fine, just need to drive the car more to keep the battery charged. Well, driving on the highway for 30 minutes wasn't enough to get it to kick back in, and on those rare occasions when I drive further and it does, it's not working the next morning.

So, I got out the voltmeter, and the battery is reading about 11.9 V after sitting, and after highway driving for 30 minutes, it only got to 12.3 V, and 30 minutes later, had already sunk back to 12.1 V. Battery is about 2.5 years old, and we're actually having good weather - highs low 70s, lows in the high 40s.

So, does this sound like a battery and/or charging problem? Or are these usual readings? Thanks for any insights!

Eric

Oscar2

What do you mean when you say the start/stop has quit working? Do you mean that when you come to a stop the engine doesn't stop? or that it doesn't start back up when the time comes to move again?

ericbergan

Doesn't stop - the A with a line through it appears on the dash. The shop says that when they observed it, it was failing an energy system check, and so the answer was to drive the car a lot to charge the battery.

tenspeed

Take a battery reading with the engine off, start the engine and take another reading.

A healthy lead acid car battery should be around 12.5 volts at rest. You should be seeing around 14.5-ish volts with the engine running right after a start. It will drop down to the low 14's after 15-20 minutes as the battery recovers.

Another thing you could do is put the battery on a charger overnight. Take it off the charger and take a reading, then take a reading a few hours later to see how much it drops. A big drop would mean that the battery is getting old or it could be a larger than average parasitic draw. Repair shops and parts stores can load test a battery.

ericbergan

Great minds thinking alike...

Earlier, realized that putting it on a charger might give me an idea of what maximum voltage it might read on a full charge and whether it would still fade so fast (about .1V an hour) if it was at full charge. So, hooked it up a couple hours ago, and we'll see what happens!

irenesbob

If the battery has been allowed to drain completely (i.e., dead battery) even once or twice it can severely reduce its life and affect its ability to ever reach full charge again.

DB22

Modern cars rely more heavily on a good battery but rarely do these cars have any indication as to the battery's condition. Older cars used to have ammeters but they were unnecessarily heavy and complex so they later started using voltmeters. Now we really need one, they don't give us one so I purchased a couple of cheap battery condition meters and keep it plugged into the cigarette lighter socket of all of my cars. You soon get to know the typical readout under most conditions and any variance from the norm can prompt imminent issues. BTW - these devices are about 10 to 15 dollars.
My 14 A6 has the socket in the armrest area but my Acura has it exposed in the center console and always visible at a glance. As of now, I loaned my daughter my meter because she is having battery issues too in an Odyssey van.

There was a VCDS mod for battery meter indication but I have never heard of it working for a C7 A6. A real shame.

uberwgn

Even if you can't turn on the battery meter in the car, you should be able to view the battery screens with VCDS:

uberwgn

Also, I'm fairly certain the start/stop function ceases to operate when the battery is weak.


These cars have huge electrical demands, Erik. It may be a long-term issue if the car sits unused.


You can do a quick scan with VCDS and view whether the BEM system is activating itself. If so, I think you need to have the battery load tested to determine if it's a problem. If not, you may need to leave the car on a battery tender device if it's going to be sitting.


Some folks claim that newer cars are going to need replacement batteries far more frequently than we're accustomed to.

ericbergan

The car gets driven daily, but mostly just around town/neighborhood. The battery has never been completely drained, or needed to be jump started.

I now have two conflicting facts - after charging the battery yesterday up to 12.7 V, this morning the voltage had dropped back down to 12.1 V. But, the stop/start function worked immediately, and so far has kept working today.

I'm going to have to monitor this over the weekend. It could be that energy management is doing some things even while the car is off, and that is why the voltage drifts down, and then stabilizes around 12.0 V, and the voltage therefore isn't a reliable shortcut way of the determining the charge of the battery?

Got a bunch of highway driving to do this weekend, which should further confuse the issue...