Battery charging

Last weekend our 2012 Golf (60k miles) started and ran, but all the dash indicator lights were on. I turned it off to restart and battery was dead. Charged battery for 3 hours, started and ran fine. I checked voltage on battery when idling, and was less than 12v. I raised the rpm to 1500 and started charging at 14.5v, went back to idle and still charging at 14.5v. Took the car to parts store for battery test, they said battery OK, alternator bad (was not at 1500 rpm). We have been driving the car, and battery is charging, just not when first started until reaching 1500 rpm. I have checked voltage at the end of a drive, still charging at 14-14.5v at idle. Check battery not running every morning 12.4v, belt seems OK.

Why would charging not start until 1500 rpm, then continue charging when back at idle? Is there any computer control of charging?

dgraham said:

Last weekend our 2012 Golf (60k miles) started and ran, but all the dash
indicator lights were on. I turned it off to restart and battery was dead.
Charged battery for 3 hours, started and ran fine.

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Check the battery terminals are clean and tight.

I checked voltage on battery when idling, and was less than 12v. I raised
the rpm to 1500 and started charging at 14.5v, went back to idle and still
charging at 14.5v.

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That is odd for several reasons, not the least of which no charge should be
about 12.6 volts with a full battery. 14.5 volts is real high. The fact you read
14.5 volts at idle after you went to 1500 rpm makes me still think you have
loose or poor terminal to battery connection. It's not normal to read such
low volts at idle.

Took the car to parts store for battery test, they said battery OK, alternator
bad (was not at 1500 rpm). We have been driving the car, and battery is
charging, just not when first started until reaching 1500 rpm.

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You may have an alternator problem. Some of the autoparts store have
very good electrical system, battery, charge, start handheld testers. One of
them uses a print out. I would go to a different brand of store and try them.
The store clerks sometime are not very knowledgeable doing tests. What was
wrong with the alternator? There should have been a reason given. It is
possible to make voltage but now power (ability to put out amps at that volts).

I have checked voltage at the end of a drive, still charging at 14-14.5v at
idle. Check battery not running every morning 12.4v, belt seems OK.

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That is all goodness. Changing alternators is the last thing you should do.

Why would charging not start until 1500 rpm, then continue charging when back at idle?
Is there any computer control of charging?

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The voltage regulator is controlled and does not charge at one fixed
voltage. When battery is charged it lowers the voltage so it does not cook
the battery. Your auto store said your battery was OK. I would look at the
terminals are clean and tight.

I recently had battery issues after accidentally letting mine drain but finally bought a cheap trickle-charger and let it sit for a total of ~30 hours (per its instructions) and everything has been good for weeks.

Maybe you need a better/longer charge?

Alternators need a certain amount of internal energy before they can self-excite and start generating electricity on their own. When they do, the voltage jumps from battery resting voltage (12.5ish volts) to charging voltage (13.5ish).

Generally the RPMs needed to do this is below 1000 or so... but for some reason your alternator is needing to get to 1500 RPM before self-exciting. Once it excites it can generate energy on its own... even at idle... which is why once you blip the throttle over 1500 and then return to idle the battery voltage stays at charging level.

Loose connections are a possibility, but since the alternator self-excites thru the BAT light let's start there: is your BAT light on the dash coming on when you turn the key to "on", and then going out when the engine starts and you rev over 1500?

battery symbol on with key on, goes out when started even when not initially charging. No error lights or indication it is not charging. Just checked the alternator connector and do have 12v at connector with key on.

When the auto parts store checked for charging, I had not increased to 1500 rpm, so would fail the test. If I increased rpm and did the test again, it would probably pass. Seems like something in the alternator, and looks like a PITA to take out.

If you have high battery voltage you may have a bad battery ground.
Take a look at the voltage from the negative terminal to a good chassis or engine ground or better yet check both as either could e the problem.
When troubleshooting many ignore the power return paths and this makes it difficult to find problems there.

I have a 2010 TDI sportwagon and recently discovered the *exact* same thing.
The card died while at the mechanic for something else.
I came to reclaim it and they said "You can't drive it! The alternator isn't charging!"
I got the car home without issue and charged the battery up.
Curious, I hooked up the VAG-COM and took it for a drive.

At startup, battery voltage showed 12.46V with a dip to 10.72 when cranking.
Generator load, however, sat at 0%. I panicked thinking this was the problem they described. However, once I started driving and the RPMs exceeded *1500* the alternator began charging! (Load while cruising ~41% and Voltage showing 13.83V)

My conclusion was that at the dealer they had started the car and left it idling while testing the other issue but because the RPMs had never exceeded 1500 that it drained the battery. This would also explain why it's never happened to me because I've never simply started the car and left it to idle.

But the question remains: What gives?
Is this normal behavior? Some form of energy savings idea?

On older cars there's an "exciter" wire to the alternator that feeds it battery voltage via the alternator light on the dash. If that wire is disconnected or broken or if the dash light is broken the alternator won't self-excite until about 1500 RPM. Wonder if you have something similar going on...

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vanbcguy said:

On older cars there's an "exciter" wire to the alternator that feeds it battery voltage via the alternator light on the dash. If that wire is disconnected or broken or if the dash light is broken the alternator won't self-excite until about 1500 RPM. Wonder if you have something similar going on...

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Interesting. I have an '82 BMW 320i on which that's exactly the case.
The alternator light on the dash is integral to the circuit.
The alternator light in the Jetta is still good, but to your point, perhaps something else in the circuit is off?

The ECU controls the fielding of the alternator. This is true of many (most) newer cars. And the field state may be 0% at idle right after a cold start to reduce the load on the engine, which is normal.

I do not have a CR car here today to check this on, but lost of cars are like this, which results in many mistaken charging system diagnostics.

oilhammer said:

The ECU controls the fielding of the alternator. This is true of many (most) newer cars. And the field state may be 0% at idle right after a cold start to reduce the load on the engine, which is normal.

I do not have a CR car here today to check this on, but lost of cars are like this, which results in many mistaken charging system diagnostics.

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This also fits perfect.
I knew in this day and age that a bulb couldn't be responsible in a car with the heart of an iPhone. I know manufactures have been working very very hard to eek every bit of performance and mileage out of modern cars including variable rate AC compressors and computer controlled charging systems.
My wife's 2012 CRV was recalled for a software patch to correct the charging algorithm which left the battery dead on occasion.

Next test: Start and monitor "Generator Load" as function of time without elevating RPMs or moving the car ...

Honda was one of the first ones to do this, back on the late '80s dual-injector TBI 1.5L Civic engines. At least that was one of the first ones I remember coming across. There is a load sensor in the underhood fusebox. No solid state processor working on those, however. It was just a timer and a load sensor to turn the field circuit on or off. Not the fancy duty cycling we have today.