Dead Battery Causing Stall?

[willbur]

Greetings All-
A neighbor's 1999 Porsche would not start at the mall. A kindly good Samaritan gave him a jump and it stalled while driving home. Would a dead battery cause this? I would expect the alternator to keep the engine running. The alternator puts out 14.2 vDC with all accessories and lights on at idle. Cranking volts with the old battery showed 8 vDC after 3 days trickle charging. Ideas?

 

[martinq]

Greetings All-
A neighbor's 1999 Porsche would not start at the mall. A kindly good Samaritan gave him a jump and it stalled while driving home. Would a dead battery cause this? I would expect the alternator to keep the engine running. The alternator puts out 14.2 vDC with all accessories and lights on at idle. Cranking volts with the old battery showed 8 vDC after 3 days trickle charging. Ideas?

Wow, okay...

So with the car running you have 14+ volts in the system? I would check for parasitic draw.

If you don't have a sensitive amp clamp for DC you can use an amp meter in series with the battery but you want to bypass the meter with a suitable jumper until the car shuts down (sleep). You want a very low current reading, hopefully less than 30ma.

Another useful tool is a thermal camera / FLIR. Scan the electrical after the car has been sitting overnight (cold), you might spot some heat coming from a module, sensor, fuse from the troubled circuit.

If it's not a fairly new / AGM battery it could be toast from being dead too long.

 

[zzyzzx]

Unless the affected person had a voltmeter when driving around we really know nothing.

 

[Astro14]

Replace the battery. A dead battery could pull down the voltage of the charging system as it struggles to charge it. Low voltage = poor running.

 

[willbur]

Wow, okay...

So with the car running you have 14+ volts in the system? I would check for parasitic draw.

If you don't have a sensitive amp clamp for DC you can use an amp meter in series with the battery but you want to bypass the meter with a suitable jumper until the car shuts down (sleep). You want a very low current reading, hopefully less than 30ma.

Another useful tool is a thermal camera / FLIR. Scan the electrical after the car has been sitting overnight (cold), you might spot some heat coming from a module, sensor, fuse from the troubled circuit.

If it's not a fairly new / AGM battery it could be toast from being dead too long.

If it were my car, I would do the parasitic draw- but its not my car. What puzzles me is that it stalled while driving- the alternator should have kept it running, He swears the connections are good, so no loose connections to cause this.

 

[willbur]

If it were my car, I would do the parasitic draw- but its not my car. What puzzles me is that it stalled while driving- the alternator should have kept it running, He swears the connections are good, so no loose connections to cause this.

I quickly tested his battery and it needed replacement. He said he trickled charged for 3 days or more so the testing was likely valid. This car sits around a lot (30k miles since 1999) and battery probably sulfated. He bought it used at 28k

 

[Trav]

Greetings All-
A neighbor's 1999 Porsche would not start at the mall. A kindly good Samaritan gave him a jump and it stalled while driving home. Would a dead battery cause this? I would expect the alternator to keep the engine running. The alternator puts out 14.2 vDC with all accessories and lights on at idle. Cranking volts with the old battery showed 8 vDC after 3 days trickle charging. Ideas?

A bad diode in the alternator can cause the battery to drain even when the engine is turned off, brushes that are just about done can cause intermittent charging issues. Both are replaceable in this unit without to much trouble. It sounds like the battery is shot, replace that first.

 

[nwjones18]

If it were my car, I would do the parasitic draw- but its not my car. What puzzles me is that it stalled while driving- the alternator should have kept it running, He swears the connections are good, so no loose connections to cause this.

A bad battery can cause a car to die/run badly even if the charging system is working. Could have an internal short or something. Need to replace battery and retest charging system.

 

[DCDG]

if you're only getting 8v after sitting 3 days on a charger there's the answer battery is no good. 8v cranking? what is the voltage initially before crank?

 

[Marcozi]

I've read of instances that when a car is jump started the electronics got a bit whacky. I suggest the Porsche owner scan he car for codes.

 

[wings&wheels]

Yes, and a weak battery (and this battery is well beyond weak) can also cause all sorts of odd issues with these cars; uncommanded window dropping/drooping, radio volume changes, etc. These cars also have a high parasitic draw when sitting unless the vehicle is locked which puts many functions to "sleep".

 

[willbur]

if you're only getting 8v after sitting 3 days on a charger there's the answer battery is no good. 8v cranking? what is the voltage initially before crank?

iIRC- it was in the low12s= maybe 12.2. Yes, he said he was going to have the battery replaced that morning

 

[willbur]

Yes, and a weak battery (and this battery is well beyond weak) can also cause all sorts of odd issues with these cars; uncommanded window dropping/drooping, radio volume changes, etc. These cars also have a high parasitic draw when sitting unless the vehicle is locked which puts many functions to "sleep".

Yes- if it were mine I would test for the parasitic draw. But what threw me is that I expected the alt. to provide sufficient DC for the car to drive, esp bc it shows 14.2 vDC with everything turned on at idle. Maybe that battery was a dead short taking all available power?

 

[Leo99]

I don't know about modern cars, but we had a 73 Cutlass with a bad battery. We knew it was bad so we had it valet parked at 7pm at Yonker's racetrack so the valet would have to deal with jump starting it while we feigned ignorance. They got it started and we were 60 miles from home. It stalled out 3 more times on the way home and the headlights barely worked. The night entailed a bag full of guns, cut payphone cords, an incompetent AAA driver, a methed up armored truck driver and finally a 4am call from a diner to my friend's mother to come pick us up. Maybe that's why I'm so adamant about replacing a battery at the first sign of weakness.

 

[wings&wheels]

Yes- if it were mine I would test for the parasitic draw. But what threw me is that I expected the alt. to provide sufficient DC for the car to drive, esp bc it shows 14.2 vDC with everything turned on at idle. Maybe that battery was a dead short taking all available power?

Possibly.

Also, if they have the original US spec H6, an H7 is a direct drop in and an H8 is an easy install requiring only a nut below the battery tray and a good lower back. More capacity is a plus.

 

[willbur]

Possibly.

Also, if they have the original US spec H6, an H7 is a direct drop in and an H8 is an easy install requiring only a nut below the battery tray and a good lower back. More capacity is a plus.

Yes I run H8 in mine