Thanks guys!

 

I'd bet brake lights and possibly ABS are drawing down the power some?

 

Couple of things:

1) When I coded up a low battery voltage detection algorithm for an automotive data logger I had to put in a time component on both the low battery voltage detection code and on the battery voltage recovery detection code.

Voltage was going up and down -- short duration dips and spikes going up and down -- and triggering spurious low battery voltage alarms and battery recovery alarms.

2) I do not recall the low battery voltage trigger level or battery voltage recovery level. These levels were configurable. Source code is at work and I do not have access to it here at home. IIRC the time component was 1 minute but this was configurable should a particular model of car exhibit longer duration "spikes" or "dips" in its battery voltage. Remember this code had to work in any OBD2 vehicle, from 1996 to present and in the USA, Canada, Europe, Japan (since IIRC 2008), and other places where OBD2 compatible vehicles exist.

3) In looking at some data logs from my Turbo over the last couple of months I see some swings in battery voltage. For instance at 9:32 into the trip voltage was 13.70V. At 9:54 it dropped to 13.13V. Then at 11:52 it dropped some more to 12:31V. And at 12:17 it recovered to 13.14V.

This trip was sometime in July of this year and here it is August is almost over and the battery is just fine.

There are a number of trips with similar voltage behavior with no pattern regarding engine RPMs, vehicle speed, intake air temperature or coolant temperature I can see.

4) Might mention I have captured real high coolant temperature spikes when monitoring my Boxster's coolant temperature. This is due to a marginal connector at the temp sensor. Under some conditions -- high g turns on rough pavement -- the connection is momentarily lost and the A/D reading spikes.

My reason for bringing this up is you must be sure the connections the data monitoring device relies upon are very sound. Any wiggle at any connector can have the voltage dropping very dramaticallly only to recover just a dramatically.

While I have gone through a few batteries over the years, fortunately I have not yet experienced a bad voltage regulator with any of my cars.

My only Porsche "experience" with a bad voltage regulator was indirect (thank goodness).

I came upon a Porsche (IIRC it was a Panamera) in a dealer service bay with what proved to be a bad regulator (or alternator). The behavior was the engine would start fine and idle ok for a few minutes, then warning lights would come on and the CEL would light up and the engine would run rough then die. The tech would turn off the key and wait. After some time -- I do not know how long the tech waited but it was on the order of minutes not hours -- the engine would start and the same behavior would appear. He did this once while I was there and I was not there more than 15 minutes or so.

Of course there can be a number of different failure modes.

I can tell you based on my experience vehicles just sitting with the engine off can manifest some sudden and dramatic battery voltage level spikes and dips for reasons I can only guess at. Various electronics in the car come on and go off and when they do this they can put a sudden big load on the car's electrical system and just as suddenly remove that load when they switch back to low power mode.

With the engine running and the electrical load stable voltage levels go up and down and can go low enough to reach "bad battery" levels but provided the level is not maintained for too long this appears to be "normal".

In your car's case, if the battery is topped up in a reasonable span of time with engine running after engine start, and if the battery voltage remains reasonable as the car is driven -- very short duration dips/spikes notwithstanding -- I do not think what you are seeing is cause for concern.

 

Thanks Macster. I'll keep an eye on it to see if this also occurs during normal street driving. Just gotta get out and drive it on the street a little I guess. I have over 30hrs of data that my car automatically stores before overwriting so I have plenty to look at. I also need to get my fuel gauge recalibrated next week as I put in a new interment cluster. That's a bit of a pain in the **** procedure as the tank has to be completely drained. Fun stuff....

 

Any update on this? I'm running lithium battery, which hates the low voltage and likes 14.2+ volts. Even voltphreaks warns of incompatibility with our gt cars. My car is 13.5v at the alternator but 13.1 at the battery. http://www.voltphreaks.com/technologydir.php

I have tried putting a diode in the sensing line to the alternator to see if I could fool it to put out more voltage but I couldn't get this to work. Any ideas??

 

I was at the shop on Saturday and pulled up voltage data from the Motec on a couple of Spec Boxsters. To my surprise the same momentary voltage dips were present corresponding with coming off throttle and jumping hard on the brakes. Must be caused by the brake lights coming on and mainly by the ABS unit load. No biggie then. The Boxsters however held the average voltage steady at around 13.75V, so roughly 0.5V higher than what I'm seeing. We are going to pull the alternator off and have it and the voltage regulator checked. Sounds like most of these cars are running a little low however, just don't know why.

 

If the round red light is flashing, it means it's not reading the correct key. Had this problem a while back.

 

Voltage readings will vary as the car revs up and as simple as the air conditioning blower motor etc can change the load a lot. So dont get too worried about 1/3 of a volt here and there.

 

For voltage issues, one thing that is frequently overlooked is the deterioration of the cable between the starter and the alternator. This cable is known to internally corrode and develop increased resistance over time. Definitely worth checking voltage drop on this cable and replacing it if necessary. After chasing the usual suspects, regulator and then entire alternator, I replaced the cable and all low voltage issues immediately disappeared.