Correct charging voltage?
 

I am trying to track down a smell that has occurred since installing an AGM battery in my 87. It's not a familiar smell, best described as warm plastic. It's in the frunk. It appears to be an issue on longer drives, i.e. 3-5 hours, the ambient temp was close to 90. Smell with and without the fresh air fans running. I don't see any toasted wiring or burnt fuses.

In an effort to track this down myself I think I've narrowed it to the battery. It's been pretty hot when the smell is most noticeable. Everything works, car starts fine, no weird aberrations that I can tell. I'm seeing voltages at the battery with the car off in the mid 13's. Cold idle 14.98 and warm idle at 14.6/7. I don't see a lot of variation with rpm changes, but I'm doing this with a multimeter in my garage.

So, are these numbers a) normal or b) indicative of a bad voltage regulator or defective battery c) none of the above, in which case any alternative theories are welcome.

"Cold idle 14.98 and warm idle at 14.6/7. "

The 14.98 volts is too high. That means that at the alternator in the rear, the alternator
is putting out 15.5-16.0 volts.

Did you mean to say mid 12's

That level of voltage drop alternator to battery will only be present with HIGH charging rate. If you do check and find that to be the case then the VR is a problem.

The battery state of charge rises during charging, accordingly, so does the value of the charging voltage also rises.

The SOC of the battery is what actually limits it terminal voltage so whatever voltage the alternator puts out, within limits, is of no matter.

My computerized home charger stops the charge cycle once the battery terminal voltage reaches, stabilizes just below 15 volts. At that point other than a slight trickle charge current the charger goes off-line and the battery terminal drops to the ~13.55V (2.25 volts/cell) standard for lead/acid batteries

No, a fully charged lead/acid battery with no load will have a terminal voltage of 13.50V.

Could you direct me to the source of that data?

you do mean 12.5 VDC don't you?

I just re-checked the battery and saw 13.10. At this point (and lively discussion) it looks like I can confidently start with the voltage regulator. Hey it will give me an excuse to powder coat the housing and fan, whoopee. This would also explain the acid damage I had previously attributed to constant trickle charge by the PO.

just went through this myself, each lead acid cell puts out 2.1 volts, so 12.5-12.6 should be what you're seeing on a fully charged battery without the car running

For interest, I just checked my car, 75 911 with a 3.2, driven 18 hours ago (5 hour trip) and the battery measures 12.5v..

Cheers

2.25 volts/cell x 6 = 13.5V

LS400 battery, Schumacher WM-6000 charger, automatically drops out of charge mode at 100% charge indication.

100% = 13.34 battery no load terminal voltage, Fluke 75 series DMM

2.15 volts/cell vs 2.25 volts/cell...theory vs practice?

http://www.powerstream.com/SLA.htm

2.25 is the float voltage of the battey. 2.1v is battery voltage.

Battery Voltage Information

http://www.landiss.com/battery.htm

It doesn't turn off it switches into float mode which is the 13.5 volts you see

13.34V was the measurement with the charger disconnected.

Same results with lawn tractor battery tested overnight.

Is this with the hi/lo pressure switch?

Charger in float mode connected to LS400 battery overnight. Read 15.7 volts this AM. Disconnected charger and the battery terminal voltage slowly, minute of so, declined from 15.5 volts to 13.7 volts

Just to save the work of taking out the alternator it might be a good idea to have that battery load tested to see if it came with a bad cell.

to end the thread, I put a new voltage regulator in yesterday and the numbers appear normal per Waynes book. Thanks for the discussion.

cool

Wasted effort, COST..??

According to this automotive lead-acid "flooded cell" battery charging systems are not compatible with AGM batteries.

Absorbent Glass Mat (AGM) Battery Information - Battery University.

5th paragraph, last sentence:

"Automotive charging systems..."