high alternator voltage due to super cold weather

[Donald]

I noticed that my alternator is putting out maybe .5 volt higher even after driving for 45 minutes. It was -8F at my house. I am thinking the alternator is doing temp. compensation for the super cold?

The car started pretty easily (thanks to PU) so the battery was not used to crank a lot.

This is just an observation.

For the Jeep.

 

[gathermewool]

I noticed higher VDC last night, too, in only 9F.

 

[carwreck]

Your Jeep (according to RockAuto part list) has a battery temperature sensor.
Per allpar.com, the battery temperature sensor measures the air temperature as it flows through the power module, using the temperature to calculate the intake air density and target charging system voltage.
____________
1987 Toronado
1999 Olds GLS

 

[SwampSurvivor]

I drove my Cherokee last night (after it sat for 2 weeks). It was 2 out when I started it.

The voltage seemed to be at about 14.5-15! Drove it around for probably 35 minutes or so allowing it to come up to temperature.

After it warmed up, it went down to normal

 

[strongt]

It's normal. Our Jeeps have a Batt. Temp Sensor as posted by carwreck. I've seen mine go over 1 volt of difference in cold or hot weather. This according to my scanquage.

 

[martinq]

Is the temperature compensation (for the Jeep) in the regulator circuit in the alternator or is there a separate box to deal with this? I've been looking for proper temperature-compensating regulators and they're all in the $300+ range.

 

[HerrStig]

Higher voltage is needed to force charging current through cold electrolyte.

 

[JHZR2]

This is the case even in older, dumber alternators. My 91 BMW, with no temperature sensor, had the voltage ar 14.3 yesterday in the cold, when it usually doesnt go over 14V.

 

[eljefino]

I've seen low 15's for voltage in GMs, but they run a few 1/10s over to charge the "maintenance free" chemistry.

This is IMO one of several reasons you see so many burned out bulbs in the winter. The others of course are it's dark all the time and people probably don't take "car care days" in the dead of winter to solve this sort of thing.

 

[strongt]

Originally Posted By: martinq
Is the temperature compensation (for the Jeep) in the regulator circuit in the alternator or is there a separate box to deal with this? I've been looking for proper temperature-compensating regulators and they're all in the $300+ range.


Voltage regulation is controlled by the PCM.

 

[meep]

ECU controls regulation in chryco vehicles. ambient batt temp sensor feeds to ECU. ECU controls the alt field coils. no separate module.

 

[SwampSurvivor]

I don't believe my XJ Cherokee has a battery temperature sensor.

As always, I don't even think it had to turn over more than once! As soon as I hit the key it fired up.

 

[caravanmike]

my 99 xj has one in the bottom of the battery tray, and as you experience, when cold mine shows .5 volts higher also. mike

 

[KrisZ]

No need for temp sensor to see higher voltages. The temp sensors just make charging more accurate.
It's a given that in really cold temps the voltage on the battery itself gets lower as well as the battery is discharged more during cold starts. By this fact alone the alternator can output higher voltage because the battery is discharged enough to take it.

That is why fully charging a dead battery by the alternator is not recommended, as the voltages really spike up causing extra heat and wear on the alternator.

 

[440Magnum]

FWIW, Chrysler products have had temperature-based voltage comp built into the electronic regulators since they came out circa 1971. First in the regulator, then moved to the PCM or in the air intake path, then later to a remote sensor under the battery itself at least in some vehicles like the Jeeps mentioned here.

I'm kinda curious as to where the sensor is in the LX-series, since they have trunk-mounted batteries ;-)

 

[NHGUY]

My 88 does that,I would imagine the sensor is in the SMEC.

 

[mechtech2]

There is a natural increase in voltage due to the cold. There is less resistance in materials [except for carbon].
And sometimes there is an additional design feature for more voltage cold/less voltage hot.

 

[SwampSurvivor]

Hmmm. I'm going to have to see if I can find that! Didnt know I had one.

 

[JHZR2]

Originally Posted By: mechtech2
There is a natural increase in voltage due to the cold. There is less resistance in materials [except for carbon].
And sometimes there is an additional design feature for more voltage cold/less voltage hot.


That is backwards.

There is a natural decrease in the voltage. And, since it is a chemical reaction, the reaction slows down and impedance grows in the battery.

But the thing is that onset of gassing goes up higher, from 2.47VPC to 2.97VPC, IIRC, so you have a higher voltage threshold before electrolyzing the water. This is needed to overcome the impedance.

 

[3311]

Originally Posted By: JHZR2
Originally Posted By: mechtech2
There is a natural increase in voltage due to the cold. There is less resistance in materials [except for carbon].
And sometimes there is an additional design feature for more voltage cold/less voltage hot.


That is backwards.

There is a natural decrease in the voltage. And, since it is a chemical reaction, the reaction slows down and impedance grows in the battery.

But the thing is that onset of gassing goes up higher, from 2.47VPC to 2.97VPC, IIRC, so you have a higher voltage threshold before electrolyzing the water. This is needed to overcome the impedance.

The decrease in voltage in a cold battery is when it is open circuit. When on charge, as in the case of the op, the voltage of a cold battery will be higher.

Conversely, a hot battery will have higher open circuit voltages and have lower on charge voltages, this can lead to a thermal runaway which will quickly damage/destroy a battery.

There is typically an inverse relationship between current and voltage in the recharging of lead acid batteries. The impedance or resistance that you refer to is the direct cause of this.