Cold Ambient Temps = Higher Alternator Voltage?

[otrfun]

Our '16 Ram 3500 Cummins (220a alternator) typically charges around 14.3 - 14.4v about 95% of the time. On occasion charge voltage will drop to 13.9v on a long trip when the batteries get topped-off really well.

This morning temps were almost zero (F) when I started our truck. Alternator voltage ramped up to 14.7v momentarily then immediately down to 12.1v (battery sag voltage after one engine start) about 4 or 5 times before it finally came on steady at 14.6v (it seemed like the voltage was exceeding some kind of threshold and was purposely being turned off, then on again for another try). In any case, it remained at 14.6v during our entire 6 hour drive on the freeway. Typically it will gradually drop (14.4v to 13.9v) as the batteries work towards a full charge. However, no voltage drop whatsoever on this trip.

After a bit of snooping online, it seems some manufacturers boost charging voltage when there's very cold ambient temps. Any chance this may be occurring with our truck?

[otrfun]

OP here. Charging voltage is now back to normal (also no more voltage "cycling") after ambient temps rose above 50 during the last few days. I guess that confirms our Ram does have a temperature compensated charging system.

Although I understand the need for higher charge voltage when temps get very low, I don't quite understand the need for the grid heater "cycling" I experienced when temps got down to zero or so. Why does the grid heater need to cycle while the engine is running? Why not remain on for a certain period like it does when it preheats before starting the engine?

[ScottG]

wolfe10 wrote:

ScottG wrote:

wolfe10 wrote:
Yes, the intake heater grid can stay on after engine starting in very cold weather.

NORMAL.

If this continues for more than a few minutes after starting-- not normal.

Yes, in most cases but if it is extremely cold out, the grids will start cycling again if the truck is stopped and idling - even if it has been running hard for hours and is at full operating temps. It may also go to a high idle under those conditions.

Wow. The good news is that we have never been in cold enough conditions to have that happen.

So-- disclaimer: No idea what happens in well below freezing conditions, as that is when we head SOUTH.

Yeah, we've learned to never be surprised when the girds light up. Even a super cold wind will turn them on out of the blue. There's a long section in the maint. manual about the logic but suffice to say, there are many conditions that engage them. Some are even emissions related.

[otrfun]

ScottG wrote:

wolfe10 wrote:
Yes, the intake heater grid can stay on after engine starting in very cold weather.

NORMAL.

If this continues for more than a few minutes after starting-- not normal.

Yes, in most cases but if it is extremely cold out, the grids will start cycling again if the truck is stopped and idling - even if it has been running hard for hours and is at full operating temps. It may also go to a high idle under those conditions.

Thanks, wolfe10 and ScottG. Good to know.

[wolfe10]

ScottG wrote:

wolfe10 wrote:
Yes, the intake heater grid can stay on after engine starting in very cold weather.

NORMAL.

If this continues for more than a few minutes after starting-- not normal.

Yes, in most cases but if it is extremely cold out, the grids will start cycling again if the truck is stopped and idling - even if it has been running hard for hours and is at full operating temps. It may also go to a high idle under those conditions.

Wow. The good news is that we have never been in cold enough conditions to have that happen.

So-- disclaimer: No idea what happens in well below freezing conditions, as that is when we head SOUTH.

[ScottG]

wolfe10 wrote:
Yes, the intake heater grid can stay on after engine starting in very cold weather.

NORMAL.

If this continues for more than a few minutes after starting-- not normal.

Yes, in most cases but if it is extremely cold out, the grids will start cycling again if the truck is stopped and idling - even if it has been running hard for hours and is at full operating temps. It may also go to a high idle under those conditions.

[wolfe10]

Yes, the intake heater grid can stay on after engine starting in very cold weather.

NORMAL.

If this continues for more than a few minutes after starting-- not normal.

[otrfun]

Just to confirm, the on/off/on/off grid heater cycling everyone is referring to is occurring while the engine is running, right? In our situation, the voltage cycled up to 14.7v, down to 12.1v, up to 14.7v, and so on, every 1-2 sec, about 4-5 times right after starting the engine. I'm used to the grid heaters activating just before starting the engine in cold weather, but wasn't aware they may continue to operate after the engine has started.

Is there a particular ambient temperature where grid heater cycling (while the engine is running) starts occurring? Never had this occur in temps down to 25-30 (just moved to the mid-west; we're experiencing our first "real" winter with the truck). Ambient temps were just above zero when the voltage cycling I mentioned in my first post occurred

[Fuller_Johnson]

sch911 wrote:
Grid Heaters cycling on and off. NORMAL!

Yup, and the Grid Heater draws around 100 amps. Enough to even dim the headlights :E

[sch911]

Grid Heaters cycling on and off. NORMAL!

[otrfun]

Appreciate all the replies. Good to know others have observed similar temperature related voltage changes with their alternators. If our alternator voltage does not drop back down to 14.4v when temps warm-up in a week I may very well find we have an electrical and/or battery problem. We'll see.

theoldwizard1, we monitor battery/alternator voltage using our Tekonsha brake controller (displays tenths of a volt). Verified accuracy with a Fluke VM. Been monitoring voltage daily since the truck was new (5 years). Voltage typically hovers around 14.3 - 14.4v about 95% of the time. On occasion charge voltage will drop to 13.9v on a long trip when the batteries get topped-off really well.

[mbopp]

Yes, higher voltage is a design feature. At one time I had a graph showing temperature vs voltage for a charging system. Now the alternators are controlled by the engine computer.

[Fuller_Johnson]

The Dodge Cummins engine has Grid Heaters in the intake for cold weather starting. This will cause the voltage to drop after starting in cold weather. The heater will cycle on and off until the intake manifold reaches 60 degrees or you go over about 7 MPH. I think that's what you are seeing.

[CA_Traveler]

My experience with a external accurate voltmeter is that alternators/regulators are rather poor at adjusting the charging voltage due to temperature. Perhaps related to how/if temperature is detected.

Both my RV charger and solar charger with external probes attached to the battery terminal adjust the bulk/absorb charging at expected.

[theoldwizard1]

What are you using to measure the voltage ? The gauges on you dash are not very accurate.

Constantly >14.0V is actually not "normal" ! After running for 5-10 minutes, the voltage should be <14.0V

Make sure all battery cable connection are clean and tight. Constant high charging voltage could also mean your battery is on its last legs.