Fluctuating battery voltage while driving
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Fluctuating battery voltage while driving
I noticed for the first time today on a road trip that my battery voltage fluctuates from a low of 12.9 to a high of 14.5V when I'm driving. It seems to level out around 13.5V. Is this normal?
Last edited by rmorin1249; Nov 18, 2016 at 10:41 PM.
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However, sometimes, for no apparent reason, my voltage will read high (14.5 to 15v) for an extended period of time. On other days, it will drift back down to the 12.8 to 13v "fully charged" level within just a few minutes of driving. I keep my car hooked up to a tender all the time when not driving. Anyone have any thoughts as to why this is?
E-Ray, 3LZ, ZER, LIFT
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From: NE South Carolina
However, sometimes, for no apparent reason, my voltage will read high (14.5 to 15v) for an extended period of time. On other days, it will drift back down to the 12.8 to 13v "fully charged" level within just a few minutes of driving. I keep my car hooked up to a tender all the time when not driving. Anyone have any thoughts as to why this is?
Details from the Service Manual: The system will enter the Fuel Economy Mode when battery temp is above 32 F and battery charge is >80%, etc. It will then set the voltage to match the battery voltage if it's between 12.5 and 13.1 volts. If those conditions are not met or the wipers are turned on (and some other parameters) it will enter the Charge Mode regulating the charging system between 13.9 to 15.5 volts. It adjusts the alternator field using PWM 128Hz control.
That is why you see the jumps in voltage. Not your fathers simple charging system!
The Service Manual indicates an 80% charged battery will start the car after not running for 30 days. Depends how often you drive the car and how long you drive when you do drive. There is typically no need to use a charger as the C7 shuts down all but a very small amount of power, even if you forget and leave a light on, after ~10 minutes.
FWIW: Had a low battery voltage (after the surface change depleted) since I got my C7 in October 2013 (~12.4 volts, no doubt a weak cell.) Bought the equivalent of the GM charger to see if that would help-no better! Used in only once that October and just recently when I replaced it with an AGM Sears battery to be sure it was fully charged.
Have been on vacation several times for ~2 1/2 weeks and it started right up after. Never had an issue but since the battery was 3 years old, replaced before winter as a precaution (when I wanted and at a discount price. Note it is not as easy to replace as a C6 etc.) It now reads as it should (with surface charge depleted) at 12.8 to 12.9 volts.
Last edited by JerryU; Nov 20, 2016 at 01:56 PM.
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Exactly, modern cars have "battery management software," which controls the alternator output. Sometimes, you will see it quickly pulsing up to 14.5-15v, and then drop right back down to 12.9v. It's all completely normal, and helps w/ battery longevity.
Drifting
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I've had it happen several times on my Silverado PU truck. Usually after about 5 hours of steady hwy driving during an extended trip. It really got my attention the first time when it suddenly dropped to around 12 volts, then occasionally jumped back up between 13 and 14 volts. The gauge voltage is usually rock steady at around 14 volts for our normal daily driving. The manual says that is fine, and I confirmed it with our local Chevy service manager.
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E-Ray, 3LZ, ZER, LIFT
Joined: Sep 2007
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From: NE South Carolina
I've had it happen several times on my Silverado PU truck. Usually after about 5 hours of steady hwy driving during an extended trip. It really got my attention the first time when it suddenly dropped to around 12 volts, then occasionally jumped back up between 13 and 14 volts. The gauge voltage is usually rock steady at around 14 volts for our normal daily driving. The manual says that is fine, and I confirmed it with our local Chevy service manager.
Details from the Service Manual: The system will enter the Fuel Economy Mode when battery temp is above 32 F and battery charge is >80%, etc. It will then set the voltage to match the battery voltage if it's between 12.5 and 13.1 volts. If those conditions are not met or the wipers are turned on (and some other parameters) it will enter the Charge Mode regulating the charging system between 13.9 to 15.5 volts. It adjusts the alternator field using PWM 128Hz control.
That is why you see the voltage jump. It's not the old simple charging system! They will do anything to get better fuel mileage!
Our X5 BMW SUV goes a step further. It has a large AGM battery (actually has a small one as well) and only connects the alternator when you're coasting or braking! There is a graphic they shows when it is charging. Kind of a simple regenerative system that uses the alternator power to help slow the SUV instead of just brakes! On a recent 3 hour trip to Hilton Head, mostly on flat I95, I had the SUV set on cruise control. Never did see it charge. Assume, like the Vette, they also measure battery condition/voltage and it was OK! Since it has stop/start and an electric water pump and steering assume I'd see some charging along the way.
Last edited by JerryU; Nov 20, 2016 at 02:14 PM.
E-Ray, 3LZ, ZER, LIFT
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From: NE South Carolina
^^^^
Yep fits cars Back In The Day NOT today! Most cars do what the C7 and C8 do TO SAVE ENERGY.
I'll condense my prior post. Today voltage is controlled by a computer:
The system will enter the Fuel Economy Mode if battery charge is over 80%. It will then set the voltage to match the battery voltage if it's between 12.5 and 13.1 volts. If those conditions are not met it will enter the Charge Mode regulating the charging system between 13.9 to 15.5 volts.
Yep that is why you see a step change!
Yep fits cars Back In The Day NOT today! Most cars do what the C7 and C8 do TO SAVE ENERGY.
I'll condense my prior post. Today voltage is controlled by a computer:
The system will enter the Fuel Economy Mode if battery charge is over 80%. It will then set the voltage to match the battery voltage if it's between 12.5 and 13.1 volts. If those conditions are not met it will enter the Charge Mode regulating the charging system between 13.9 to 15.5 volts.
Yep that is why you see a step change!
E-Ray, 3LZ, ZER, LIFT
Joined: Sep 2007
Posts: 35,010
Likes: 12,393
From: NE South Carolina
I'll add my observation.
I never use my battery charger. I've been retired since 2000. Typically drive my DD Vette the ~30 miles into town ~3 times a week. On the way into town the battery is below 80% charge. (It will ONLY charge to 80) so sitting for a few days will be below!) See above post as why.
On the way into town it's bringing the battery up to 80% and voltage is observed at 14.5 to 15 volts. Soon after I return home I see a step change in battery voltage to about 13 to13.5 volts. The computer is controlling output to just supply the power needed to operate my AC, Sound System etc. It will not "overcharge" past that 80% charged battery.
You can look it up, but it's NOT energy efficient to change much past 80%. That is what most modern cars are doing to same what energy they can.
I never use my battery charger. I've been retired since 2000. Typically drive my DD Vette the ~30 miles into town ~3 times a week. On the way into town the battery is below 80% charge. (It will ONLY charge to 80) so sitting for a few days will be below!) See above post as why.
On the way into town it's bringing the battery up to 80% and voltage is observed at 14.5 to 15 volts. Soon after I return home I see a step change in battery voltage to about 13 to13.5 volts. The computer is controlling output to just supply the power needed to operate my AC, Sound System etc. It will not "overcharge" past that 80% charged battery.
You can look it up, but it's NOT energy efficient to change much past 80%. That is what most modern cars are doing to same what energy they can.
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Good discussion and thanks for all the info.