13.5V alternator when driving and idling

Joined
Sep 23, 2017
Messages
433
Hey guys so I recently fully charged my battery in my hummer h2 since it sat for 1-2 weeks and the battery was fully dead. I put it in the truck today and then I remembered that my radar detector does a voltage reading so I turned that on and I noticed that my voltage would only be between 13.5 -
13.6v while driving with no Ac/fan and the stereo shut off. I even shut the lights off and same thing, when I got home I let it idle and it was between 13.5-13.6v

is this normal because my battery was just recently fully charged? Or is something wrong? The alternator is about 5 years old and it's a China one so it could a about to go but just wanna make sure what values I should be seeing. I'm also going to be getting a HID kit very soon so I don't want anything to go wrong with that especially
 
I would get a better reading off of the battery posts with a real meter. Since you said the battery was completely dead, it could be damaged, or the alternator could have something wrong with it, but I wouldn't trust a voltage display on a radar detector.
 
Last edited:
That is not out of the realm of a normal output from the alternator. I see your in canada, what was the ambient temperature during your readings? So many factors play into the output of the alternator which is likely controlled by the pcm in this vehicle, one of which is ambient temperature and under hood temperatures in many cases.
 
Originally Posted by 5AcresAndAFool
That is not out of the realm of a normal output from the alternator. I see your in canada, what was the ambient temperature during your readings? So many factors play into the output of the alternator which is likely controlled by the pcm in this vehicle, one of which is ambient temperature and under hood temperatures in many cases.



Temp was around 10 Celsius and I don't think the vehicle was at operating temp but probably near
 
Originally Posted by apollo18
Originally Posted by 5AcresAndAFool
That is not out of the realm of a normal output from the alternator. I see your in canada, what was the ambient temperature during your readings? So many factors play into the output of the alternator which is likely controlled by the pcm in this vehicle, one of which is ambient temperature and under hood temperatures in many cases.



Temp was around 10 Celsius and I don't think the vehicle was at operating temp but probably near


13.5 volts sounds right under those conditions.
 
13.5 is not enough to keep a modern battery charged. Most GM will run at least 14.5 and may go up to 15 volts when it is cold. Some GM vehicles have a sensor mounted at the battery and once the computer determines the battery is charged it will reduce the voltage to 13. something range. It also has a desulfation mode where you can see over 15 volts for some time. This system is called RVC or SARVC. There may be later versions. These are supposed to provide longer battery life AND lower fuel consumption.

You need to research the internet and see if you have one of these systems. You need a real voltmeter, attached to the alternator output terminal or battery if you can get to it. Simple alternators are rapidly disappearing.

Rod
 
Lots of alternators put out about 13.5 my sprinter alternator hangs right at that. They usually burst higher after a start then come right down.

A standard alternator without something like a Balmar kit on it will NEVER fully charge any battery regardless of distance driven.
Put it on a smart charger after a road trip and watch it take a charge.

A basic flooded lead acid battery is fully charged at 12.7 volts, starts to degrade itself around 12, and is basically dead at 11.5.

UD
 
It might be OK.
GM has a complicated charging scheme.

https://www.motor.com/magazine-summary/are-you-smarter-than-a-smart-charging-system/

https://www.motor.com/magazine-summary/trouble-shooter-july2015/

https://www.motor.com/magazine-summary/trouble-shooter-september2015/

Quote
There are two types of RVC systems in use-integrated RVC and stand-alone RVC (SARVC). Integrated systems use a battery current sensor to inform a body control module (BCM) how much the battery is being charged or discharged.

SARVC systems (found mostly on trucks) do not use the BCM for operation. They have a generator battery control module mounted to the negative battery cable, to interpret battery current and voltage and battery temperature inputs. The battery current sensor is internal to the module. This module also directly controls the generator L terminal duty cycle instead of the ECM/PCM.

The purpose of the RVC system is to maintain the battery state-of-charge at 80% or above and support vehicle loads. To accomplish this, it has several different modes of operation. Six common modes of operation are Charge Mode, Fuel Economy Mode, Voltage Reduction Mode, Start Up Mode, Windshield De-Ice Mode and Battery Sulfation Mode.

The PCM/ECM (generator battery control module on full-size trucks) controls the generator through the generator L terminal control circuit. It monitors the generator performance though the generator field duty cycle signal circuit. The signal is a 5V PWM (pulse width modulated) signal of 128Hz with a duty cycle of 0% to 100%. Normal duty cycle is between 5% and 95%. The ranges between 0% to 5% and 95% to 100% are for diagnostic purposes.

The control module enters Charge Mode whenever one of the following conditions is met:

Under WOT conditions and when the fuel rate (sent by the ECM/PCM) is greater than 21 g/S and the throttle position is greater than 90%.
The headlamps are on, low or high beam.
The wipers are on for more than 8 seconds.
The electric cooling fans are on high speed.
The rear defogger is on.
The battery SOC is less than 80%.
When one of these conditions is met, the control module ramps up the voltage slowly to a level between 13.4 to 15.5V (depending upon the mode of operation the system is presently in) at a rate of 8mV to 50mV per second.

The control module enters Fuel Economy Mode when the following conditions are met:

The calculated ambient air temperature is above 32°F.
The calculated battery current is less than 15A and greater than -8A.
The battery SOC is greater than 80%.
The generator field duty cycle is less than 99%.
This mode's targeted generator output voltage is 13.0V. The control module will exit this mode once the criteria are met for Charge Mode.

The control module will enter Voltage Reduction Mode when the following conditions are met:

The calculated ambient air temperature is above 32°F.
The calculated battery current is less than 2A and greater than -7A.
The generator field duty cycle is less than 99%.
This mode's targeted generator output voltage is 12.9V. The control module will exit this mode once the criteria are met for Charge Mode.

After the engine has started, the control module sets a targeted generator output voltage of 14.5V for 30 seconds (Start Up Mode).

The control module enters Battery Sulfation Mode when the battery voltage is less than 13.2V for 45 minutes. Once in this mode, the generator battery control module will set a targeted output voltage between 13.9 and 15.5V for five minutes. The control module will then determine which mode to enter depending on voltage requirements.

In RVC Mode, the control module bases the charging voltage on battery SOC, which is estimated during a key-off event every eight hours, after three voltage measurements every 24 hours thereafter, and then monitored constantly while the ignition is on. These voltage measurements are then compared to estimated battery temperature, as battery temperature vs. battery voltage directly corresponds to battery SOC. While the engine is running, the system uses both the battery voltage and estimated battery temperature to determine the battery current in and out of the battery. The control module then regulates the charging voltage to keep the battery above an 80% SOC.
 
Battery resting overnight in the morning should be 12.5 volts. Charging should be around 14ish volts. Have someone crank the engine and if it goes below 10 volts the battery is bad. This is how I have done it over the years. YMMV.
 
Load (not voltage) test the battery? or if it is more than 1, needs to do it one at a time.

Load testing is the only reliable way to check if the battery is still good and can hold charges.

After you know the battery is good, then you can determine if the alternator is good by loading up the electric, turning all electrical/electronic components in the car/truck.
 
Heres a pretty standard table outlining voltage, vs state of charge, vs specificities gravity per cell.

Batteries all vary a bit and higher end one will post their exact parameter so a charge profile can be specifically created.

[Linked Image]
 
Thanks! I actually checked the battery posts with multimeter with the vehicle running and was hitting 14.2-14.3V I believe which I assume is good enough. It's odd because even my gauge in my truck has the need at about 13.5-13.7 ish but I guess the battery posts are most accurate

I assume I'm good then?
 
Originally Posted by apollo18
Thanks! I actually checked the battery posts with multimeter with the vehicle running and was hitting 14.2-14.3V I believe which I assume is good enough. It's odd because even my gauge in my truck has the need at about 13.5-13.7 ish but I guess the battery posts are most accurate

I assume I'm good then?


You're good.

UD
 
13.5-13.8 is right about where both the lexus in my sig will run with low load and moderate temperatures, Tundra around 14.2-14.5 just like my Mercedes. I think it can just vary depending on the way the vehicle is set up, but that seems in "the range" of normal to me
 
The proper way to measure charging voltage is across the battery terminals. There's likely a lot of resistance between the battery, cigarette lighter, and radar detector, so your reading may be artificially low. It doesn't take much to cause voltage drop at 12-13v.
 
My 1999 S-10, 2012 Frontier, and 2015 Pathfinder ramps up to 14.6V immediately after startup and then falls to 13.8V after about 10 seconds with a good battery.

So it seems to me your situation is about average.
 
Last edited:
yeha i checked the terminals,
at first with the multi it said 12.4v ish, and then when i went to start it dropped to 10.73v and then jumped to 14.1v with the alternator running
 
My older euro vehicles are between 12.5-13.5V, with very low rated alternators.

My Accord Hybrid is really strange. Headlights on it will keep 14.5V, rest of the time, the accessories get 12.5v and the Lead acid (not used for starting) battery drops to 12.0.

My bmw and our odyssey tend to keep right at 14.1v consistently. They don't always drop.

My 98 S-10 jumps quickly to 14.5-14.6, but then will drop down over time. But I don't really usually see much lower than 13.9V.
 
Back
Top