OBD Voltage readings

[meerschm]

got a new scangauge II and used it to keep an eye on the TB for a recent trip.

must have left something on overningt ( I suspect a dual USB charger). would not start in the near-zero darkness.

scangauge read 11.6 volts, which is really pretty much out of juice (was enough for instrument panel indications and lights)

after a jump, the OBD volts indicated 15 volts, and by the end of a 20 minute drive had dropped to 14.7 volts.

on the 4 hour drive home the next day, running voltage ended down around 14.2 or 14.3 after turning the engine off, volts read 12.5 or so.

do you think this is normal, and what do you see for operating OBD voltage behavior?

 

[Blckshdw]

I've never thought to mess with mine when the engine's not running, but my voltage is usually 14.2-14.5 under various conditions. Was your USB charger actually charging anything or just plugged in and sitting there? How old is your battery? I think that, combined with how cold it was overnight, could be more important factors.

 

[The_Roadie]

Those voltages look normal. The alternator 's voltage regulator does vary on purpose with its temperature, and tapers off as it warms up. If I remember the shop manual details correctly.

 

[JerryIrons]

I think engine rpm speed can play into the alternator charging as well, ie idle speed vs 2000 rpm and what is using power.

2nd vote on how old is your battery, and if not maintenance free, have you topped off with distilled water and put a charger on it ? Or, how are the water levels if you can get into it.

 

[meerschm]

The battery is only a couple months old. not 500 miles on it. Interstate dual terminal 1000 amp cold weather job.

I hooked up a charger on Christmas Day, to make up for weeks of idleness.

the culprit I suspect is the 12v to double USB.

nothing was plugged into it, ( a cable was hanging which my wife did slam in the door for a minute before returning to the car. doubt the gasket would have shorted the cable, and it worked on the drive home)

but it was installed in one of the 12 v sockets in the dash. (I do wonder how it could suck enough energy to run the battery down. could have been somthing else, like a jammed power seat switch.)


Thanks for the responses.

 

[JerryIrons]

Your voltages seem fine to me, other than that 11 volt event. Just for peace of mind you could check for parasitic drain. It's never happened to me, but it's possible for an alternator to charge correctly but still have a bad diode and drain the battery with no key in ignition.

 

[budwich]

I don't think your 12.5 reading is good after a 4 hour drive although it depends on what it (scangauge) is actually reading (ie. the circuitry involved). Get a meter and verify at the terminals. A "good" battery reading is in the 13.x range. As for your usb charger plugged in... probably not a good idea. Again depending on the circuitry in the system... some DC to DC "things" just use a resistor (bridge) to drop voltage to "proper levels" which may draw more than expected even when nothing is on the other end.

 

[meerschm]

http://www.exide.com/Media/files/Do...ery Charging & Storage Guidelines 5_9_13.pdf

says normal open circuit voltage for an automotive battery should run 12.5 to 12.75 or so for a charged battery

How to charge sealed lead acid batteries for engineers.

looks like the voltage to add charge to the battery is higher, (14 plus usually. my 15 volts makes sense for 0-10 degrees)

After some light reading, I am lucky the battery did not freeze solid. probably was a little slushy inside, which is why the tow truck had to leave the jump hooked up for a minute or two before it would turn over.

The OBD does not measure open circuit voltage ( electronics have to run to operate the OBD and ScanGuage)

 

[budwich]

I don't think that what the chart says or at least that's not how I would read the "12v" one. IF you look at the first row for auto type cells, it indicates that 50 SOC is about 12.5v. After a 4 hour run at any kind of drive speed, I would think your battery should be fully charged and not at 50%. Further note this information appears to be for "storing batteries", it may not apply.

Anyways, if you are happy with the answer you found for your question, that's fine.

 

[meerschm]

it also says 90% SOC open circuit is 12.77

I will check the battery with a meter, but just now it is howling and getting colder.

thanks for the input.

 

[IllogicTC]

I agree on the USB plug contributing to parasitic drain, especially since you said it's been sitting there for days/weeks. Obviously the input is 12 volts. The output on a USB port is 5 volts, if I remember correctly. To achieve this, there is generally actually several small resistors, a few capacitors, usually a transistor or two, and even diodes, or maybe a small transformer or an IC. It's more than just a basic resistor in-line with the circuit to achieve the appropriate voltage; that's how we have the full-time-on LEDs (some which even change color through the use of a voltage drop or a small IC when something's plugged in). While having a device or devices plugged in would obviously increase drain (with a dual charger, possibly up to 20 watts), under no load there will always be loss. That's why I never leave any device plugged into the adapter which does not have a discrete power switch to enable/disable the circuitry within.

 

[meerschm]

Those voltages look normal. The alternator 's voltage regulator does vary on purpose with its temperature, and tapers off as it warms up. If I remember the shop manual details correctly.

Thanks,

this makes more sense for the voltage than my first guess of changing as the battery charges up. no way for the generator to know what the state of charge of the battery is. internal temperature compensation inside the alternator makes more sense, and matches the battery charging guidance my google dug up.

 

[meerschm]

drove to work today, she started right up.

outside temp (on the HVAC control) read 4. Intake air read 6

OBD voltage reading of 15.0 dropped a bit to 14.7 by the time I got to work, (intake air made it up to 20 by then)

and just to tie it all up, I did check the battery voltage with a meter after driving home. 12.75 volts. the OBD voltage dropped to around 12.5 indicated after I turned off the engine, before the ScanGauge turned off.

 

[meerschm]

just a follow up.

measured the current of the USB charger adapter (with nothing connected to the USB charging ports), and it only takes 15 mA. (there is an LED on the device) must have been something else left on to run the battery down.

 

[bobdec]

This is an exert from a rather long article on alternators.. Mentions GM used temperature sensor in the regulator..

If the CS series of GM alternators weren’t smart enough for you, we have the latest systems called Regulated Voltage Control (RVC) showing up on the scene in the mid 2000s. Why a new system? Batteries are sensitive to temperature. The lower the battery’s temperature, the lower its chemical and electrical reactivity. This means the battery puts out less voltage in cold temperatures and needs more voltage to charge. Conversely, a battery puts out more voltage and needs less voltage from the alternator in warm weather.
While Chrysler did indeed use an external temperature sensor to sense battery temperature for its PCM-controlled voltage regulation, GM always used a thermistor built into the voltage regulator to sense underhood temperatures for charge rate compensation.

complete article. http://www.motor.com/magazine/pdfs/042010_09.pdf figure 2 in pdf is interesting.

 

[meerschm]

thanks for the article.

I do not think the generator in my 2003 I6 is quite so sophisticated. (CS series, not the RVC)

it does wait a bit when first starting when it is very cold (single digits as far as I can tell), and start to charge once the engine is running ok.

and I can see that the voltage depends on ambient temperatures. (does run at 15 volts when it is way cold outside and closer to 14 when it is warmer)

but it does not look to me like it ever drops off to float, (13 volts)

Do not think there is a current sensor on the wires, or anyway to calculate the actual battery level of charge. just charges to a temperature compensated (based on temps internal to the generator) voltage level, reports load level to the PCM, and responds to a PCM command to turn off or turn on.

the engineering in more recent vehicles is getting more sophisticated.

 

[bobdec]

Agree, the internal regulator controls alternator max voltage output. And the PCM does some on/off control added to the mix based on software. The alternator current is dependent on total load (battery and accessory resistance). In reality system voltage lower than 14 volts is due to alternator not keeping up with the load (low RPM or excessive loads) or the PCM taking control. I seem to notice in my '07 the voltage drops to near 13V during hwy driving. Maybe I have the PCM float voltage software. I getting ready to purchase some better scan s/w, going to check if it provides the PCM output to PIN L (alternator on/off) of he alternator during HWY runs. By the way looked at '02-07 books and the same two alternator pins are used in all years F to PCM and L from PCM. So any control would be different software.

 

[The_Roadie]

My impression from reading the shop manual cover to cover is that the PCM can suppress the alternator output, and that's only used for letting engine RPM stabilize after a very cold start. The PCM can read the field duty cycle of the alternator, which is a measure of how hard the alternator's working. If the PCM measures the voltage drooping, even while the alternator's at 100% duty cycle, the PCM concludes the alternator's losing the battle, possibly due to excess load or damaged alternator (dead diode). Then the PCM goes into a 3-4 stage load shedding algorithm (interior lights and entertainment system, HVAC blower and AC compressor, external lights last) in order to keep as much of the dwindling battery energy as possible to keep the engine running to allow the driver to limp to a place of safety or repair. The load shedding function is NOT mentioned in any literature or the Owner's Manual I've ever seen, just the shop manual. I've also never tried to trigger mine to confirm how it works.

 

[meerschm]

That morning last week when it was 4 I can verify that the generator did not kick in right away. did not keep records, but it was noticeable. could have been ten seconds or so.

you could easily see the headlights brighten up, and I was watching the OBD voltage, which stayed in 12.x range until it kicked in, then went up to the 15 volts.