Grimmer
Well-Known Member
The battery management system (BMS) will adjust the alternator's output based on how much power it thinks the car needs...
Right after start up, it has typically been in the high 13's through the mid 14's and then varies while running, pretty much regardless of RPM. However, I did notice once (with the new battery) that after it sat with an external battery charger/maintainer connected for a couple of days, when I started it up the dash voltage was in the 12's, i.e. battery management must have figured the battery was already full and basically turned off the alternator completely. That didn't last for very long. The BMS, in addition to recharging the battery, also anticipates high electrical needs (like running the cooling fans, lights, etc.) and increases the alternator's output accordingly.
This is part of the reason why the service manual (and probably the owner's manual) advises against charging the battery by attaching directly to the battery posts. Doing so will bypass the sensor the BMS uses to measure power in and power out of the battery and gets it out of sync with the battery's actual state of charge.
I have seen the voltage reading in the cluster vary from 12.X volts to about 14.8 volts with the engine running, always changing... but it spends most of its time between 13.5 and 14.5 ~ish. Second largest chunk of time between 13 and 13.5. Only occasionally on the outer fringes (high or low).
If you have FORScan capabilities, you can reset the battery management system when you install a new fully charged battery. If you don't have FORScan, you can leave the car to sit static (with minimal electrical load, e.g. no significant parasitic draws like a dash cam, no opening doors, key FOB locks/unlocks, etc.) for 8 or 10 hours (if I'm remembering correctly) and the BMS will attempt to self-reset / re-calibrate. I would have to go back and re-read the system description in the Service Manual for the details...
Right after start up, it has typically been in the high 13's through the mid 14's and then varies while running, pretty much regardless of RPM. However, I did notice once (with the new battery) that after it sat with an external battery charger/maintainer connected for a couple of days, when I started it up the dash voltage was in the 12's, i.e. battery management must have figured the battery was already full and basically turned off the alternator completely. That didn't last for very long. The BMS, in addition to recharging the battery, also anticipates high electrical needs (like running the cooling fans, lights, etc.) and increases the alternator's output accordingly.
This is part of the reason why the service manual (and probably the owner's manual) advises against charging the battery by attaching directly to the battery posts. Doing so will bypass the sensor the BMS uses to measure power in and power out of the battery and gets it out of sync with the battery's actual state of charge.
I have seen the voltage reading in the cluster vary from 12.X volts to about 14.8 volts with the engine running, always changing... but it spends most of its time between 13.5 and 14.5 ~ish. Second largest chunk of time between 13 and 13.5. Only occasionally on the outer fringes (high or low).
If you have FORScan capabilities, you can reset the battery management system when you install a new fully charged battery. If you don't have FORScan, you can leave the car to sit static (with minimal electrical load, e.g. no significant parasitic draws like a dash cam, no opening doors, key FOB locks/unlocks, etc.) for 8 or 10 hours (if I'm remembering correctly) and the BMS will attempt to self-reset / re-calibrate. I would have to go back and re-read the system description in the Service Manual for the details...
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