After reading about headlamp failures, I got re-interested in logging the voltage of my 12V battery during vehicle operation. My logging period lasted about 7 hours and included an initial cold start, and three periods of vehicle operation. The logger resolution is 50mv and the interval between measurements is 10 seconds. Plot is attached. Garage temp was about 5C at start-up and outdoor temp was about -5C. What is interesting is that the charging voltage during vehicle operation was very consistent at about 14.75-14.85 volts. I expected the voltage to decrease as the battery space warmed from exposure to cabin air (cover panel was removed exposing thermistor to cabin air). As a point of reference, this vehicle does not have a headlamp lifetime issue.
The voltage is indeed consistent. The voltage is at the high end of what's allowable for an AGM battery, but still OK. These results suggest that you don't have to log voltage to characterize the charging scheme for your vehicle. One reading in the READY mode with a DVM tells all. This lets us compare one vehicle to another, which is perhaps important to owners with headlamp failure issues.
Exactly. That's also why the often-reported-on-PriusChat "bad 12-volt battery kills your mileage" effect would be so easy to quantify, if anyone were of a mind to. You would just need one measurement of the charging current going into the battery, while comfortably stopped and in READY, as driving conditions don't change it. -Chap
