richs said:
I've just gotten one of those USB car chargers that displays the input voltage that it's getting from the car's 12V power outlet. I'm going to try to use this to monitor the voltage of my 12V battery over time and try to predict when the 12V is starting to fail so that I can replace it proactively while I can still drive it. The USB charger only cost $10, so why not give it a shot?
Maybe because it would not allow you to monitor the 12 V battery's resting voltage as you hoped
I installed a voltmeter in the auxiliary power port below the center of the dashboard over 3 years ago hoping to do what you would like to do.
richs said:
Anyway, immediately upon plugging it it, it displayed 14.1V, which I assume is the voltage of the DC-DC converter, not the voltage of my three-year-old battery. How long do I have to wait before the DC-DC converter shuts off and I can get a reading from the battery itself? Can I do something to manually shut off the DC-DC converter while I'm sitting in the car or will I need to take this reading from outside of the car, before opening any doors?
The high-voltage system remains on for at least 30 minutes (I gave up waiting). However, the 12 V power to the auxiliary power port turns off before the high-voltage system, so the voltmeter would turn off before the DC-DC converter. Even if the DC-DC converter turns off before the auxiliary power port, the 12 V system voltage would higher than the 12 V battery's resting voltage which is what you would like to measure. It takes hours after being charged for the 12 V battery's voltage to reach its resting voltage.
The voltage at the battery terminals agrees very well with the voltage at the auxiliary power port.
I have measured the resting voltage at the 12 V battery terminals by shutting the frunk only partially in the evening so that I could open it the following morning without turning on the high-voltage system. I have also connected wires to the 12 V terminals on the motor electronics under the rear cargo floor, and have dropped these wires out the bottom of the car so that I could measure the voltage between them without opening the hatch. Another approach I've tried it disconnecting the high-voltage disconnect to the right of the frunk box which prevents the high-voltage system from turning on when a door, the frunk, or the hatch is unlocked or opened. The following morning, the 12 V battery's resting voltage is displayed by a voltmeter in the auxiliary power port after a door is opened. All methods are a pain in the butt which makes monitoring the 12 V battery's voltage over time difficult.
Several i3 drivers have installed a
Bluetooth battery monitor that can be read without opening anything. The problem I have with installing a battery monitor like this is that it constantly consumes 1 mA of current adding to the vampire loads that slowly discharge the 12 V battery. However, it's probably the best way to monitor the resting voltage over the long term.