How long does the DC-DC converter run after you power off the car?

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?

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?

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?

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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?

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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.

As it turns out, my 2017 REx is acting a bit differently from what I thought. My initial assumption about the DC-DC converter seems to be wrong. This afternoon, I left the car off for a couple hours, not plugged into an EVSE. I then opened a door and plugged the charger into the center console 12V power port before powering up the car. It read 12.5 volts. As soon as I hit the car's power button, it jumped up to 14.1 volts. Later, I tried this again with the rear 12V port in the cargo area and got the same result.

This seems to indicate that my car, at least, doesn't power up the DC-DC converter immediately upon opening a door. (It might eventually power it up to charge the 12V battery, but I haven't waited around to see if that happens.) I'll just bear in mind that getting a good reading on the battery voltage has to be done after leaving the car powered off and unplugged overnight, and before hitting the power button. So, while using a cheapo USB charger with a voltage readout isn't an ideal way to monitor battery health, it may just be good enough to avoid a tow due to a dead battery.

I wonder if the behavior of the DC-DC converter was changed at some point during the production run of the i3, or maybe in a software update. It doesn't make a lot of sense to power it up when just opening the door.

richs said:

This seems to indicate that my car, at least, doesn't power up the DC-DC converter immediately upon opening a door.

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The DC-DC converter on our 2014 BEV doesn't start immediately upon unlocking or opening a door, the frunk, or the hatch. However, the high-voltage system and various electronic controllers do start up immediately. I leave my voltmeter plugged in, so what I see is the voltage plummeting from whatever the current 12 V battery voltage is down to as low as 11.8 V within 5 or so seconds at which point the DC-DC converter has booted and the 12 V system voltage suddenly jumps to ~14.3 V.

When I first noticed this behavior, I assumed that the 12 V battery was failing because the load of several controllers booting simultaneously caused its voltage to sag significantly. I asked our BMW dealer to check the health of the 12 V battery during our biennial service visit. The service manager told me that the 12 V battery was behaving normally and that the behavior that I described is normal for an i3. Over 3 years later, the battery hasn't failed, so it must have been relatively healthy.

richs said:

I wonder if the behavior of the DC-DC converter was changed at some point during the production run of the i3, or maybe in a software update. It doesn't make a lot of sense to power it up when just opening the door.

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I wouldn't be surprised if this occurred because always charging an AGM battery while an i3 is on might not be the best behavior to maximize its life. It might be better for the 12 V battery for the DC-DC converter to cycle on and off to maintain the charge level between a lower and upper limit to allow the battery's charge to vary under load as happens on ICE vehicles with alternators and voltage regulators. BMW might have changed the i3's charging algorithm to be less aggressive at maintaining a full charge.

Another explanation might be that because we drive our i3 so infrequently and for such short distances (only 10k miles total since November, 2014), its 12 V battery might typically be at a lower charge level than that of the average i3 due to vampire loads that gradually discharge the 12 V battery while an i3 is off. If so, when the high-voltage system starts, the controller responsible for 12 V battery charging notices that the battery is discharged below a certain limit and boots the DC-DC to charge the 12 V battery which happens more frequently than in other i3's like yours.