Surprising voltage measurements at accessory power sockets

Owning two cars whose 12V batteries are never used to start an ICE, I know how easy it is for their 12V batteries to deteriorate without notice until they are incapable of providing the power sufficient to boot the various controllers required to drive. So I bought a 12V voltage monitor that plugs into an accessory power socket, the idea being that I would notice when the 12V voltage began dropping enough to indicate a deteriorating 12V battery before it deteriorated sufficiently to leave me stranded.

I expected the i3's 12V voltage to be ~14V when the DC-DC converter is charging the 12V battery and ~12.5V otherwise which would match the behavior of our other car. I also expected the voltage at the accessory power sockets to be 0V when the car isn't in the Ready or Radio state. But so far, every measurement I have made suggests that the accessory power sockets in our i3 are not only always powered but that the DC-DC converter is always on; i.e., the voltage monitor always measures 14.0-14.3V whether I'm driving or whether the car has been parked and locked for several hours. I verified the voltage measurements with a VOM.

This doesn't seem right. The life of a 12V AGM battery would be shortened by being continuously float-charged at 14V. Having the DC-DC converter always on could discharge the Li-ion battery pack if a significant 12V load were connected to an accessory power socket. Looking in the lousy Owner's Guide and in the i3 Technical Training manuals, I haven't been able to find any discussion of when the accessory power sockets are powered. I think I recall reading that those of you who want to power a surveillance camera from an accessory power socket have not been able to do so because they become unpowered several minutes after an i3 has been shut down and locked. This doesn't seem to be happening with our i3.

Have any of you measured the voltage at an accessory power socket while driving, while parked with the car off, after several hours of being parked and locked, etc.? I may have to take out the frunk box so I can measure the voltage at the 12V battery terminals to try to figure out what's happening.

I haven't measured the voltage, but I have a dashcam permanently wired to the socket circuit. I noticed that it turns on any action- even opening the charging door. Once turned on, it stays on for about 15 minutes. The same goes for turning "the ignition" off- the sockets stay hot for another 15 minutes. They turn off eventually, and I suspect that charging of the 12V stops at the same time.

gt1 said:

I haven't measured the voltage, but I have a dashcam permanently wired to the socket circuit. I noticed that it turns on any action- even opening the charging door. Once turned on, it stays on for about 15 minutes. The same goes for turning "the ignition" off- the sockets stay hot for another 15 minutes. They turn off eventually, and I suspect that charging of the 12V stops at the same time.

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That explains why the socket was hot when I entered our car after it had been parked for hours.

I did leave the gauge plugged in so that I could see it through the window without entering the car. Maybe I didn't wait long enough for the socket to power down. I'll try again.

More information… The accessory power sockets do power down after the car has been shut down and locked for a period of time (~20 minutes?). The socket powers on without the DC-DC converter running when the hatch is opened, so the 12V battery's voltage is displayed by the voltmeter in an accessory power socket. Its voltage is very low initially (~12.1V) but gradually climbs to ~12.45V after ~30 seconds. I don't understand this behavior.

When I unlock and open the driver's door, the accessory power sockets power on, but sometimes with and other times without the DC-DC converter running. I haven't figured out what determines whether the DC-DC converter runs. When the DC-DC converter doesn't run, the 12V battery's voltage ramps up just as it does when the hatch is opened.

I plan measure the voltage at the 12V battery's terminals to determine whether there's enough resistance in the accessory power socket circuit to result in low voltage readings at the accessory power ports.

It appears that the i3 runs the DC-DC converter enough to make the rather low capacity 20Ah 12V battery sufficient, in effect using the capacity of the high-voltage battery pack to power 12V accessories under some circumstances. I hope I learn how to detect when the 12V battery is weakening to the point of needing replacement before refusing to boot the controllers into the Ready state.

The voltage climb may indicate that the battery is charging, but I would expect it to go higher than 12.45. How do you know when the DC-DC converter is operational?

gt1 said:

The voltage climb may indicate that the battery is charging, but I would expect it to go higher than 12.45. How do you know when the DC-DC converter is operational?

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I'm guessing that the voltage at the accessory power socket reflects significant electrical resistance between the 12V battery and the accessory power socket. I could verify this by measuring the voltage at the 12V battery terminals, but that requires removing the frunk box. This resistance causes the voltage to drop significantly when the 12V current increases (e.g., when the interior lights illuminate, the entertainment system boots or awakens from sleep, etc.). After several seconds, I can hear a relay switch off at which point the measured voltage climbs.

When the DC-DC converter turns on, the 12V voltage jumps immediately to 14.0-14.3V, the voltage required to charge a 12V lead-acid battery. The DC-DC converter also turns on when the high-voltage battery pack is being charged which charges the 12V battery. This essentially eliminates the need for a separate 12V battery charger assuming that the DC-DC converter remains on when the high-voltage battery is fully charged.

I believe that the socket is on a dedicated circuit, with a small load such as voltmeter the voltage will not drop in the wire. If the other loads are energized at the same time, the voltage will drop at the battery.
BMW is not known for doing things the simplest way. I wouldn't be surprised if the relationships between the high voltage and 12V batteries are fairly complicated

Back to the initial reason for the thread, maybe the car monitors the battery and it will let you know if it needs to be replaced. Not the answer you were looking for, but being not a standard (yet) battery, I would think BMW would make sure you don't get stranded or at least design it to turn on the DC-DC inverter if there is not sufficient charge.

It's my understanding that the DC-DC power supply only runs if you are connected to the EVSE OR if the car is in the ready state. It IS possible to fully discharge the 12vdc battery, and then, it will take awhile to get a recharge started. What I'm told happens is that there is an 'emergency' 12vdc charging circuit that is activated if the 12vdc battery cannot power the computer. Then, once the battery has sufficient charge to run the control logic, it will switch to the 'normal' charging routine where the DC-DC circuit will work again, along with the vehicle high voltage supply to recharge the main battery pack.

IOW, if your 12vdc battery is not capable of running the computers, you can't recharge the car, and its fallback circuits will only try to charge that battery up until that can run the computer, and then recharge the main batteries. It doesn't matter what the state of the main battery pack is at that point...it needs the 12vdc path to run the logic boards.

Now, this information came near the first introduction of the car, and I guess, it may have changed.

davidzi3 said:

Back to the initial reason for the thread, maybe the car monitors the battery and it will let you know if it needs to be replaced. Not the answer you were looking for, but being not a standard (yet) battery, I would think BMW would make sure you don't get stranded or at least design it to turn on the DC-DC inverter if there is not sufficient charge.

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Quite a few i3 owners have reported being suddenly unable to enter the Ready state of their cars. Some have reported that starting charging with an EVSE didn't help which is surprising since my voltage measurements indicate that the DC-DC converter turns on when charging starts which should have charged the 12V battery enough to enter the Ready state. These owners had to have their cars towed to a BMW dealer. The diagnosis was a bad 12V battery. Some have reported noticing a warning about low voltage or insufficient power, but the warning was so poorly worded that it wasn't obvious that a bad 12V battery was to blame.

My voltage measurements indicate that the DC-DC converter stays on when the car is shut off until the accessory power sockets are shut down some time later (~20 minutes?). Occasionally, but not always, the DC-DC converter is turned on when a door is unlocked and opened. Maybe it turns on only when the 12V voltage is below a certain voltage. The DC-DC converter is certainly on far more than the DC-DC converter in our 2000 Honda Insight hybrid which has no alternator and charges its 12V battery from its high-voltage battery pack via its DC-DC converter just like our i3.

jadnashuanh said:

It's my understanding that the DC-DC power supply only runs if you are connected to the EVSE OR if the car is in the ready state.

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My voltage measurements indicate that the DC-DC converter is on in other situations as well such as after the car is shut down while the accessory power sockets remain powered (~20 minutes after turning the car off) and when unlocking and opening a door, possibly only when the 12V voltage is below a certain level which makes sense. In the second situation, I can hear a relay close at the same time that the voltage jumps from 12.xV to 14.xV.

For practical purposes...the time from when you wake up the car to when it shuts itself down may more accurately define when that can run verses the READY state...anyway, from what I was told, it does not run outside of that window unless it is connected to an EVSE and you can run the 12vdc battery down as a result. I do not know if you can easily switch the car back into the storage mode that they use when transporting the vehicle from the factory, but my guess is that that shuts off everything (clocks, timers, etc.) so you do not have as big of a chance of draining and possibly damaging the 12vdc battery. If you could activate that, it might be a good thing while leaving the vehicle for months. I do not know what that takes, or what might have to be reprogrammed once it was returned to 'normal', though...might be onerous.

jadnashuanh said:

I do not know if you can easily switch the car back into the storage mode that they use when transporting the vehicle from the factory, but my guess is that that shuts off everything (clocks, timers, etc.) so you do not have as big of a chance of draining and possibly damaging the 12vdc battery. If you could activate that, it might be a good thing while leaving the vehicle for months.

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If I knew how to do that, I would do so when we leave our i3 in storage for several months each year.

A BMW tech told me about the orange high-voltage disconnect that is easily accessible under the driver's side frunk cover. He suggested that I disconnect this disconnect before I disconnect the 12V battery negative cable to protect the high-voltage system from a sudden shutdown of the 12V system, but that doesn't prevent 12V loads from discharging the 12V battery. I would think that enabling/disabling the BMW storage mode would not require removing the frunk box to gain access to the 12V battery so that the negative cable could be disconnected/connected, but maybe that's what's required.

Thanks. I didn't realize it was an issue. Hopefully mine lasts the 30 months before my lease is up. In my ICE cars i usually replace batteries after 36 months, so my fingers are crossed it lasts as long as a regular ICE battery since it isnt starting the car.....

Embarrassing, but I don't even know where the 12V battery is located. Is it Lead or LiPo?

The 12vdc battery is in the 'frunk' near the right-hand side of the vehicle and is an AGM, lead-acid battery, similarly sized to what you might find in a motorcycle. It can be seen here: http://www.realoem.com/bmw/enUS/showparts?id=1Z23-USA-04-2014-I01-BMW_i-i3_60Ah&diagId=61_3350#61217635788 AGM batteries usually do not need much of any maintenance until they die.

jadnashuanh said:

The 12vdc battery is in the 'frunk' near the right-hand side of the vehicle and is an AGM, lead-acid battery, similarly sized to what you might find in a motorcycle.

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If you remove the cover to the left of the frunk box and look toward the back side of the frunk box, you can just see the battery. It's much more accessible with the frunk box removed.

It does not appear to be an standard automotive-size battery.

jadnashuanh said:

It can be seen here: http://www.realoem.com/bmw/enUS/showparts?id=1Z23-USA-04-2014-I01-BMW_i-i3_60Ah&diagId=61_3350#61217635788

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It looks the warranty replacement battery is a considerably larger standard battery (i.e., flooded cell, not AGM) with twice the capacity. If true, I hope our battery is never replaced under warranty, and I wouldn't buy a replacement from BMW. Mazda Miatas have an AGM battery which might fit. If not, Odyssey might make an AGM battery that would work.

I've been using AGM batteries in our 2000 Honda Insight hybrid since the original battery died. I feel that they're superior to flooded cell batteries because with no liquid electrolyte, they don't outgas corrosive vapors or leak corrosive electrolyte, and their self-discharge rate is much lower.

jadnashuanh said:

AGM batteries usually do not need much of any maintenance until they die.

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My experience has been that no maintenance is ever needed. With no liquid electrolyte or corrosive fumes, battery terminals stay clean.