Strange Power Loss - No Faults

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Nanniepoo

Member
Joined
Dec 12, 2023
Messages
6
This past weekend, due to our other car being out of commission, we took our i3 out for a long drive to visit family, deciding to just rely on the REX. Typically we don't use the REX often, but it's put in some work over the last couple weeks extending some of our drives.

REX kicked on, on the highway, on the way out, no problems.
We charged a bit at our destination, so we headed back electric only to start.
Again on the highway, the REX kicked on. Unfortunately I can't really remember the exact situation, but a little bit after, I noticed the car (in cruise control) had slowed down into the upper 50s as someone went whipping past us. I took over control, however then I noticed the "available power bars" had diminished, and we were up against it. We kept losing speed until I pulled over into the shoulder when we got into the 40s. When stopping/regenning, I noticed the bars fill back up again. Trying to accelerate, the bars dropped back down. I tried a power cycle thinking it was some weird software bug, but things did not improve. We decided safest place was not on the highway, and try to limp to the next exit. We made it on the shoulder, but basically by the time we reached the exit, it really felt like best we could do was coast. I pulled off again to allow backed up traffic to pass. Decided to push forward to at least get to a safe parking area, and we coasted in to the gas station. The best we could accelerating from a red light was 5 mph, until we hit the downhill after the intersection. A bunch of restart attempts, during which REX turned on everytime, but bars actually got worse, to where I thought we were going to be stuck as there were no bars available. Through some frantic googling, somewhere I found someone recommended starting REX in emissions test mode, which I did, and we immediately got all bars back. (of note, it sounded to me like REX started, coughed, and then started running again). Decided to use the opportunity and headed to a very close level 2 charger. Charged up for a while, but not full, and headed for home, avoiding the highway as we knew we didn't have the range. REX did kick on and everything was normal for the rest of the drive. We had our 1 and nearly 3 year old with us, so tried juggling most time efficient but safest way to get home.


Obviously a lot of things happening at once, but to me, the battery SOC looked like it was right at the REX threshold. The only reasons I could think of for power loss are:
Low SOC - I don't think this happened
Low battery temp - 77F out and had been driving a while
High battery temp - I don't think I've ever read about this happening to anyone, but did cross my mind that highway + REX charging at low SOC causing issues thermally? Still wouldn't think that would happen on a pretty mild day. And I would think that would trigger some warning? Low battery temp is pretty expected, but a high battery temperature induced derate surely should drive a warning. Also why would REX in emissions mode fix this instantly.


So my questions are:
In general, any ideas/experience with what happened?
Could my memory possibly have deceived me and REX did not start and SOC fell and that's why we lost power? Would REX not starting when it expected cause a warning or any kind?
Is it possible the REX can start but not actually charge and that's why we lost power? And would this cause or not cause a warning?


Any input is appreciated, even if it's take it to a dealer to see what they can find, because at this point I have no confidence in using the car in a situation we'd need the REX. I have ISTA for our E70, but have yet to try hooking it up to the i3. Also have a cheap, but BMW specific, scanner that I can see if there are any faults stored that wouldn't have caused any dash lights/CELs.
 
I'm guessing that the BMS is not calculating the charge level correctly and has overestimated the actual charge level. When your REx engine starts automatically, the charge level isn't 6-7% but is only 1-2% which is when the power bars start to disappear. The REx generator won't generate much power until the REx engine is warmed up, so during the first 360 seconds that the REx engine is running, it won't be producing much electrical power, so the charge level would not increase and might continue decreasing.

This BMS problem has happened to many i3 owners. I have read that BMW dealers had a way to recalibrate the BMS, but that doesn't seem to be something that they all do now. Some owners have tried to run the charge level down to near 0% followed by charging fully. Maybe do this a couple of cycles to try to teach the BMS what voltage corresponds to a displayed 0% and what voltage corresponds to a displayed 100%.

The best workaround would be to enable Hold State of Charge so that you could manually turn on the REx engine before the charge level drops too low, maybe at 20% or higher. The BimmerCode smartphone app communicating through an OBD to WiFi or Bluetooth adapter could do this easily.
 
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I'm guessing that the BMS is not calculating the charge level correctly and has overestimated the actual charge level. When your REx engine starts automatically, the charge level isn't 6-7% but is only 1-2% which is when the power bars start to disappear. The REx generator won't generate much power until the REx engine is warmed up, so during the first 360 seconds that the REx engine is running, it won't be producing much electrical power, so the charge level would not increase and might continue decreasing.

This BMS problem has happened to many i3 owners. I have read that BMW dealers had a way to recalibrate the BMS, but that doesn't seem to be something that they all do now. Some owners have tried to run the charge level down to near 0% followed by charging fully. Maybe do this a couple of cycles to try to teach the BMS what voltage corresponds to a displayed 0% and what voltage corresponds to a displayed 100%.

The best workaround would be to enable Hold State of Charge so that you could manually turn on the REx engine before the charge level drops too low, maybe at 20% or higher. The BimmerCode smartphone app communicating through an OBD to WiFi or Bluetooth adapter could do this easily.

Thanks alohart! That makes a lot of sense to me.
 
What year is your car? What was the SOC reading when the REX kicked in?
2015
Not to sound obtuse, but REX turned on presumably at whatever SOC it's supposed to, unless the calculation was incorrect as alohart mentions. It was right at the indicator on the instrument panel.
 
2015
Not to sound obtuse, but REX turned on presumably at whatever SOC it's supposed to, unless the calculation was incorrect as alohart mentions. It was right at the indicator on the instrument panel.
sounds like the calculation was incorrect then. This has come up for other owners before.
 
Years ago I happened to repeatedly have an experience similar to yours. BMW assistance could not understand the problem. The last time this happened to me, the car refused to even start again after stopping and I couldn't even close the windows again. This way I discovered that the problem was the 12V battery (original). Once that was replaced the problem never arose again.
 
Years ago I happened to repeatedly have an experience similar to yours. BMW assistance could not understand the problem. The last time this happened to me, the car refused to even start again after stopping and I couldn't even close the windows again. This way I discovered that the problem was the 12V battery (original). Once that was replaced the problem never arose again.
As far as I know the battery is original (I've owned since 2018 and have not changed it). Maybe I'll do that just as preventative maintenance. Thanks!
 
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