2014 i3 Rex - Power Bars Disappear Between 9%-13% SOC

[kgorczyn]

Greeting All – Apologies for the longer post.

TLDR;
• 2014 i3 REX Losing Power between 9-13% SOC
• Software Fix was applied in 2019, Fixed temporarily, reduced range, problem is worse now in 2021.
• Possible to check Cell Voltage of each individual cell?


Back in June of 2019 I took my car (2014 REX - 52k miles) to the BMW Dealer to address the abrupt and almost complete power loss when the REX turns on and battery hits 4.5% - See figure 1. I showed them the BMW Service Bulletins on the issue and they agreed the software update should fix - and it did for about 6 months. I tested it well; I drove it at 80mph on the highway (when I know the REX can't keep up) and battery hit 2.5% and I still had full power bars. Fixed I said. Looks like the fix was short lived…
Link to what I showed them -> https://www.bmwblog.com/2017/04/17/bmw-address-issue-incorrect-state-charge-readings-i3/)

The software update BMW performed in June cost me $350 and was not covered under any warranty. It also cut my EV Range by 20% or so. Where before the GOM would show 90-100 miles (I would get 70-80 actual) post update GOM shows 70-80 (and I get 45-60 actual) - I wonder if my battery deteriorated further and BMS can't account for the loss? BMW official report said 74% capacity remaining/test passed back in June 2019 – That was 2.5 years ago and about 30k less - Electrify app says SOH is in the upper 70s - I'm guessing this is not that accurate or is it possible for battery health to be repaired?

Fast forward to today/end of 2021, I lose my power bars anywhere between 9% and 13% - Wayyyy before the REX even turns on. Luckily, I am coded for HSOC so I can manually turn on the REX and get home every time. See Figure 2.

What's the HV Battery voltage supposed to be when the REX kicks on? I go from full power bars to 1 or 2 when my HV Voltage drops to <300v. If it hits <280v all power bars are gone, regardless what the Battery % is. More importantly, see figure 3. Notice the big delta between Cell Voltages. The top and bottom number are usually with 0.01v of each other until I hit <18% SOC as displayed on the dash. Is there a way to get a readout of these voltages on all the cells/modules?

I’m suspecting more battery degradation – but I don’t want to pay more diag fees until I’m sure battery is bad.

Thoughts?

Figure 1

Figure 2

Figure 3

 

[3pete]

Fast forward to today/end of 2021, I lose my power bars anywhere between 9% and 13% - Wayyyy before the REX even turns on.

Reading the bmwblog link you posted (I don't have a REX), you shouldn't be losing power blocks before 3% SOC, so this definitely seems wrong. I know you're trying to avoid stealership charges, but this seems to me like it should definitely be covered under the 8 year HV traction battery warranty. In fact I'm shocked they charged you for the original 'fix' but maybe they took that more as a "software upgrade request" as opposed to you saying "a warranted component of my vehicle is misbehaving".

Tom also mentions a lack of knowledge from the dealers with regard to the REX I3. It's possible they just see your car as being out of the 4 year warranty and they don't realize BMW is still on the hook for any components. (My dealer was not aware of this until I pointed it out in a discussion at one point.)

BMW official report said 74% capacity remaining/test passed back in June 2019 – That was 2.5 years ago and about 30k less - Electrify app says SOH is in the upper 70s

I took my car in for a capacity test because Electrified said 67%. They did a software upgrade, ran the test and said it was at 73% capacity. Somehow this process made Electrified think my SOH was now 80%, so it clearly does not always match with BMW's assessment of capacity.

More importantly, see figure 3. Notice the big delta between Cell Voltages. The top and bottom number are usually with 0.01v of each other until I hit <18% SOC as displayed on the dash. Is there a way to get a readout of these voltages on all the cells/modules?

This also looks wrong to me. It almost seems like maybe cell balancing isn't happening, either due to a glitch or because it can't.

I would advise reconnecting with your dealer, maybe with your Service and Warranty Information pamphlet in hand and a bookmark on page 7.

Or the other approach that might get traction is to feign ignorance of technical details and just recount the EXTREMLY harrowing story of how you were transporting a basket of puppies and newborn babies to a nearby hospital in a thunderstorm when your car lost power on a trecherous segment of high speed expressway.

 

[alohart]

More importantly, see figure 3. Notice the big delta between Cell Voltages. The top and bottom number are usually with 0.01v of each other until I hit <18% SOC as displayed on the dash. Is there a way to get a readout of these voltages on all the cells/modules?

This also looks wrong to me. It almost seems like maybe cell balancing isn't happening, either due to a glitch or because it can't.

Top balancing is used by the BMS which means that the voltage of weak or low capacity cells could drop considerably at low charge levels compared with other cells even though the cells are balanced at high charge levels.

It looks like at least one cell is weak or has a considerably lower capacity than most other cells. I think when the voltage of even 1 cell drops below a certain voltage, further discharge is not allowed by the BMS. This in effect reduces the usable capacity of the entire battery pack. I would think that the BMS would adjust its idea of usable capacity when this occurs so that the charge level would better match reality.

To me this is good evidence that the battery pack should be replaced under warranty. Hopefully, BMW's capacity test doesn't discharge the cells beyond the low voltage cut-off enforced by the BMS. If this capacity test allows the voltage of some cells to drop below this cut-off, the measured capacity could be higher than the actual usable capacity.