BMW i3 Precharge blocked

[Garion-Momberg]

Hi,

I’m experiencing a strange issue that I can’t seem to resolve. The BMS is indicating “Error: Precharge Blocked”, yet all other status messages appear normal—there are no welded contacts, no emergency mode, and no active errors. Has anyone encountered this issue before?

When I start up the system, I can hear a contactor close and immediately open, followed by the Precharge Blocked error. I’ve checked the capacitor, and it is seated correctly. I’ve also opened the battery, unplugged all cables, waited for a while, and reconnected everything, but the error still appears. Additionally, I’ve pressed Reset DTC multiple times, yet the error persists.

Has anyone dealt with a similar situation? Any guidance would be greatly appreciated.

For reference, I have the 94 Ah version of the BMW i3.

Thanks in advance.

[dalathegreat]

Post screenshots from the More Battery Info page!

• Verify 33Ohm resistor on LV connector
• Verify that HVIL is seated next to HV connector

[Garion-Momberg]

I have verified the 33 Ω resistor and HVIL.

To be completely transparent, I have ported your battery emulator code to Beckhoff’s TwinCAT. I believe this has been partially successful—excluding the precharge blocked error—since the BMS appears to be communicating. For example, when I remove the HVIL jumper, the BMS reports an "Interlock error," as expected. Additionally, I am reading all cell voltages in the range of 3.599–3.611 V. Overall, everything seems to be functioning except for the contactors closing.

I understand that not using the exact system you specify could be causing this issue. However, given that I am successfully reading battery voltage, cell voltages, and statuses, I thought I might reach out for your advice. I completely understand if you prefer not to assist, but if you have encountered this issue on your system, perhaps I could try the same troubleshooting steps you used.

With this in mind, here is so more information:

• Isolation external: Not evaluated
• Isolation internal: OK
• HV Interlock: OK
• Precharge: Error Blocked
• Emergency mode: OK
• Battery status disconnecting switch: Contactors open
• Battery status error disconnecting switch: Contactors OK
  If you need anymore information, I will do my best to provide it. Thank you for taking the time to read this message!

[dalathegreat]

I suggest connecting standard Battery-Emulator in parallel to your implementation, to make sure you have a working baseline. After verifying that battery is working with normal Battery-Emulator, continue to troubleshoot your own implementation. The i3 is very picky when it comes to CAN timing, having some of the messages delayed by an additional 3ms can make or break it.

[Garion-Momberg]

That’s interesting, because I was wondering about the timing beyond the obvious cyclic CAN message transmissions (e.g., 20 ms, 100 ms, 200 ms, etc.). The Beckhoff IPC operates with a deterministic scan cycle, which I’ve set to 500 μs.

In my case, I also need to queue messages, since the CANopen interface module I’m using only supports transmitting one CAN frame at a time, with a transmission time of 230 μs per frame. The queue is typically cleared before the next 20 ms transmission cycle, but if the timing is closer to 3 ms, as you suggest, I may need to revisit this—especially since there are moments when multiple messages accumulate in the queue.

To clarify, if a message were delayed by 3 ms, would that result in the situation I’m currently seeing—where the contactors do not close, yet I still receive CAN frames from the BMS? As I mentioned, the battery information continues to update as usual.

[dalathegreat]

Yes, I am unable to help you since you are not using code I can review.

Please use a standard LilyGo board with latest 10.4.0 software, and evaluate if your battery is OK. I cannot help you!

[Garion-Momberg]

You have given me an avenue to pursue. Thank you. If you would like me to share the code once it's working, I would be happy to do so.