CMU CAN ‘spoofing’

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MickeyS70

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Feb 23, 2022
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Location
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I had a long running issue in my 2012 Imiev where cell 88 voltage started to creep up compaired to the rest of the pack. This offset wasn’t changing under load therefore I deducted that the problem is CMU12 and not the cell itself.

I have replaced the LTC chip on the very same board before so I knew the work involved but have no longer access to a lift and therefore put off the repair. The issue has affected winter range significantly as cell 88 stops the charging process way ahead of the other cells.

Encouraged by @pievs post using a CAN ‘man in the middle approach’ to correct a faulty temperature reading, I used the same approach to correct the voltage accordingly and got my range back.

https://myimiev.com/threads/main-traction-battery-upgrade-i-miev.5458/

In theory this method could also be used to ‘fool’ the BMU to work with NMC cells however correcting SOC on the BMU/EVECU side of the CAN bus seems to be a better option.

It’s also worth mentioning that later I-MIEV versions (2017 onwards??) have the BMU moved from under the bench into the main battery pack..
 
Great job to fix the issue with a CAN bridge MIM from CMU to BMU.

Do you think it is an issue with the LTC chip again, or maybe some component or solder joint in the voltage measurement circuit?

i wonder if anyone has opened a late model pack to inspect the cells and BMU? Why did they move the BMU--to keep us from having access?
 
Do you think it is an issue with the LTC chip again, or maybe some component or solder joint in the voltage measurement circuit?
Probably a solder joint as replacing a SMD chip was a first for me...

i wonder if anyone has opened a late model pack to inspect the cells and BMU? Why did they move the BMU--to keep us from having access?

See some photos in links below

https://www.evmonitor.info/2018/07/mitsubishi-miev-pack-details-cmu.html
https://www.evmonitor.info/2018/07/mitsubishi-miev-battery-pack.html

The voltage formula described in the link didn't work for my 2012 IMiev, I used cell_Voltage=(MSB * 256 + LSB) / 200 + 2.1V (thanks to @coulomb for figuring this out)

I gather most EV's actually have the BMU now inside the HV pack, maybe Mitsu just followed the trend..
 
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I'd seen Nikolay's work before but didn't realize it was a 2017, good find.

The CMU board appears to be the same; the purple wrapper on the cells appears the same as LEV50(N); also same BMU.

i suppose it would be easier to keep the BMU in the pack near to the current sensor, isolation sensor, voltage and temperature sensors, and Pack CAN buss, and then just use vehicle CAN buss to talk to the EV-ECU, rather than run them all to outside the Pack. Keep it simple.
 
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