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BMS Shunt Activation Question...

Posted: Wed Sep 27, 2017 6:43 pm
by goin4bro
I have a 2011 Leaf battery pack that is now powering a 2008 Mini Cooper hatchback. I am using the Leaf's BMS and get readings of about 4.1v on each of the 96 cells. In testing the third party charger, the battery voltages climb nicely, but I have a few cells that got to 4.3v without any shunts ever turning on. I have tried charging both with and without +12v on the yellow wire (pin 21 of the pack connector - master charge relay engaged). When charging, I do see negative amperage on the current transducer, so I think the BMS should be able to tell that we are charging and it should attend to the shunts.

The only piece of the puzzle I cannot test or simulate is the connection of the BMS to the VCM. I suspect there might be a command from the VCM to the BMS to initiate the cell balancing. Does anyone have intimate knowledge of what it takes to force the BMS to start balancing? Without functional shunts, all my efforts to use the Leaf's BMS will have been for nothing. And I've put some ridiculous effort into it! :|

Thanks in advance!

Re: BMS Shunt Activation Question...

Posted: Fri Sep 29, 2017 1:10 pm
by dwl
There are others on here who know far more about the Leaf BMS than I do but what I have picked up over discussions on balancing and in depth look by one expert is the Leaf BMS has very low shunting capability. It seems the shunt IC is current limited by a 430 ohm resistor which makes sense to minimise dissipation and if more shunting is required then an external driver is required per cell pair. It seems the current design is only intended for packs which are expected to be closely balanced.

You may be missing the signalling to back off the charge when any one cell pair rises above 4.12V. If the pack is not well balanced then a manual balance may be needed to get them very close (at lower voltage).

This post goes into more details

Re: BMS Shunt Activation Question...

Posted: Wed Oct 04, 2017 1:48 pm
by goin4bro
"You may be missing the signalling to back off the charge when any one cell pair rises above 4.12V. "

Where would I catch this signal? Is there something that happens when one cell gets over 4.12V?

I am aware of the 430 ohm resistor situation and all that it implies. I simply cannot ever see shunts being applied either via LeafSpy or via my own querying of the BMS using CAN/Arduino.

I manually discharged the few cells that were over 4.2V, but the overall pack is out of balance to the tune of 300mv (highest minus lowest). I have correctly modified the BMS and its wiring to change the pack output from roughly 400v to roughly 100v, so basically have the entire pack divided into four equal 100v packs sitting in parallel. Thanks to Turbo3 for his posts on the inner workings of the electronics in the BMS.

Any further help would be appreciated!!!

Re: BMS Shunt Activation Question...

Posted: Mon Aug 26, 2019 9:13 pm
by Tremelune
I'm currently installing a Leaf drivetrain into a classic Mini, and it would save me a thousand bucks if I could use the Leaf BMS instead of buying an Orion...Might I ask how you got the BMS to work at all? Do you have a build thread or anything?