Ghost in the machine

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phildw56au

Member
Joined
Sep 10, 2021
Messages
11
Location
River Heads, Qld
A couple of months ago, I replaced 3 weak modules in my 2012 24kWh battery - specifically, cells 65, 73 and 81 (as identified by Leaf Spy) would drop 200mV+ below the rest of the pack, triggering turtle mode even though the battery would still have ~40% SOC.

I couldn't source new modules, but managed to get some reasonably healthy second-hand ones.

The car's battery behavior has significantly improved, however, two months later, when I look at the battery state using Leaf Spy, I find that the same three cells - 65, 73 and 81 - still report as weak, compared with the rest of the pack.

Unlike the earlier situation, the voltage differential is now closer to 70mV, but it still impacts on the reported energy remaining as the SOC gets low.

So, my question is ... is it likely that the persistent low voltage reporting of those three cells (and just those three cells - not the even-numbered cells in any of the modules) is being caused by a faulty BMS? If so, is it possible to 'reset' the BMS to address this issue?

Any thoughts from the wisdom of the forum would be appreciated.

Cheers, Phil
 
howdy phil, how close did you match the new cells/modules to the rest of the pack when you did the swap? The balancing current is only about 10mA in the BMS, so it would take a long time to ever correct an imbalance.

If they started out even and have now diverged to 70mV from all the rest, then they were likely weak modules. But it is quite unusual for just the odd cells to be affected and makes me think there could be an issue in the BMS/LBC. Can you do some cell capacity testing on the old modules--if they are okay modules then that would point to an LBC issue.
 
Hi Guys. It's been a while since I posted this topic starter, and I'm now ready to drop the battery pack out (again) and have a closer look at what's going on with it.

The voltage difference between the troublesome three and the rest now drops to around 150mV.

Reading through some other posts, I found one (created by the very capable Turbo3) detailing the innards of an early BMS (just like mine), and in it I noted that the chips monitor cell voltage levels in sets of 8.

Curiously, the three cells that give me grief - 65, 73 and 81 - are exactly 8 cells apart from each other. Btw, only one of these were replaced - the other two were simply relocated from elsewhere within the pack and were at almost identical voltages, so never exhibited any faults until they were placed in these specific locations.

Anyway, this time I'll take Leaf Spy voltage readings immediately before dropping the pack, and then get inside with my voltmeter and read the cell voltages to see if the BMS is reporting truth (and if it is, then maybe the BMS is causing the cells to drain) or fiction (and if it is, then maybe the BMS is causing the error).

Either way, methinks I replacement BMS will probably be in order, along with the purchase of a battery pairing tool.

Thanks for your thoughts on my dilemma.

Cheers - Phil
 
Dala and Turbo3 added battery pairing to LeafSpy Pro, so you should be able to pair a replacement BMS/LMS to your pack using the latest build:

https://mynissanleaf.com/viewtopic.php?f=44&t=32819
 
Were the removed modules found to be weak or defective or out of balance?

Sounds like there could be an issue in the LBC. Did you disconnect all the LBC connectors before swapping the modules? What about the big disconnect latch at the center of the pack? Just looking for clues. Those AFE chips are self-powered by the modules/cells that they are monitoring. One might fail or be defective, but 3 going out at the same time..?
 
phildw56au said:
Reading through some other posts, I found one (created by the very capable Turbo3) detailing the innards of an early BMS (just like mine), and in it I noted that the chips monitor cell voltage levels in sets of 8.

Could you provide a link to this? I'd like to know more about the inner workings of the BMS just out of curiosity. Thanks.

FWIW, I agree that the problem is probably in the electronics or wiring. If known good cells are now reporting a lower voltage after just being moved to their new locations that almost certainly means the problem isn't with the cells themselves but with the charging of them or the reporting of their state.
 
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