I have been experiencing very strange issues with a battery pack of a RAV4 EV model 1997.
The modules seem to fall in 2 groups.
One group (lets call it group LOW) is reporting relatively low module voltages compared to the rest of the pack (lets call the others group HIGH). This is especially visible at the end of a charge, where group HIGH is at around 14.3V while group LOW is at about 14.0V.
After a week of non use
Sometimes it looks different
Once I start driving it takes couple of kilometers and both LOW and HIGH are at about the same 13.0-13.7 depending on current load and state of charge.
The modules in group LOW are the first to reach end of discharge after about 60 to 90km.
And just to make matters more complicated sometimes the LOW group has voltages higher than the HIGH group.
At the end of discharge
Now the "funny" thing - the LOW group are modules #5, #6, #7, #8, #17, #18, #19, #20.
This is funny because the LOW group is electrically in middle of the halves of the battery pack.
So the pack electrically looks like this:
01,02,03,04,05,06,07,08,09,10,11,12 [disconnect] 13,14,15,16,17,18,19,20,21,22,23,24
Initially I thought that this was due to temperature differences and location of the batteries in the pack... but after looking at the location map of the modules this seems very unlikely.
On the image below I have mapped all the problematic modules with pink/orange color... as you can see there is not clear relationship between location and problem.
Battery pack map of module location
All modules in the battery pack are connected to a Battery Sensing ECU - lets call it BSECU. Inside the BSECU there are two almost identical boards, each one responsible for reading half of the modules in the battery pack.
Some photos of a dissected BSECU:
BSECU with two, almost identical boards
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Manual fix?
Back of board 1
Back of board 2
Each battery module sense wire is connected to the BSECU via resistors in series
Could it be that the BSECU is drawing more current from the modules "in the middle" of each of the halves in the pack?
I am thinking of rotating the batteries - so modules 19 and 20 change places with 23 and 24. Modules 17 and 18 change places with 13 and 14. Then I can test charging and driving and monitor the development....
Any suggestions on what else to test before dropping the pack and rotating the modules?