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REC and WS500 User Forum

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Wrong voltage measurement on most positive cell


I'm having trouble with a Q BMS. It is being used in a lab setup with a varying number of battery cells.

When connected and configured to 16 cells the cell measurements works fine, with 14 cells the 14th cell voltage measurment is lower than actual, and with 12 cells the 12th cell voltage is so low (2.8V vs 3.3V actual) that it screws up the SOC estimate and the BMS goes into alarm.

According to the manial the BMS should support configurations of as little as 5 cells, why can't it handle 12 or even 14?

-Br Marius

Do you have the jumper installed from the last cell used to the last cell possible?

see diagram excerpt from manual below

Uploaded files:
  • Q-Wiring.png

Yes, positive side of the last battery is connected also to the last terminal on the connector, like shown on the figure.


it sounds like at some point the cells were wired up incorrectly and the circuitry damaged.  The BMS contains very sensitive ADCs that measure accurately to several decimal points and do not like to be wired out of sequence.  There are some internal fuses you can check.  If you grab a torx# 10 screwdriver you can remove the top half of the case and remove and test them.  They are black +-10mm square with a bit of silicone to help hold them in place.

If they are ok, you may need to return it to REC.  They have a repair-return program.

Just realized, the Active BMS has fuses that are user replaceable but the Q series does not.