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Internal resistance values are incorrect (by a significant amount)Updated a year ago

This problem can be caused by

  • Backwards current sensor

  • Current sensor wiring issue (or no current sensor.)

  • Loose cell tap wire, internal damage to the BMS (blown cell tap fuse)

  • BMS has not been able to measure the resistance yet and is using default values

  • Bad cell

  • Cells at low temperature

  • Loose terminal

  • Incorrect busbar compensation settings

  • Skin effect


Resolving the issue:

Step 1.  If any trouble codes are present, first resolve those codes before proceeding.

This is particularly true of any open wiring fault codes.

Step 2.  Ensure that a current sensor is being used.

If no current sensor is used, the BMS cannot calculate internal resistance values and default values will be shown.

Step 3. Apply a load or charge to the pack to allow the BMS to calculate resistances.

If all cells are reading the internal resistance same value, the BMS may be using the pre-programmed default internal resistance numbers. These values are pulled from the “temperature compensation” table which can be found in BMS utility. Since the BMS relies on external changes in current for calculating internal resistance of cells, apply a load or charge to the pack to allow the BMS to calculate resistances. Sometimes multiple changes in current are needed within a short timeframe. 3 to 4 pulses of more than 40 amps typically are enough for the BMS to calculate resistances with.

Step 4.  Ensure that the current sensor is installed in the correct orientation.  

Using the BMS utility, ensure that current going into the battery pack (charge) shows up as negative (i.e. -8.5 amps) and that current being discharged from the pack shows up as positive. Current sensor polarity can be inverted in software under “General Settings” if backwards.

Step 5.  Determine if busbar compensation is being used.  

Open the Orion BMS utility, download the profile from the BMS if necessary, go to the cell settings tab, click cell population settings button and find the cell in question. Look in the last column and verify the busbar compensation setting in the last column. Incorrect settings here can artificially cause the cell to appear as if it has a high resistance when the cell is under load. For testing purposes, it is recommended to turn busbar compensation off (i.e. set the value to zero.)

Step 6.  If cell resistances are all jumping randomly, ensure that skin effect is not present.  

Skin effect is caused when a switching mode power supply or converter is used which places a significant AC component on the battery wiring. This effect can cause observed changes in wire resistances and can cause erratic resistance readings. Please see the “Skin effect” section in the wiring manual for more information.

Step 7.  Follow the procedures for diagnosing a high resistance cell in the troubleshooting section “High Resistance Cell.”

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