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Cell voltages appear incorrect (when under load)Updated a year ago

When cell voltages are correct while sitting, but appear incorrect when the pack is under load, the cause is usually a loose terminal on a battery, a high impedance connection between two cells (such as a long cable, fuse or disconnect), not enough cells on a group, or a cell tap wire that is either not attached or loosely attached.


Resolving the issue:

Step 1. Open the BMS utility, connect to the Orion BMS and go to the Diagnostic Trouble Code screen.

Look for “open wiring fault” or “low cell voltage fault” error codes (labeled “open cell voltage fault” in older versions of the utility). These particular error codes indicates that the Orion BMS has detected a wiring problem which causes many other faults and means that the BMS is aware that cell voltage measurements are wrong. Refer to the “Wiring Fault Error Code” section under Diagnostic Trouble Codes later in this manual for steps to correct this error. If other error codes are present, address them as well using the information found in the Diagnostic Trouble Codes section of the manual. If there is a possible wiring fault, do not leave the cell voltage taps connected to the BMS unit for long periods of time as it may drain the cells or damage the BMS.

Step 2. Ensure that at least 4 cells are wired in the affected cell group, totaling a minimum of 12v.

If fewer than 4 cells are wired in a “cell group”, the BMS cannot accurately measure the cell voltages, please refer to the wiring manual to correct the wiring or for more information on ensuring wiring is correct.

Step 3. If fewer than 12 cells are wired into the affected cell group, ensure that each of the unused wires in the cell group is connected to the positive terminal of the highest potential cell.

Please refer to the wiring manual for more information on correct cell tap wiring.

Step 4. Inspect the battery pack for loose or corroded terminal to busbar or terminal to cable connections.

Loose or corroded terminals may pose a very dangerous condition that can lead to fire and should be addressed immediately. Terminals can often oxidize or otherwise corrode without being visually obviously and can present a fire hazard since they can heat up very rapidly from extra resistance.

Step 5. Inspect the wiring around the cell in question for any planned high impedance connections.

If there are any relatively high resistance connections directly adjacent to the cell such as long cables, long busbars, safety disconnects or fuses, see the wiring manual to determine if busbar compensation can be used to correct the measurement error or if the wiring needs to be changed.

Step 6. If no long busbars or cables are present adjacent to the cell, check to see if busbar compensation is improperly setup.

Busbar compensation can be viewed on the Cell Population Settings dialog which can be viewed on the Battery Profile tab under Cell Settings -> Cell Population Settings. Busbar compensation is the last column and is expressed in mOhm. For diagnostic purposes, busbar compensation can be set to zero so that the real, uncompensated cell voltages are displayed on the live cell data tab.

Step 7. Taking all proper safety precautions, measure the cell voltage while cell is under load using a multimeter.

If safe to do so, place the multimeter probes directly on the cell’s terminal (not on screws, bolts or other parts of the cell as they may not be making good contact with the terminal). If the cell voltage matches the voltage in the Orion BMS and the cell is weaker than the rest of the pack, the Orion BMS unit is working correctly and the cell may weak and needing replacement. See the “High Resistance Cell” for more information on determining if a cell is in fact weak and needing to be replaced.

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