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White Paper - Cell voltage reversal

Direct connection to a capacitive load can reverse cell voltage, damage BMS

Problem anchor

Under these conditions:

  • a battery composed of cells with unequal internal resistance,
  • a load characterized by a large capacitance and low resistance,
  • no precharge

the surge current results in an instantaneous reversal of the voltage of cells with the highest resistance.

The cell may be handle that short-duration reversal in voltage; however, that voltage reversal, may damage the BMS:

  • In a wired BMS, it can damage the circuit at the input for that cell (if no fuse) or blow its fuse (if provided).
  • In a distributed BMS, it can damage that cell's cell board (if no fuse) or blow its fuse (if provided).

The problem is exacerbated by:

  • low temperature (higher cell resistance)
  • low SOH cells (high resistance)
  • particularly large capacitance (long duration)
  • low resistance load (little current limiting)
  • big gauge wires (no current limiting)
Mechanism anchor

When the load is first connected, a surge current charges its capacitance.

The peak of that surge current can be very high, as it is limited only by the internal resistance of the cells and of the load.

That surge current, dropped across the internal resistance of a cell, results in a drop of that cell's terminal voltage.

If the current and the cell internal resistance are both sufficiently high, the voltage drop can exceed the Open Circuit Voltage of the cell; in that case, the terminal voltage of the cell will be reversed (the positive terminal will be more negative than the negative terminal).

That voltage reversal, no matter how short-lived, may damage the BMS connected to that cell.

No precharge reverses cell voltage
Example of a 2-cell battery, connected to a low-resistance, high capacitance load, without precharge.

Note that, if the resistance of all the cells is exaclty the same, the volatge will be divided equally, and no cell will be reversed.

That may be possible with just a few cells in series, but becomes less and less likely with more and more cells in series.

Solution anchor

When connecting a battery to a capacitive load, start with a precharge stage, to charge the load slowly and at a limited current; then connect the battery directly to the load.

 

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Davide Andrea, Elithion, 2/16/10

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