8.3.2.2.1 Cell Amplifier Headroom Under Extreme Cell Imbalance

For cell voltages across (VC1 – VC0) that are less than approximately 2.64 V, extreme cell-voltage imbalances between (VC1 – VC0) and (VC2 – VC1) can lead to a loss of gain in the (VC2 – VC1) amplifier. The cell imbalance at which the loss of gain occurs is determined by Equation 3:

Equation 3.

Assuming VC0 = VSS, it can be seen that when (VC1 – VC0) > 2.64 volts, the voltage across (VC2 – VC1) can range up to the limit of 4.4 V without any loss of gain. At the minimum value of (VC1 – VC0) = 1.4 V, an imbalance of more than 900 mV is tolerated before any loss of gain in the (VC2 – VC1) amplifier. For higher values of (VC1 – VC0), increasingly large imbalances are tolerated. For example, when (VC1 – VC0) = 2.0 V, an imbalance up to 1.33 V (that is, (VC2 – VC1) = 3.33 V) results in no degradation of amplifier performance.

Normally, cell imbalances greater than 900 mV will signal a faulty condition of the battery pack and its use should be discontinued. The loss of gain on the second cell input does not affect the ability of the system to detect this condition. The gain fall-off is gradual so that the measured imbalance will never be less than the critical imbalance set by Equation 3.

Therefore, if the measured (VC2 – VC1) is greater than (VC1 – VSS) / 0.6, a severe imbalance is detected and the pack should enter a fault state which prevents further use. In this severe cell imbalance condition comparisons of the measured (VC2 – VC1) to any overvoltage limits will be optimistic due to the reduced gain in the amplifier, further emphasizing the need to enter a fault state.