A mixed chemistry battery having a first battery cell having a first chemistry and a second battery cell having a second chemistry that is different than the first chemistry is provided. The first battery cell is connected to the second battery cell in series. The mixed chemistry battery includes a battery monitoring system configured to obtain a first SOC of the first battery cell and a second SOC of the second battery cell and based on a determination that an absolute value of a difference between the first SOC and the second SOC is greater than a threshold value, obtain a first capacity retention rate for the first battery cell and a second capacity retention rate for the second battery cell; and update the second SOC based on the first SOC, the first capacity retention rate, and the second capacity retention rate.