A mixed chemistry battery including a sensing cell having a first chemistry, a battery cell having a second chemistry that is different than the first chemistry, where the battery cell is connected to the sensing cell in series. The mixed chemistry battery also includes a battery monitoring system configured to monitor a current flow through the sensing cell and the battery cell and to calculate a state-of-charge (SOC) of the sensing cell. The battery monitoring system is further configured to calculate a SOC of the battery cell based at least in part on the SOC of the sensing cell.