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A four state reconfigurable battery state of charge estimation algorithm combining the open circuit voltage (OCV) and its estimation and current integration is proposed in the present paper. In general, the open circuit voltage requires a few hours to become completely stabilized, when 100% confidence indicator is obtained. The proposed algorithm in its first state, works when the battery current is zero and 5 minutes within the OCV estimation process. For the second state, during constant current charging the battery state-of-charge (SOC) estimation is based on the previously measured look-up table that stores the battery voltage versus battery state-of-charge. In the third state the battery SOC is estimated by battery charging voltage vs. charging current characteristic stored in a look-up table. Finally, for the fourth state, during variable discharging current and regenerative breaking the battery SOC is estimated by current integration method. The four states of the estimation algorithm are self-identifiable and from here the reconfigurability property. The proposed algorithm is amply documented by experiments on a 48 Vdc, 55 Ah battery pack typical for mild hybrid electric vehicles.