A method of determining battery degradation retroactively using historical data is disclosed. The method includes the steps of collecting state of charge (SOC) and DC ampere data for a predetermined time period; determining a delta (Δ) SOC based on the data collected; creating a set of SOC regimes having a size based on ΔSOC; filtering the SOC data and determining a set of points which indicate a charging or discharging event; and calculating overall Coulombs associated with each charging or discharging event and for each event, producing a timestamp and Coulombs associated with each event.