Systems and methods for monitoring and controlling a battery are disclosed. Systems can include a battery having an output voltage and an output current when delivering power, a load driven by power delivered from the battery, battery output voltage and current sensing circuits, and processing circuitry coupled to the battery output voltage and current sensing circuits. The processing circuitry may be configured to obtain sensed battery output voltage and current values, and to estimate values for at least three battery state parameters by concurrently fitting the voltage and current values to at least two different equations.