A device to determine a state of a battery is disclosed. One or more transistors provide a resistance between first and second nodes. The one or more transistors are configured to conduct a supply current from a battery between the first node and the second node. A measurement circuit measures the voltage generated between the first node and the second node. The measurement circuit further measures the supply voltage. A calculation circuit generates an estimate of the supply current based on the voltage measured between the first node and the second node and the resistance of the one or more transistors. The calculation circuit generates an estimate of the state of charge of the battery based on the measured supply voltage and the estimate of the supply current.