Bidirectional current sense

A method for sensing bidirectional currents charging and discharging one or more batteries without requiring an external sense resistor, comprising the following steps:

• (1) providing one or more batteries to be charged and discharged, a power transistor, a charge sense transistor, a discharge sense transistor, a regulated cascode circuit for a charge current, a regulated cascode circuit for a discharge current, and an analog-to-digital converter (ADC);
• (2) mirroring to the charge sense transistor a small fraction of the charge current flowing through the power transistor;
• (3) regulating a voltage drop of the charge current over the power transistor and over the charge sense transistor to be the same by the regulated cascode for the charge current;
• (4) measuring a feedback current of the voltage drop regulation of the charge current by the analog-to digital converter;
• (5) mirroring to the discharge sense transistor a small fraction of the discharge current flowing through the power transistor;
• (6) regulating a voltage drop of the discharge current over the power transistor and over the discharge sense transistor to be the same by the regulated cascode for the discharge current;
• (7) measuring a feedback current of the voltage drop regulation of the discharge current by the analog-to digital converter;
• and
• (8) determining actual charge of the one or more batteries by a difference between integrated charge measurements and integrated discharge measurements.

A circuit for sensing bidirectional currents charging and discharging one or more batteries without requiring an external sense resistor, comprising:

• an input port for a system voltage;
• a port for a combined current sink/source, wherein one or more positive terminals of the combined current sink/source is connected to a drain of a power transistor and one or more negative terminals thereof are connected to ground;
• a charge control circuit, regulating a voltage drop of a charge current over the power transistor and over a charge sense transistor to be the same;
• a discharge control circuit regulating a voltage drop of a discharge current over the power transistor and over a discharge sense transistor to be the same;
• said power transistor, having its source connected to a system voltage and its gate connected to a gate of a charge sense transistor and to a gate of a discharge sense transistor;
• said charge sense transistor connected between the source of the power transistor and an input of said charge control circuit, wherein a small fraction of the charge current through the power transistor is flowing through the charge sense transistor;
• said discharge sense transistor connected between the drain of the power transistor and an input of said discharge control circuit, wherein a small fraction of the discharge current through the power transistor is flowing through the discharge sense transistor;
• and
• a charge/discharge gate control circuit connected to the gate of the charge sense transistor, to the gate of the power transistor, and to the gate of the discharge sense transistor.