After experimenting a while with the above circuit, I found that low side current sensing with only about 20mV signal required a lot of care in circuit design to minimize drift and offset. The TCA271 was OK as you can add an offset nulling circuit but I didn't want to have to trim the circuit so I started to look for a suitable alternative opamp. In searching I discovered the Texas INA219 high side sensor. This is available as a module from around $2 from China up to around $10 from Adafruit. The INA219 not only includes a suitable high side current sensor but also includes a bus voltage sensor, which I need for this circuit, and a 12 bit ADC. It connects to a microprocessor using the I2C bus. I have now changed the circuit to put the MOSFET on the low side of the sensor input. I am waiting for delivery of the INA219 modules that I have ordered and will report on the results when I have them. The microcontroller will remain an Arduino mini pro but the code will now read the input over I2C from the INA219, driving the MOSFET as per the current design. More to follow....