USB transistor constant current charger for 2xAA

NB The Set LED1V9 battery is not part of the circuit but just a means of setting the LED forward V to about 1.9 in this model. The LED provides a roughly steady voltage at the base. Negative veedback from the voltage across R3e maintains a constant charging current until, as the 2 AA cells reach full charge their voltage rises to about 3.1V and the current falls.