okay if the power seems inadequate, how would you know whether it is okay or inadequate? symptoms? it would seem okay but not when GUI loads, sd card errors at startup also indicate low power!
voltage after the power supplies (connected on the board after the the micro-usb) should be 5V, a higher current is needed, depending on the board you are using whether it is A or B? for the A unit
300mA 500mA min is required and as for unit B you need 700mA min! i don't know the rating of the batteries you are using! then again providing 5 volts at the micro-usb inlet isn't sufficient enough, even a higher voltage source be connected to the micro-USB, check the current at the Pi board power points, test voltage at the points labelled TP1, TP2, it should be between 4.75V to 5.25V lower or higher values would create problem! multimeter helps a lot in studying the connections and the ratings!
see the power rating, as well as discharge rate and %age, with load connected
the power supply design isn't direct 5 V, is the requirement, but you need a higher voltage input 6-20V, because board uses switch mode power supplies that can generate 5V, but input of 5V isn't supported, your Pi Unit is powering up due to the high current that NiMH batteries provide!
refer to this: http://elinux.org/images/6/6f/PSU_7805_v02.jpg
more on microUSB and power supplies: http://www.raspberrypi.org/archives/260