You only need 1 volt over the battery terminals, top left are + and bottom right are ground, to get it to detect that batteries are present and enables recording. It doesn't need much current so a 1kOhm resistor is enough to any power source that has 1 volt or more. If one battery cell are faulty it could have a total breakdown and not let any voltage pass thru it, or the battery contacts in the scanner could be dirty or have come loose from the circuit board.
/Ubbe