LCD Display Different Battery vs USB

Other Hardware Displays
1

I have a 4x20 LCD where I am ruining into something new; when the Arduino is connected to USB power, the display is perfect and easily visible; when connected to battery power, the display is almost unreadable (like contrast knob need adjusting). Unfortunately the way it is mounted I don't have easy access to the contrast knob.

I am powering with a 6 x AA battery pack and fresh batteries; I've also tested with single 9v.

Suggestions?

2

Clearly the voltage supplied to the LCD differs under these two circumstances.

Measure it with your multimeter at the supply terminals to the LCD (pins 1 and 2) in each case, tell us what it shows.