Mega on battery

I have a SparkFun LCD backpack that I have a simple hello world program working on. However, it only seems to work when I have the Mega powered through the USB. I have a 9v battery connected to the 2.1mm barrell connector. and the board is powered up properly.. but nothing displays on the LCD (which I have powered with a separate battery pack (4 AA batteries)

This is my simple code //DEMO

uint8_t reset[]={0x7C,0x00}; uint8_t demo[]={0x7c,0x04}; uint8_t reverse[]={0x7c,0x12}; uint8_t blight[]={0x7C,0x02,0x30}; void setup() { //Serial.begin(9600); //Serial.println("Preping Serial 1"); Serial1.begin(115200); Serial1.write(reset,2); delay(10); Serial1.write(blight,3); Serial1.write(reverse,2); delay(10); Serial1.print("Hello World!"); //Serial.println("hello world on lcd"); delay(10000); Serial1.write(reset,2); //Serial.println("Screen should be clear now"); delay(10000); Serial1.write(demo,2); //Serial.println("Splash displayed"); }

void loop() {


yep.. grounded properly etc.