Simcom900 shield not enough power from fully charged 12v 18Ah Battery?

Hello, i am using simcom 900 shield with aruino mega and i have also 1602 LCD screen connected, LINKSPRITE ttl camera,sd card module,RTC clock, button and everything works well when powered from USB.

The problem is when powered the arduino from external FULLY charged battery which is 12V 18Ah.

When the gsm modem tries to connect the LCD screen starts to dim...dimms more and after 3-4 second shuts down and also the arduino resets. It seems like there is not enough power,current.

But how is that possible to work with USB power but not with ext battery? I was thinking USB power sometimes could not be enough, but no it works with USB power but not with the ext battery.

I have read somewhere to solder a capacitor somewhere on the GSM shield but has anyone already one that? What will you recommend guys? I need to make this work with battery not with USB or DC adaptor.

Thanks in advance!

I am using this GSM shield

Any help...? ::)