Hi Arven,

If you have one of your t-rex battery at 11.1V and 2100ma and add another one of the same size. That will give you a total of 22.2V and 4200MA. That will you the same total as one one of my battery and will be far less than what I paid for my battery. I am looking at picking up more 11.1V 2200MA battery and build my own packs for my 600. When you do pick up a 600, you will be happy you did!
I hope this help you out! Have a great week.

SLewis