SIM900 Conducted Emissions Fail
As a rule High frequency signals will always take the path of lowest impedance to get back to the source (or radiate). I can't speak for your whole setup because you have currently only posted a schematic. 

I can tell you this, if its conducted emissions that is you problem on the battery cable, then you are making a nice antenna out of you battery cable. So applying the rule: the current's most preferred and lowest inductance path is through your cable and radiating out into the world. To find out if this is the case, increase the high frequency impedance of the cable. This can be done by adding inductance, either by ferrite or by adding inductance on the PCB with inductors that attenuate high frequency "noise". 

If it is your board radiating, then you probably don't have a sufficient ground plane or you have made some nice slot or dipole antennas. But I'll digress on speculating since you haven't provided any information on the PCB.