A problem on Relay Power loss
While there are relays that will do what you want, these are not the ones.
Better would be either a suitable low Rdson MOSFET per battery, or a properly rated relay. 

Relays of this type can generally be expected to be junk for practical purposes at LV DC and dangerous junk at mains voltages. See the pinout footprint in the datasheet below - isolation is much reduced by placing the common contact between the coil contacts. OKish on LV, terrible for mains. 

You need to say what Voltage and MAX current you are switching.
The relay is dated at 28V DC at:
10A in SPST mode with resistive load.
7A in SPDT mode with resistive load.
5A in SPST mode with inductive load.
3A in SPDT mode with inductive load.    

What current MAXIMUM does your copter draw?
What is your operating voltage?
What voltage drop do you measure across the relay contacts during operation?
[If you have not measured it, why are you asking questions about it?]

At 10A the contacts MAY drop 1V.
At 50A = 5V !!! :-( (and the relay would die)

While the 100 milliOhm contact resistance specification (see below) is a 
.  

I found this data sheet - the same in details as Edsign links to but not as nicely presented.

The contact resistance is shown as "not more than 100 milliOhm" - Agh !!!!!!!!!!!!!!
At 10A that would drop 1V at maximum allowed resistance.
At 50 A = 5V drop.  



Added:

OP advises.
Ioperating ~= 50A
Battery = 3S LiIon.
Vbattery = 3.6V x 3 = 10.8V nominal. (  9V min, 12.6V max) 

The chosen relay is totally unsuited to the task.  You either need 


A  50A relay. This is not hard to achieve at your voltage but somewhat power hungry. A car horn relay may work depending on operate period allowed. 
OR 
A low Rdson MOSFET per battery.   


A MOSFET at 50A will dissipate
I^2 x R = 2.5 Watt per milliOhm of Rdson.
So even a 5 mOhm FET dissipates 12.5+ Watts - not undoable but almost makes a properly rated relay attractive. (But, see below, a 0.4 milliOhm Rdson MOSFET -  1 Watt at 50 A.

You can buy 30V, 300A, 0.4 milliOhm Rdson MOSFEts for $US3.43 each i nquantity 10 (!!!). Infineon IPT004N03L. Utterly awesome. Interesting package . 8 power SFN - looks unusual but has drain tab on 1 side, 8 pins on other. Pin 1 = gate, 2-8 = source. 

Good starting search here