on the series connectors what + and whats -?

[sharkattack99]

i was looking at the series harness i bought and it pretty obvious that the red wire is the positive and and black one is negitive (red arrows) so for the first battery i just match up the colors, but what about the second battery there both black (green arrows) how do i know i have it plugged in right?

 

[Mickey]

on the connector with the green arrows positive is the wire from the other battery connector