Connecting multiple batteries to a battery tender / trickle charger
The Cellink battery has a 12.5V cutoff option so wire with supplied harnesses and an use adequate charger for the main battery. 

A 10W battery “tender” may or nay not be adequate because it would need to regulate current to raise the voltage to 14.2 slowly from periods of discharge .  But a smart “tender “ will work.

If it is a simple constant voltage “trickle charger”, that may cause over current protection cycling off and on.