Paul, to connect two batteries of the same voltage in such a way as to maintain that voltage, they should be connected in parallel, that is, +ve to +ve, and -ve to -ve. This still gives the same voltage, but doubles the capacity, in your case, 12v 14ah.

 

It is my understanding that you should charge each battery individually, but others may have more information on that. 

 

Peter. 

i have two 6v 4.5amp batteries in my boat connected together  for 6v x 8 amps  and i charged both  at the same time so far after 3 months no problems

Paulyou could have all sorts of problems if you connected the two in parallel, like if one battery were to discharge, the other would try to charge it upbecause the internal resistance of the discharged one would be lower. As I see it, the best thing to do would be to carge each one individuallyHaving done that, I suggestyouconnect them up using the negative as a common ground or earthand connecting the positives through series diodes there are plenty of butch diodes available that can handle 7 to 10 amps forward current. this would ensure that you always had the correct polarity available to your motor and electronics.   If one battery were to take the lion’s share of providing power to the motors and go flat, you would have the other available as a spare in this case, the second battery would not charge the first because there would be a reverse polatrity diode acting as a non- return valve in the way, but it would be OK to provide power to the motors and ESC’s.  If access is a problem to awkwardly placed batteries, my suggestion again would be to install a charging harness at the same time with a nice convenient connection socket for your chargerjust keep a weather eye open for corrosive discharge from a charging battery, in other words, ventilate.

        Good Sailing…………………………………………………Mike Davidson