Originally Posted by GrantR
If you are going to put in 2 batteries, I am curious why you would not put in 2 x 12 volt deep discharge batteries. That would provide more overall capacity.
you are incorrect that 2 -12 volt battery have more power. Typical 12 volt (group 27 or group 31)battery are about 70 amp hours at 12 volt = 140 amp hours total
2 - 6 volt battery in series will be over 200 ~ 260amp(total) hour depending on the exact battery. you still can only draw a lead acid battery down to 50% without reducing the life expectance of the battery regardless of the brand or voltage so keep this in mind with your looking at your 12 volt battery needs.
the other issue with 2 -12 volt battery is if one of the two battery fail and have a internal short it kills both battery and you have nothing. If one battery has a bad cell your going to over charge that battery and it will end up being the cause of both battery failing. Its less likely with a 6 volt to have a battery cause both to fail from a internal short because its not wired the same way where one can feed current to the other .