Originally Posted by
driveon
Hi dnej and fox, the first time you connect them, if they are at different States of Charge then there will be an “equalising” of the voltage levels but even if you had two identical types of batteries with different SoC, the same thing would happen so there is no real difference as to the whether the two batteries are the same or not.
Further more, if you are charging these two batteries, once connected in parallel, using an alternator, then the current from the alternator will be high enough to keep the voltage at both batteries higher than the State of Charge of either battery so there will be no flow of current from one battery to the other.
Last but not least, even when connecting any form of batteries in parallel, once they have reached the same State of Charge, whether it be by one battery discharging down to the other or by both batteries being fully charged, provided they are left permanently connected together, they will charge and discharge at the same level all the time and will act as one large battery.