i have a stock batt under the hood and the 1800 in the back. i'm gonna put another in the back so ther willl be two. so there will be an 1800 and a 2400 connected pos to pos and neg to neg in the trunk.
it wont matter as long as you stay all kinetik. i believe they are AGM so you have to stick with that you cant put a flooded battery back. its not good to mix old betteries with new b/c the old one will try and recharge from the new one and bring down the life of the new one