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I have one bad battery I'm wondering if I should replace both battery's or just the one i know is bad

Jimmy - why throw away a good battery if its good?

Suggest you do a little internet research on what a boat starting battery (engine) and a house battery (boat accessories) is and how it functions with the boat switch. Mark each battery physical location, based on the position of the boat switch a starting battery and ahouse battery

Place that new battery that you buy into the starting battery location and the older battery that's still good your house battery. location. Make sure you mark that new battery that you buy with a purchase date and the warranty .

Thanks that is what i thought

Jimmy - The next step after establishing the boat start/house battery configuration and depending on accessories (volts drawn) is a BlueSea ACR. good luck

Unless your good battery tests 100% you should replace both batteries. In a car or truck if there is 2 batteries and one is bad and the other tests okay but not 100% the new battery will only be charged to the level of the old battery. I am not sure on a boat with an ACR switch but I always replace them in sets in my trucks.