How do I add second battery to my 2010 JK 2-door?

Yes, you can buy a dual battery tray from many manufacturers. Then if you search around, you can see how to wire up the dual batteries. I would use a battery isolator on the charging circuit and possibly a high current switching system to switch between them if you want redundancy. Or if you are wanting just higher capacity, you can connect them together in parallel. BUT I must warn you that you should only parallel the same type, capacity and age batteries together lest you want problems because they need to charge at the same rate and drain at the same rate to work properly in parallel mode. Never put them in series or (end to end) like flashlight batteries are or you'll damage your electronics when 24 Volts blows everything up. Parallel keeps the 12 Volts but potentially DOUBLES your current capacity or standby time with accessories like stereos, fridges or inverters and lights.

RR
 
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