I have been contemplating a second battery in my vehicle. Been contemplating it for some time, but never done anything about it. My inaction is due to the sad state of my electrical knowledge. I neither know how to add a second battery, or if it will even yield the benefit I hope. What I want -- is that I have considerably more Ah to spend while the engine isn't running, for using a (ham or CB) radio, or recharging electrical devices, using a computer, etc. So, another battery in parallel (obviously want to retain 12v). And as a side benefit I'd like to be able to start from the secondary battery if the primary fails--or at minimum, to recharge the primary from the secondary. (Although I've read that deep cycle can be used to start engines.) But this brings up a load of questions. Do any of you have experience here? -- Would the second battery be like the first, or would it be a deep cycle battery? If a deep cycle, will it play nice with the standard battery? -- Would I need to get a larger, beefier alternator, and what would that do to MPG? -- Would I be able to isolate the 2nd battery so leaving a light on overnight would not discharge both? In general, what are the pros and cons of adding a second battery to a vehicle? I would like to make my vehicle a bit more useable as a bug-out option. And it seems that "more electricity" would be a good start.