Anyone really know about truck's electrical system or a site where I can ask?

Usually if the truck is off and the cig socket is on the assy side or key on side, then the ckt is broken back to the battery.

You are going to carry truck batteries in the cab?

 

It depends on how that specific model of truck is wired.

If there is a diode in the circuit, power will not flow from the battery back through the socket to other circuits.

This could be very dangerous. If your truck has an inverter, and there is no diode in the circuit, the inverter will be trying to pull some power from the battery, through the socket and its rather small wiring.

Even just a 1500 watt inverter can pull up to 125 Amps of current, which would fry the small wires in a cigar socket.

Even without an inverter in use, you have no control over how much amperage will be flowing from the battery through the socket, except for deciding what will be turned on. But if anything goes wrong, you are going to do major damage, as most of the fuses in the truck are closer to the factory power sources, not the devices they are feeding.

This is really just an all around bad idea. However, more than likely, there are diodes in your trucks circuits that would stop your plans anyway.

Do you own this truck? If so, the solution is to add additional batteries on a dedicated rack and have them wired properly with large, heavy gauge cables and battery isolaters/diodes so that electricity flows in the proper direction at the correct time.

If you do not own the truck, and you are wanting additional 12Volt power when parked, use a sealed battery like you said, and unplug it when you need to use it. You can get adapters that have 3 sockets and a cord with alligator clips to attach directly to a battery. Before anyone else gripes about the dangers of a battery in the cab, there are safe 12 volt sealed lead acid batteries that do not emit fumes, and will not spill anything if jostled or knocked over. Not very different than laptop batteries with 7000mAH of juice stored in them, or 12V rechargeable spot-lights.

 

Short answer: It depends.

Longer answer: It depends on what you are trying to do.

Some lighter sockets lose power when the vehicle is shut down. Those will definitely NOT do what you want.

Some stay powered at all times. Those, you should be able to feed power back through, but they are at the same limits as drawing power; typically 10 Amps or so.

I have never seen a lighter socket with a diode in line. All diodes have a voltage drop across them, and that translates to heat when the socket is being used.

If your truck has an inverter installed, then the inverter connection would be the appropriate place to attach an extra (sealed) battery; fused, of course.

 

It seems to me that the aux. battery would take forever to charge through the 16g wires used in the power socket and vice versa. It would be like having 16g jumper cables.

And yes a battery in the cab, if it's done right is not an issue. Dodge Durangos have tandem under the passenger seat, VW bugs had unsealed batteries under the rear seat and the campervans had them under the front seats, I've seen Peterbilt day cabs with 4 batteries in a box in the cab under the passenger seat.

I wouldn't just have it sitting on the floor however, imagine a 60-80 pound battery flying through the cab in an accident. Batterie boxes are cheap.