Charging through the ciggy plug is possible, but certainly not the best, safest or most effective method.
First limitation is the vehicles original wiring. Long and thin with too many connections, voltage drop becomes a real issue, and the lower the voltage, the lower the charge rate making it to the distant battery. Most ciggy plug receptacles can realistically handle about 10 amps or less, and the plugs themselves usually can handle less than that. It is a spring loaded connection requiring steel springs to pass the current. Not an Ideal conductor, it heats up, causes more resistance, and heats up more. Finally the springs lose their spring, and the problem compounds on itself, until the center coil spring melts into the plastic housing.
Second id the fusing. A depleted battery, even a small one, can pull enough current to blow a fuse. Plug in an 11.5 volt battery to the system providing 14.4 volts, and enough current can flow to blow a 15 amp fuse, and most ciggy plugs are fused for only 10 amps or so.
Hopefully this is not something which requires you to run to a service department, but fuses are not cheap, and if they are, they are chinese, and will likely NOT blow at their labelled rating.
Third is if the vehicle's ciggy plug is not live with the ignition off, and you leave this battery connected, this battery can be powering things you might not want it to be powering. The stereo might still work with the key out of the ignition, the windows might too. Depends on the vehicle. All sorts of weird things can happen if there is an additional power source feeding power from an area not intended.
Generally a smaller battery hooked through a ciggy plug will not cause a load dump situation on an alternator if it is removed. It is possible, and newer vehicles with massive amounts of electronics in an attempt to make everything convienent, easy and idiot proof, actually make things more complicated.
My older vehicle, I can and do decide to remove my hungry house batteries from the alternator circuit making the alternator go from producing 60+ amps to about 10. No issues. I do this because it will cause the belt to squeal at higher rpms and I don;t like listening to that
If one really wants to charge an additional lead acid battery inside the vehicle, a dedicated circuit should be wired up. Connected to the main battery, fused at the main battery, and close to the auxiliary battery as well. The wiring should be 10 awg. The battery should be an AGM.
Wires should Not be run through the door seal!!!. This will not only damage the door seal, it can also cause a direct short and fry various parts of your vehicles electrical system. This is why you fused at both ends of the wire though.
There are all sorts of firewall pass throughs from the factory. Just have a look and find a way to get wires behind the dash from the engine compartment. Again, not difficult.
In general Ciggy plugs are standard, and convenient, but are poor connections, and if a connections needs to be reliable, and pass more than 75 watts for any length of time, a better method/ connector should be utilized.
I recently acquired some Anderson powerpoles that did not even get warm passing 30 amps.
the 12v SAE connector on the bottom of the photo is just for size reference. I do not trust these SAE connectors to pass more than 15 amps when new, and not more than 10amps when older and well used
I also recently acquired a Blueseas Ciggy plug and outlet that are built to a much higher standard:
http://www.amazon.com/High-Quality-...8&sr=1-2&keywords=blue+sea+1011+12v+dc+socket
They require very modest skills to attach to existing devices. If you need to pay someone else to replace a fuse for you, then do not attempt to do so.
I bought this for my 90 watt Laptop DC to DC car adapter. It has gone through 4 ciggy plugs I salvaged from other devices over the last 3 years. I have not yet wired it up because the last time it failed all that was required was a new center coil spring and fuse. I also took my dremel and ground some teeth into the ground springs which push against the walls of the receptacle. It is when the "tip" spring pushes the connector out of the receptacle that most of the heat issues develop. If the tip spring remains compressed, the connection is much more electrically and physically sound. The 'teeth' I dremeled into the ground springs keep the tip spring from backing it out of the connector, at least so far.
