Hi again Gregrae
I feel little confidence in the idea of an electronic circuit to achieve the results you require. If you look at the kinds of electrical devices that are used in automobile power regulation you will see that they are very robust, both in the sense that they survive mechanical shocks like collisions and the vibration of the road, and in the sense that even the high voltages that sometimes leak out of faulty spark plug leads will not easily destroy them.
Your requirements could be met by a relay switching the charging curent from the main battery into a secondary battery. I gather you understand that an electromagnetic switch is available for this job, but want to try using electronics to do the job as a learning project.
I think that it is very hard to design a good electronic circuit for this job because of the very delicate nature of control electronics and the very rough electric and mechanical environment of an automobile. Of course it is not impossible but it might be very far from economical.
However here's a suggestion for your first step.
Build your voltage detector using whatever technology you want and try and get it to a reliable stage. Remember that the functioning of your car will depend on the circuit you build, when deciding how reliable it really is. An 'occassional' fault could easily destroy your battery.
However I also think that a transistor circuit to do this job would be a complex and difficult design job, and that transistors are the wrong choice when there are easy-to-use packages such as op-amps available. I have a suggestion for the circuit, which I have drawn and attached below.
I hope someone else comes along and looks at my circuit and helps with some good ideas for protecting the rather delicate electronics from the rough environment. Already I see that I have omitted a zener diode, rated for breakdown at 18V, which should be connected across the power supply leads to the op-amp to protect the op-amp from high voltage spikes.
You'll find as you go along that in electronics designs you'll want to use the fewest possible components, often in an imaginative way, to get the job done. You'll also find that a lot of thought has to go into design work.
I don't guarantee that this circuit is the best for the job - in fact I'm sure it's not.
As always, I recommend "The Art of Electronics" by Horowitz and Hill, which is the best text ever written on electronics. You could buy a copy and read it from cover to cover.
