Car battery charging system

[Barwick]

I'm looking to hook up a secondary battery to my car, strictly for powering an external accessory (a portable cooler) when the car is not on.

I know how to set it up so that the secondary battery is disconnected from the main system while the car is fof, but the question I've got is, how do I keep this secondary battery from drawing all sorts of power from the main battery when the secondary system is very discharged? I imagine when the secondary battery is at 20% charge, and the primary system is at 99%, the secondary system will start to immediately draw as much current as it can from the main system, in order to recharge itself? Or how does this work? There's going to be two batteries, one at 12.3V (fully charged), and another at maybe 11.1V (20% charge?), won't that screw something up?

Also, will I need a separate battery charger for the secondary battery? Or do I just slap on a power and ground wire from the main system, and the main system will handle the rest?

 

[RODALCO]

Keep them as two separate batteries. Ok, the negative can be connected permanenty from both batteries. The positives should remain separated.
I don't know what capacity your car alternator has, but an option is to charge your car as per normal and have a high power, 60 Amps or more heatsinked powerdiode in series with the secondary battery charging lead.
Ok, you have to allowe for a 0.7 Volts drop in the diode and heat, hence the heatsink.
You can consider building a voltage sensing circuit which senses that the secondary battery has recovered voltage enough and closing a relay contact in parallel with the series charging diode, for a full top up while the car is still running.
Otherwise tou have to look at fitting a second alternator.
Good luck Raymond