Battery Charging

David the problem with hooking your second battery up in the method you describe, is you are not getting the full charge into the battery from the alternator. You are only getting the potential difference between the two battery voltages. The alternator only senses the battery that is most fully charged. This could result in requiring quite a time to fully charge.

The cheapest and best solution for your case is to get a dual battery switch preferably with a field isolation switch built in.
Supercheap carry these switches and they are not that expensive. I don't know if the one they carry has the field isolator switch.
These switches are designed to be hooked up with battery cable.

The switch can have these possitions.
OFF
Battery One
Battery Two
Both
or just the first three.

What you would do is start on your main battery, and then switch over to your second battery.
Now, you would have to check with an auto electrician if the alternator you have is safely switched at idle without a field switch. Most alternators at idle are ok, but don't take this for granted as some Hitachi models are not and will require a field switch before change over.

Just thinking about this, a battery switch with BOTH on would be better if you discharge the second battery to much. Some alternators require sufficient voltage to sense to start charging. If you flattened the battery to much then the alternator would not sense and charge the battery. If you had BOTH on the switch, the fully charged battery would then start supplying some charge to the second battery, and once it had come up somewhat, you could go back to idle and change the switch over to that battery on its own to get full charge. This system would need to be totally independent of your present dual battery system, or would need to be incorporated into your dual battery system to override it and work as described so you got full charge into the battery from your camper.

As to how long, well it depends on the battery size and the size of your alternator and the state of charge. An amp meter would be a useful device in the circuit so you know what is happening in that second battery. I think at the Supercheap etc. they can be had for around $15.00

I am not exactly rapped in using jumper cable leads. These do damage the terminals to ensure a proper connection, can come off or not make a good connection which could fry your alternator. A good eye terminal would be better solution with a wing nut terminal.

david if money is an isue you can alway swap the batteries over placing the 1 from the camper in the vehicle to charge it & placing the carged one in the camper, bit time consuming but might be a tempery fix till you can aford abetter solution
Regards Bruce

David.
The rate that a battery charges is determined by the voltage aplied to it and its state of charge at the time. The alternator does not sense the charge of a battery but puts out a constant voltage. your alternator is capable of charging all 3 of your batteries without any problems if you voltage regulator in your alternator is adjusted corectly it will charge any batteries connected to it at a maximum of 8 amps per battery. The way the wiring in the vehicle is arranged limits the charge by using the resistance of the wire from the alternator to the battery, So if you want to charge the second and third batteries you must run a new wire from the large terminal on alternator to the other batteries via a switch or a relay operated by the ignition cct. Make sure that the wires are run in a way that the insulation wont rub through and start a fire, if you can find a piece of old garden hose to run the wires through this will be as good as conduit. The time the batteries take to charge is calculated by the capacity divided by 8 so you n70 will take 9 hours from dead flat. dont let anybody tell that you can charge a battery faster than 8 amps without damaging it it is simply not true. Eric.

hello david
1 a suggestion as your 2 sub batteries pack up you change them to deep cycle i know about extra cost but it is well worth it.
2 i wired my own second battery myself (deep cycle now but originally just a normal battery) it comes from main batteryto second battery via a full time or constant heavy duty solenoid which i can control with a switch under the dash .it is wired with battery cable and from it i run my van on 12 v0lts plus 1 fridge in back of car uhf radiohf radio when fittedand touch wood its 18 months old and still performing (century battery $175 but worth every penny) i believe that as long as you run 6mm+ cable to your plug you could put a switch under the dash to swap from charging second car battery to charging van battery with out to many problems and a lot of expense (this is assuming that you can do the work yourself and save the dollars it is not hard work you just need the know how cheers steve

David,
As you have stated your Liemack fridge is NOT power hungry at only 1.8 amp/hrs, I have a 50ltr Liemack fridge/freezer running @1 and -14 degree's with the same low power requirements as you have posted.

I fully aggree with Eric and Phil, however the leads must be capable of delivering without a power drop. "Ozzie's" idea of the 3 way switch to charge the camper trailer battery and/or the second car battery would be of great benifit to you, however with a Manual switch you have to Remember to turn it on as required....