Auto Electricians advise required

nothing at all that is the original duel battery setup with the cole hersy switch (bat 1 bat2 both and off) countless thousands of vehicles set up like this including my 80

Ditto
As long as the "car" battery remains connected to the vehicle wiring, especially whilst the alternator is working.
regards
Donk

David,
reading your post, you want the original starter battery to always remain the starter battery, you want to add another battery in parallel with the starter battery maybe to assist to run the fridge etc...
then put a 'large capacity' on/off (2 way) switch in line between the two batteries, as it is there only to connect/disconnect the second battery, a 3 way switch is not required because the (+) lead is either connected or disconnected by the 2 way switch.
Use the identical size battery lead as the existing leads, running (-) to (-) on both batteries, not (-) to earth on the second battery.

I myself have a question similar to David.

I have a dual battery set-up using a rotronics isolator and I want to put a 3rd battery in my camp trailer and charge it while driving only.

Can I just run a 6mm twin sheathed wire from the main battery thru a fusable link to an anderson plug at the rear of the truck and just plug the 3rd battery from the trailer into it while driving. (Connecting parallel from main to 3rd)

Would that have any affect on the second battery or main battery?

Vinnie

Adding a flat battery across a fully charged battery does NOT flatten the fully charged battery. Therefore one can connect the trailer to the car before starting the engine without concern of flattening the car battery if the trailer battery is flat.

A number of Vans I have worked on use a relay which connects as soon as the ignition is turned on. Even if the secondary battery was flat from overnight use, the car still starts as it _Affordable_Storage_Drawers.aspx all current from the charged battery.
Regards
Donk

You can do as you suggest but for best results you should ensure both batteries are of the same type eg. cranking batteries or deep discharge etc. to ensure they both receive full charge. If you mix batteries (which you can do) then one of them probably won't reach full charge - but you may never notice anyway :)

You can make you own cheap and very simple semi-auto dual battery system if you prefer: I gave details of it in post #16020

Mike Harding

There is no reason why you can’t parallel as many batteries and different types of batteries as you need and depending on how big you alternator is, you are using large enough cable between the batteries and providing you drive long enough, there is no reason why you can’t FULLY charge all the batteries.

As long as your vehicle can supply 13.8 volts or higher then you will be able to fully charge any and all your batteries.

The only thing to watch, for starting the vehicle, make sure you have a cranking battery with 12 volts or more in it and you won’t have any problems starting.

If you know you have auxiliary batteries at different levels of charge just make sure the motor is running before you connect them to the vehicles power supply. EG If you have a caravan or trailer with a partially discharged battery or two in it, leave the motor running while you couple up to the vehicle. This way the alternator is the thing supplying the power to the discharged battery and not your charged cranking battery.

The only draw back with a manually controlled battery connect, is that you will eventually forget and end up with two flat batteries and usually at a very inopportune time.

Cheers

Gaday Dave. That's the system I've used on the past 3 cars & works pretty well. Ideally you would fit 2 batteries of the same size & construction so they accept charge at the same rate but is not compulsory. With a standard Toyota batt & an 85 amp hour deep cycle connected together all the time I was getting 3+ years out of them. If only parked for 2 nights in one spot I would not bother Isolating the switch but could still start the diesel without trouble. I estimate each batt after 2 nights of fridge & lights was about half full. Alternativley if you had an automatic isolator system the aux battery would be dead flat over the same time & we all know thats what shortens batt life. To avoid the possibility of 2 flat batts & to maximise the usable power available you could concider fitting a variable voltage disconnect switch so that the fridge & lights would be turned off around 11.7 volts. This would give the maximum run time with just enough power to start your car, certainly a longer run time than just one aux batt isolated on it's own.
Run 35mm cable between them for improved starting & if you are going to fit a winch the switch should be rated at 200+ amps. (never use the red key type)
My little experiment of removing the starting batt & replacing with two 115 amp hour deep cycles is now 2 years old & they said it wouldn't work ;-))
Cheers Craig.........

With a lot of late model vehicles the radio is turned off during cranking, so to prevent the rear batteries from trying to start the engine, take a power feed from the radio power (check that it is being switched off when you crank) feed to a constant duty solenoid and use it to isolate the batteries when the engine is shut down or when cranking. If needed, use a double throw double pole switch and attach main battery feed to one side and radio feed to the other and solenoid to the centre which gives the ability to bring extra power in when winching or starting. I also hook the winch into the aux batteries so the main is engine only until needed. Barnray

i recon use a deep cycle second battery instead of a normal car batt. Car batts hate being discharged and are not designed for it. Use a deep cycle and fully isolate the batts with solenoids etc..so they are not running together as this will eventually wear one battery out. When you are camping you can be rest assured you can use the battery right up without any problems shortening it's life. If you discharge a car batt right down 5 to 10 times it will be stuffed, they should not be discharged more than 2 - 5%. Slightly off topic but I hope it helps.

Hi Dave,
Our mate Neil has this setup in his farm truck. He's got two identical crank batteries (mine and his :-)) We used a marine switch for isolation should that ever be needed - got a nice one on special at Whitworths for $35. But you never need to isolate them for overnight. Give me a call if you want to check it out. Very important to have the pos cables thoroughly insulated with a huge degree of paranoia as a short circuit can be a disaster with those thick cables.

Cheers
Phil