Triple battery set up question.

[bushnbash]

Hi all I currently have a dual battery setup with out a dc-dc or bd-dc . I have a portable battery set up at pictured with a Projecta Intelli-Charge DC to DC. My question is would it be better to conect the battery box up to the starter battery or the second battery?
Would I be thinking right that it would be better to connect it to the starter battery as it would charge quicker than connecting to the second battery especially if the second battery wasn't fully charged?
KARB110DC.jpg
TIA

[Cuppa]

Yes connect it to the starter battery.

[my third 256]

if you have power to the back does it realy matter as you dont need to run extra wire to back
and as it isnt used when driving it will charge happily
isnt slower charge better than fast charge

[the evil twin]

If you are talking about with the engine running IE there is an Alternator supply then it won't matter a rats which battery as your DC DC charger will 'see' the Alternator.

If you are talking about engine stopped and you have no battery management or isolator it won't matter a rats which battery as your third battery DC DC charger will drain them both somewhat equally

If you are talking about engine stopped and you have a battery manager or isolator that disconnects the cranker if the aux goes low then connect it to the Aux not the Cranker as the DC DC charger on your third battery will drain whichever one it is connected too.

No matter what you do the 3rd Battery can only ever charge at the max of the Projecta device IE 7 amps or 10 amps or whatever model you have so the state of charge of the other batteries won't have any noticeable affect when the Alternator is running UNLESS you have a large capacity unit (say above 25 amps)

[bushnbash]

If you are talking about with the engine running IE there is an Alternator supply then it won't matter a rats which battery as your DC DC charger will 'see' the Alternator.

If you are talking about engine stopped and you have no battery management or isolator it won't matter a rats which battery as your third battery DC DC charger will drain them both somewhat equally

If you are talking about engine stopped and you have a battery manager or isolator that disconnects the cranker if the aux goes low then connect it to the Aux not the Cranker as the DC DC charger on your third battery will drain whichever one it is connected too.

No matter what you do the 3rd Battery can only ever charge at the max of the Projecta device IE 7 amps or 10 amps or whatever model you have so the state of charge of the other batteries won't have any noticeable affect when the Alternator is running UNLESS you have a large capacity unit (say above 25 amps)

Sorry about my lack of knowledge and probably stupid analysis on how I think it would work but hear goes.............
Yes the 3rd battery has a PROJECTA IDC25 DC/DC 25amp wired into the battery box.
There is also a IBS Battery Monitor System and also a big on/off dial under the bonnet, which I was told by the previous owner was to be used when using the winch.
I was thinking that if the third battery was connected to the starter battery it would get a better charge then being connected to the second battery.
Especially if i had been camping for a few days and the fridge was running off the 2nd battery and it was at say 50%(12.2v) . If the 3rd battery was connected to the 2nd battery wouldn't it have to wait until the second battery got charged enough before it would start drawing power from the 2nd battery? or would the 3rd battery draw all the power and the 2nd battery would not charge to it cappacity until the 3rd battery was fully charged. Thanks for any imput as I can quite get my head around it.

[the evil twin]

The 'big on/off dial' is probably a Cole Hersey switch.
Most people use one to connect the Aux battery in parallel with the Cranker for winching by bypassing the IBS system (as the IBS will not allow high current connection between batteries).

As for what charges what...
When you connect batteries in parallel (E.G. your 3rd battery to the Aux or Cranker) they appear to be just '1 big battery' as far as the charging source.
Unless you have a system such as IBS it will mean no battery has any priority over another no matter whether one is say 50% and the other say 75%.

If there is no charging source and you connect a DC/DC device (like your 3rd battery) to your 50% depleted Aux your 3rd battery at 75% will continue to deplete your Aux because that is what DC/DC devices do (take a lower DC voltage and step it up).
If you didn't have the DC/Dc device the lower battery (in this scenario the Aux) would actually 'steal' charge from the higher battery until they equalise

If a charging source becomes available, lets say you run the engine to turn the Alternator, then both batteries will begin to charge.
If there was no DC/DC they would charge at the individual Battery Charge Acceptance rate (C.A.R.) until they hit the max available current from the source.
Because you have a DC/DC on the 3rd battery it changes the scenario slightly as it 'MAY' steal some of the Aux battery share depending on lots of factors.

Also don't get trapped by the difference between Battery Isolators and Battery Managers.
Most Battery Managers are actually only Isolators IE they disconnect (isolate) one or more batteries to prevent a load (fridge or whatever) discharging them all.
These are really isolators and calling them a Manager is just a sales pitch.
A true Battery manager is quite complex and expensive and is actually capable of priority charging and load shedding etc.

Bottom line is no battery in parallel with another will 'wait' for the other to charge first assuming all other things are equal.