Triple battery set up

Rosscoe,
Thanks for the reply. I have a battery box and was intending to vent outside. The main issue I am not sure of is the whole charging 3 batteries set up and whether the third battery will drain the others if not isloated and any other potential problems. In other words, will the third battery when in use drain from the other batteries via the output on the alternator.
Lachlan

Lachlan,

My logic (which sometimes aint great) say yes, you need to isolate the battery. Effectively you're connecting it to the main battery in parallell and any load will draw on the main battery as well as the rear mounted battery. Lots of people use a simple manual switch BUT if you forget to change it over when you stop you could end up with a dead cranking battery.
The automatic smart separators (Piranha, Sure, Rotronics etc) do this when the ignition is off and or when the main battery drops below a preset limit.
The other way is to connected the rear mounted battery to the vehicles auxiliary battery assuming it's separated from the cranking battery by a smat solenoid/separater.

With your isolator switch closed you are effectively connecting the batteries in parallel and they will both have 14.2 V (typically) applied across them from the vehicle's voltage regulator. The third battery when connected is again in parallel and will have the 14.2 Volts across its +ve and -ve terminals. Now, the only problem is that if all three batteries have a low state of charge the alternator will try and pump full current at the 14.2 volts until the charge level rises and the internal resistance of the batteries increase.
So what's current rating of your alternator?

The issue is when you stop. Because, unless you indivdually isolate the batteries you will draw current from each in the same way you applied current during the charging cycle. Worst case - you flatten the cranking battery. Not a good idea! So you need discipline to make sure you isolate.
When I am in holiday mode I rank very low in that department so I go automatic when I can afford it and because I work in an industry where automation is the norm, I have a lot of confidence in brand name electrical and electronic hardware.

Rosscoe,
Not that I am doubting you, but reading an earlier post it saud about joining the batteries in parrallell and then trying to charge:
"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" this is why I was thinking of hooking straght up to alternator.

Lachlan,

This is probably correct to some degree, but under normal circumstances the voltage regulator looks at the average voltage of the batteries and typically maintains this voltage at about 14.2 V. At this level, as you point out you never achieve a fully charged battery.
What I can't tell you (coz I don't know) is what "raw" voltage is generated at the output terminals of the alternator. If its around 15V or more this could overcharge and damage the battery or at least "boil" off the electrolyte.
I think there is a response later on this thread where someone has done exactly what you want to do.
You will still need to isolate your auxiliary batterioes when you stop.
Let's know what you decide to do and how it goes.
Good luck with your project and have fun.

OzyGuy.
The modern charging systems use a diferent set up to the older units. in the older machines the alternator was connected directly to the battery, but in the vehicles with fusable links the alternator output and the referance for the regulator go to a distibution piont usualy where the fusable links are and then a feed is taken from there to the battery. The result of this is that when the alternator is working hard the output can be more than 1 volt higher than the mian battery this is why it is best to connect to the alternator, the deap cyle battery needs the extra voltage. Eric.

Connecting to the + positive terminal on the First Battery from the alternator as I suggested gets you the same result as connecting to the Alternator direct and has the benifit of being able to disconnect the storage battery simply and without interupting supply to existing battery system.

Hoffy,
I've been looking at these but couldn't get much detail on how they can claim 100% charge. I know they use pulse technology but was lead to believe by a lady at the 12V shop in Sydney that they only put out 14V and they are not "smart".

Sounds like you're happy with yours but from what I can see all their main function is to compensate for voltage drop between the vehicle voltage regulator and caravan battery and save you rewiring the caravan.
I can't see how they can fully (100%) charge the deepcycle van battery at 14V.
Can you tell me more.
P.S. I've been quoted $78.00 fitted by an auto electrician for a device with similar claims. I am confused.

Rosscoe