Gharging third battery

" better to connect the trailer battery directly to the alternator is because the alternator output voltage is a little higher than the starter battery voltage this makes a huge difference to the charge rate of the trailer battery."
Incorrect advice as you can destroy the regulator. Modern vehicles are beyond the scope of the majority of DIY people.

"keep the voltage at the ignition switch constant"
Again incorrect advice. Never seen a system that works like that. Every regulator senses battery voltage.

"Marine type switches are a real danger to alternators" is an incorrect statement. They have a field switch on them so need to be wired correctly.

Autolec, haven't seen you in here before. Your screen name implies you are a qualified auto-electrician. If that's the case, how about telling us all how to do the dual/tripple battery thing properly, cheaply. You could also add your professional opinion to quite a few recent threads about a variety of "power" questions. I'm sure we'd all appreciated the advice of a practicing professional.Be good, or be quick.
Rohan (Sydney)

To do the tripple thing cheaply would require a battery manual change over switch or suitable relay after a Redarc battery isolator type unit.

Not particularly in favour of the tripple thing as there is no way to effectively charge more than two batteries cheaply. It can be done and there is equipment available if are prepared to spend the money.

I'm a simple fellow, and don't mind the odd (slight) inconvenience, so I'm happy to swap the trailer and vehicle second battery when required. It takes less than 10 mins. and what's 10 mins. every 2 days when you're camping, eh?Be good, or be quick.
Rohan (Sydney)

Suggest you fit 50 amp Anderson plugs on short leads to each battery to avoid having to do and undo battery terminals or nuts and bolts up each time on the battery leads.

Got to www.surepower.com My battery Isolator is one of their units and 10 years on not one failure or short or power surge like I keep hearing about and cant understand if it is done right the first time

Battery isolators like the Surepower ones do not let the batteries get fully charged. The design using diodes is an antique and outdated system.
Buy something simple that works well like the Redarc units made in Australia. More Redarc units fitted than any other brand
Rotronics is ridiculously overpriced

Interesting, Ok Why & What?
Why are diode types not letting batterries fully cahrge and What is different in a Redarc set up. How do they stop the batteries from draining to each other to "equalize" on charging or recieve a sudden surge on start up from the alternator.

If you do not have an external regulator that can be adjusted to overcome the between .6 volt to 1.2 volt drop that occurs with diode type isolators your batteries will never be fully charged. The .6 volt is approximately 25% of your battery capacity and the 1.2 volt drop is approximately 40% of your battery capacity. The majority of the diode isolators only have the .6 volt drop. Diode type battery isolators should be avoided. Go to the Redarc website and read the information there. Better than me typing it all out
http://www.redarc.com.au/sbi.htm
The Redarc is also among the best units for spike protection which is essential on modern vehicles. Gone are the days you can plonk in a few relays or a solenoid. We get vehicles in weekly with blown up alternators and electronics due to DIY electrical twin battery installations

Dozer how do you people dream up this sort of stuff. If you use a proper twin battery charging system like the Redarc there is no power going to the second battery until the vehicle is started. Hooking two batteries together especially one that is fully charged and one that is or could be discharged and of a different capacity or type as you suggest is not at all a smart thing to do and is considered to be dangerous.

Hi Autolec
Whats with the attitude about us people?
Next you will be saying the only way to start a car with a flat battery is to put a new one in...how does it go..its too dangerous to put two disssimilar batteries together, especially when ones flat and ones full. I agree it doesnt lengthern their lives...we do abuse these batteries by the way we use them.
Does not your battery isolator join the batteries together anyway?? ok its a smart one, only after the alt is charging and the main is above x volts will it come in and join the flat second battery to the full main....
This is how most work anyway, i shouldnt presume your aussie made Redarc works thesame as an ARB smart solenoid or a Pirahna, but chances are it does....
Heres something for all who are interested to consider... i use a second ($30 from the wreckers) alt to charge my aux battery-no connecting either system together at all, both batteries love it, but how to recharge the 3rd in the van????? Best way is ac charger, but without that its either swapping the aux for the van battery or the jumperleads trick. Trying to charge it by connecting the trailer plug and using smaller gauge wire than a jumper lead will introduce resistance and heat when heaps of amps try to equalise the voltage- not good at all for the wire insulation, plug or surrounding areas.
Andrew Wollongong

Dozer your information is technically incorrect and would cause failure in modern vehicles. Your technical expertise is that of an ill informed amateur.

Hi again Brian
Sorry for squabbling and name throwing on your thread.
There is a specially built 3 way isolater available in the states, called a "hell roarer" or something like that. It would be perfect for you but you would probably have to sell the Nissan to afford it. Because you are intending to go outside the manufacturers of second battery isolaters design, by putting in a third, and not having it there all the time means you wont find a standard fix. I have done thesame with my caravan, and get around the difficulty of a depleted battery by swapping or jumping the second battery (not the starting one that controls my injection etc)
My advice is to persue an auto electrician or three for the best information and fix.
IMHO all the systems listed here work including that old reliable diode one from the Arc - the sure power. If it didnt, it wouldnt still be for sale.
The questions you asked originally are summed up like this...
When batteries are joined together, they try to equalise their electrical pressure. (Voltage) this is sensed by the alternators voltage regulator, and as a consequence,the alt makes more current (amps) or flow of volts available to each of the batteries. Adding a third will do thesame thing, but remember the variable here-the resistance of the length of wire and plug you are using.
It can be thought of (by us novices) as 3 water tanks on a farm. Your pump can supply pressure and flow, and depending on which valve is opened, will fill one or all together. The new electronic isolaters are like the valve that fills the first tank full before opening a valve to fill the second tank. Some of the first tank will backflow to the second tank until their levels are thesame and then the pump will fill them both up. If tank 2 is empty when the valve is opened, a greater rush of flowing water will come from the first tank, if the second tank was nearly full, a small flow of water will result. Add a third tank over in the next padock, and the small hose that joins it to the second tank will dictate how much flow gets to it to fill it. Even if you use thesame size pipe as used with the other tanks, it will still effect flow rate, slowing it down. In thesame way, the third battery will be slower to charge than say the second because of its proximity to the alternator (or pump)

There is another way of looking at this Brian. When possible, have the trailer connected to the vehicle, so both trailer and second batteries are joined, that way, the trailer battery wont go flat as fast or as bad.Driving the vehicle daily will help to keep the second battery full and this will assist the trailer one when they are reconnected after the drive. Because neither is depleted, there will be minimal flow of amps between them, and less harm to either.
Andrew Wollongong 94 fzj80 landcruiser plus caravan and 3rd battery.

Hi Tony,
what (type/brand) batteries are you running with your outfit?

Sounds like a bloody good solution too

Eric your information is technically incorrect and downright dangerous advice and people should NOT follow your methods. We see weekly vehicles electronics and alternators destroyed due to wiring methods like you have posted