Third battery set-up

[PeteV]

OK folks, I've read most of the long thread of the other double battery set-up, and am asking a separate question cos mine is slightly different yet a similar topic. I have a near new STR 2013 D22. I've bought a Projecta 150Amp isolator kit with all the bits and cable included. I have a toolbox in the tub. So my plan is: attach the isolator to the inside of the toolbox, run 6m battery cable from the +ve Main terminal to the main cranking battery on the drivers side. I'd attach the cable to the drivers side chassis to save cable length. I then attach the +ve AUX terminal on the isolator to the aux (3rd) deep cycle battery which will sit in a battery box inside the toolbox as well. As per the instruction manual, of course noting to connect the necessary earthing for the AUX as well as the isolator cord coming off the side (to the chassis). So I'm basically following all instructions, the only difference being I have the isolator in the back near the 3rd AUX deep cycle rather than in the engine bay. I don't want to mess with the existing original 2 batteries which I believe to be both crankers as per other threads. I want to leave that alone. So that way the car cranks as per normal and the deep cycle in the back powers whatever I connect to it.

My questions - (1) am I doing the right thing? I prefer to have the isolator in the back rather than the front in the engine bay, I don't see the difference, either way there's 6m of cable between the cranker and the 3rd aux deep cycle. It also means it's less likely to get wet during a river crossing. I'm no electrician so friendly advise welcome. (2) Has anyone else done this? (3) Could anything go wrong with any of the batteries this way? I'm assuming the micro processor in the isolator controls the action and will ensure the crankers don't drain or get compromised. Constructive criticism welcome. Cheers.

 

[glennm]

I received some good feedback from members when i asked about setting up a 3rd battery.
http://www.navara.asia/showthread.php?t=27967
Hope it helps.

 

[Old.Tony]

There's (electrically) no difference to the cranking activity if it's back there or up front: while cranking over, the isolator will be off, and not feeding power to the aux in the back.

There will be a small difference in voltage sensed by the isolator. Some might say that is going to make a difference, but in reality I don't think it's going to be worth worrying about. Over 6m - if you're using good heavy cable (8G is what I use, 21mm2 - you could go up to 4G) - your voltage drop is going to be something like 0.2 to 0.3V. The cut-in on the isolator is typically 13.2V (when the voltage up front exceeds 13.2V, the isolator switches on) and on the end of that cable you'd need to have 13.5V on the cranker before the isolator started supplying power to the aux.

Now that MIGHT have been a problem at some point in the past, but I don't think it'll trouble you in your car. Your car will start and within seconds the alternator will have the cranker's surface voltage back over 14V - so do NOT worry about the isolator position.

Oh, and if you do intend to use the chassis for one of the electrical paths, be prepared to find the aux battery dead flat one day (and probably on the day that you need it the most). I strongly advise that you run a cable for positive to the isolator and a cable for negative just to be safe, sure and confident in its reliability.

What I would do is put a great honking fuse in the way, as close to the battery as possible.

Oh and water crossings aren't a problem if done properly. Properly = wading bra, slow steady pace in 4LO 1st or 2nd gear and keep a bow wave up = not so much water in the engine bay. Just to be on the safe side, you could make sure all the electrical things are clean and then spray them with that blue sealer that auto electricians use that I can never remember the name of.

 

[bods]

All of the above. The other thing to keep in mind, the aux battery won't get fully charged without a dc-dc charger or an inverter and multi-stage charger setup, as even the isolator you have, although smart in the way it supplies power, isn't the same as a proper dc-dc charger....

There's nothing wrong with the way you want to set it up though and if you want to use the chassis as an earth, it might be worth adding a couple of extra earths from the body to the chassis and neg terminals of the crankers to the chassis too

 

[Saitek]

The only other thing I would suggest is using a sealed battery in the box in the tool box just to prevent any possible leaking or explosive gas build up inside the box just in case of spark and such.

 

[PeteV]

Ok thanks everyone, just one question, Tony says "Oh, and if you do intend to use the chassis for one of the electrical paths, be prepared to find the aux battery dead flat one day (and probably on the day that you need it the most). I strongly advise that you run a cable for positive to the isolator and a cable for negative just to be safe, sure and confident in its reliability."

Not sure where this fits in. I look at the installation diagram (hope the attachment worked) of the Projecta and I see a fairly simple setup. The Isolator has two main connections, the MAIN + and the AUX +. The MAIN + goes to the Cranker +ve point. The AUX + goes to the 3rd Deep Cycle +ve point. There's nothing left, except the little cord that comes off the Isolator which is the earth and I plan to feed that through the toolbox down and attach to the chassis. Is that needed? Could I just attach it to the toolbox with a bolt? Now the only other earthing I see is the one that needs to go from the 3rd Deep Cycle to the chassis. The kit comes with a thick black battery cable so I planned to run that down from the -ve point on the AUX to the chassis. The other 2 batteries in the engine bay stay the same without interference.

I'm a little unsure what Tony's talking about, you obviously know your stuff. The only cable I plan to run ALONG the chassis is the one I described earlier, labelled (2) in the diagram. What am I missing folks?

 

[bods]

All he was saying is the factory earths aren't great, so if you don't upgrade the earths from the under bonnet batteries, the aux won't charge correctly as they earth will have a bad connection. You can either run cable all the way to the front, as suggested, or add extra cabling under the bonnet to achieve the same thing and earth the aux to the chassis under the tub

 

[Old.Tony]

I'd run a cable from (1) to (4) (which is also (6) as well). Same size as the positive lead - the same amount of power has to flow along BOTH cables!

 

[PeteV]

So my neighbour is a sparky and I think I have the idea and the reason why. He's suggested I run the earth cable from the AUX in the tray along the chassis (similarly to the +ve cable running from the AUX +ve to the Crank +ve) and connect it up to the earth (black) of the cranking battery. Is that what you guys are saying? Should I duct-tape the two cables as I cable-tie them to the chassis? Will that affect the electrics running through them (i.e. is the rubber coating enough)? Cheers.

 

[Old.Tony]

Yep, that's what we've been saying.

Coat them as much as you like. If the cable is suitable for the power going through it (in other words, you bought it big enough!) it won't heat up so insulating it further is no drama (and even better for it). Just run it along out of the way of things or risks and you won't have a problem.