Dual battery system isolator question

Submitted: Tuesday, Aug 27, 2013 at 11:26
ThreadID: 104069 Views:2250 Replies:5 FollowUps:3
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Hi all, have a relatively simple question regarding dual battery systems.

How important is it that the isolator is in close proximity to the (cranking) battery?

The reason I ask is that I have limited real estate under the bonnet of my Territory (SZ diesel) to place the isolator and the only patch of unclaimed engine bay that seems suitable for it is behind the airbox on the passenger side of the car. Unless I get creative and mount it behind the driver’s side headlight or whatever…the place I was going to put it was claimed by the circuit breaker for my electric brake controller…
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Reply By: Member - bungarra (WA) - Tuesday, Aug 27, 2013 at 12:22

Tuesday, Aug 27, 2013 at 12:22
My opinion is that the distance between the cranking battery is not a big issue providing that the wiring is of sufficient size to prevent volt drop....Having said that the cable size is very relevant either side of the isolator..so really the simple answer is that it is not a big issue...if all the cables are selected correctly.

But I would be aware that if the isolator was very close (adjacent almost) to the crank battery then a fuse/breaker may not be needed (i.e. a very close, short well protected wire probably doesn't need fuse protection to the isolator)

If the isolator was some distance from the battery then fuse/breaker protection would be important (at the take off from the cranking battery)...simply because the greater the distance of wiring and the various routing paths, the greater the risk perhaps

I don't have any protection on either of mine from the source...but then they are very safely positioned and their length is no more than 300mm from the source. (cranking battery)

Having said that I do have protection on the other side of the isolator ...at the receiving battery end (on the receiving terminal).....simply because it is a live feed from both directions....the only "dead spot" is really between the terminals within the isolator.
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AnswerID: 517152

Reply By: olcoolone - Tuesday, Aug 27, 2013 at 15:00

Tuesday, Aug 27, 2013 at 15:00
An isolator is just an isolator so it can be put anywhere, we fit them in caravans to control the three way fridges...... but as said above make sure you use good size wire.

Fit it where ever it's the easiest.
AnswerID: 517158

Reply By: tazbaz - Tuesday, Aug 27, 2013 at 15:42

Tuesday, Aug 27, 2013 at 15:42
Any Auto Electrician can work it out. Take your car to one, and relax.
AnswerID: 517161

Reply By: Road Warrior - Tuesday, Aug 27, 2013 at 17:58

Tuesday, Aug 27, 2013 at 17:58
Thanks guys. At this point I will be fitting it myself however if the obstacles become too much for my level of knowledge I'll get an auto leccy to do it.
AnswerID: 517166

Reply By: troopy 47 - Tuesday, Aug 27, 2013 at 20:02

Tuesday, Aug 27, 2013 at 20:02
Just about to wire dual battery in the tray of new dual cab landcruiser was going to use isolator near crank battery with 100amp fuse and 6b&s wire to run from crank battery to tray any ideas if that size wire will be OK.
AnswerID: 517179

Follow Up By: kerrydenis - Tuesday, Aug 27, 2013 at 20:25

Tuesday, Aug 27, 2013 at 20:25
Hi, If you are using your second battery just as a charge battery use about the same size cable as your OEM cable, but if you want to use it for starting as well use a size or two larger to allow for voltage drop. Cheers Denis
FollowupID: 796758

Follow Up By: olcoolone - Wednesday, Aug 28, 2013 at 08:10

Wednesday, Aug 28, 2013 at 08:10
I would use a smaller fuse, the wire you are using is only small and there is no need for a 100 amp fuse....try a 50 amp fuse.

There is no way you will pull 100 amps and especially with 6B&S.

In most installations I use 3B&S and if you intend jump starting or winching of the aux battery a 150 - 200 amp fuse.

What size isolator are you using?
FollowupID: 796776

Follow Up By: Member - Phil G (SA) - Wednesday, Aug 28, 2013 at 18:33

Wednesday, Aug 28, 2013 at 18:33
There's a lot of variables, but I think what you are proposing will most likely work fine......but......

If you are fitting a AGM battery in the tray be aware that most AGMs have a limit on initial current. Most 100A AGMs want no more than 25Amps. With 4B&S (bigger) cable I've measured 45 amps going into a Remco 100Ah AGM in the tray - and that battery died before 2 years because the initial current was too high. So it's not such a bad thing to use lighter cable. Using 8Ga cable is fine in my experience - and it will limit the initial current that goes to the AGM.

Alternatively, fitting a CTek D250S or Redarc BCDC20 as your isolator will cost more but will limit the current to no more than 20 amps and prevent the AGM from losing capacity. It will also upsize the voltage so most of the time, the battery gets a full charge.
FollowupID: 796816

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