fuses/circuit breakers fo dual battery setup

Submitted: Wednesday, Jan 23, 2008 at 01:47
ThreadID: 53812 Views:34383 Replies:10 FollowUps:11
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Hi All

I have read a lot of posts regarding this issue but still believe I do not have an answer.

I have a second battery installed in the back of my wagon.

It is mounted in a marine battery box with a 120amp dual battery controller mounted in the box.

The battery box is connected to the main battery via anderson plugs and the wire is dual insulated 16mm2 approximately 120 amp cable.

I am confused as to what fusing/circuit breaker I need, if any. Bear in mind the dual battery controller is mounted in the box with the 100 amp AGM battery.

This is only a charging circuit and will never be used for starting the engine. I will use jumper leads for this if required.

I am concerned about an explosion if the circuit breaker/fuse is mounted in the box with the battery.

My questions are:

Do I need any sort of fuse?

If so, what shoudl i use, manual reset circuit breaker, auto reset circuit breaker or fuses?

Where should these be mounted if required?

Is this setup with dual abttery controller mounted in the battery box safe?

Any help would be appreciated.

Cheers

Rob
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Reply By: gilghana - Wednesday, Jan 23, 2008 at 06:11

Wednesday, Jan 23, 2008 at 06:11
Hi Rob,
I have a very similar set up to yours, except I have a sterling battery to battery charger doing the business in the battery box. Like anything else fed from the main battery/charging circuit it should be protected (e.g. if it was to short circuit). I have a 30 amp circuit breaker which is not yet fitted but will be this weekend!!! This I will put at the main battery to feed the second battery arrangement. It should be at or very close to the main battery to protect in case a cable was to short out maybe through chaffing or something. I just used manual reset klockner mueller circuit breakers (hosehold type) to feed out from my battery box, and same type I will install to feed the battery box. Far more knowledgeable people on this forum let me know that a normal household CB is just fine with 12v, and I tested it - they break very fast and of course you don't have to faff about with replacement fuses.
Hope that helps,
Gil
AnswerID: 283239

Follow Up By: Mainey (wa) - Wednesday, Jan 23, 2008 at 07:30

Wednesday, Jan 23, 2008 at 07:30
Gil,
You suggest: ""Far more knowledgeable people on this forum let me know that a normal household CB is just fine with 12v, and I tested it - they break very fast and of course you don't have to faff about with replacement fuses""
Gil, as I understand it they are NOT the way to go at all . . .
but as you say the people who advised you are "far more knowlegeable" :-(

Ever asked why there is not a 'fuse' between the Cranking battery and the starter or the Cranking battery and the Alternator as they carry very high amperage currents too ??

To be absolutely correctly installed there should be TWO fuses, (in a dual battery system) one fitted to each (+) terminal at EACH END of the (+) battery cable between EACH and every battery.

Reason being, the current WILL flow from BOTH batteries along the battery cable in the event the battery cable 'earths' on the body, NOT from only one direction or from only one battery.

A 'fuse' is fitted for protection in case the power cable 'earths' on the body, however (I believe) if the battery cable is securely fitted to eliminate this problem a 'fuse' is not required, that's why I don't use a fuse on my dual (3) battery system !!

THAT'S NOT A EXCUSE FOR YOU NOT TO USE A FUSE !!

It's my choice and I accept all and every risk involved.
I will RECOMEND using circuit breakers at each (+) battery terminal.
Mainey...


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FollowupID: 547854

Follow Up By: Member - Roachie (SA) - Wednesday, Jan 23, 2008 at 08:22

Wednesday, Jan 23, 2008 at 08:22
I agree with Mainey..... I have a 2nd and 3rd battery/ies in the back of my dual cab's cargo box. One of these is identical to the one under the bonnet (ie: I have 2 x Supercharge cranking batteries). That big sucker is permanently joined to the starter battery by a 00 gauge cable (about the thichness of my index finger) and is fed through a length of reinforced hose. It travels along the chassis rail and up through a pre-existing hole in the right mudguard. I have put a "sign" at both terminal ends to warn anybody/everybody that this cable is live at both ends.......

The 3rd battery is more typical of most blokes' set-ups and is charged from the main battery via a Arrid Twin Charge.
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FollowupID: 547865

Reply By: Member - Kiwi Kia - Wednesday, Jan 23, 2008 at 07:37

Wednesday, Jan 23, 2008 at 07:37
I don't want to muddy the waters but...
A circuit breaker is really to protect against short circuits and the only advantage it has over a fuse is that it can be reset. It will carry arround twice it's rated current for a LONG time before it trips.

I would not use a household 230V circuit breaker. The 12 volt ones from Repco or similar would be a better choice or use an inline fuse and carry a couple of spares.

AnswerID: 283244

Reply By: Member - Bucky (VIC) - Wednesday, Jan 23, 2008 at 07:57

Wednesday, Jan 23, 2008 at 07:57
Drop Derrek from ABR ( advertiser at this site ) an email

I am sure that he will have all your answers for you !

Cheers
Bucky
AnswerID: 283247

Follow Up By: Mainey (wa) - Thursday, Jan 24, 2008 at 10:07

Thursday, Jan 24, 2008 at 10:07
Link to Dereks recomendations

Article By: Derek Bester Updated: 1 Jan 2007

"Fuses
You may fit fuses at either end of the positive battery cable but they will need to be rated at 100 amps and are very expensive.
Most owners do not fit fuses on these cables for the following reasons:

Vehicle manufacturers do not fit fuses between the battery and the solenoid of the starter motor.
There is very little chance of a short circuit between the battery and isolator.
Fuses are a cause of voltage drop and will decrease the charge rate of the auxiliary battery"


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FollowupID: 548122

Reply By: Member - Bucky (VIC) - Wednesday, Jan 23, 2008 at 08:00

Wednesday, Jan 23, 2008 at 08:00
Derrek from www.sidewinder.com.au an email, he is very helpfull

He advertises on this web site as ABR.

Cheers

Bucky


AnswerID: 283248

Reply By: Robin Miller - Wednesday, Jan 23, 2008 at 09:08

Wednesday, Jan 23, 2008 at 09:08
Hi Rob

From your description you don't need high currents and it appears you have a voltage relay type controller and hence use of 16mm2 cable to feed the system.

The main source of ignition in this system would be from operation of the controller or shorts on 16mm2 cable which could allow a 1000 amps to flow. (depending on fusing)

I would not mount controller in the same box unless well ventilated.

When using 16mm2 cable for its low resistance and hence faster charging its important to not then limit things by use of a fuse system that has a high resistance so everything is best sized according to the job.

It appears you don't need high currents and system is not feeding a trailer so 8mm2 cable sounds like it would have done the job, and I would use a plain fuse (e.g. 60 to 100a blade) immediately on the positive output of each battery as master fuses.

I would then use a similar type fuse on each feed out of your 2nd battery sized accordingly (Typically 30 amp)

Don't know your controller and presume it has its own fuse protection, if not use similar fuse on its input and manufacturer would have reccomended the size.










Robin Miller

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AnswerID: 283256

Reply By: Robbates - Wednesday, Jan 23, 2008 at 09:27

Wednesday, Jan 23, 2008 at 09:27
Thanks for the information so far.

I have spoken to Derek (purchased the dual battery controller from him) and he said to install 50 amp auto reset circuit breakers which I am leaning towards.

Just wanted to hear from others as I have spoken to two Auto Electricians here in Perth and got two different views, both of which did not make much sense to me.

I frogot to mention that from the battery box in the rear I have anderson plugs connecting to my caravan and running the fridge when driving and when stopped for extended periods will run the lights (fridge will then be on gas).

Rob
AnswerID: 283258

Reply By: Member - Mike DID - Wednesday, Jan 23, 2008 at 09:34

Wednesday, Jan 23, 2008 at 09:34
Every source of high current needs to be fused.

The Fuse size must be LESS than the current rating of the SMALLEST cable fed by that fuse. So if you tee off smaller cables, they, need to have smaller fuses.
AnswerID: 283261

Reply By: Gronk - Wednesday, Jan 23, 2008 at 10:51

Wednesday, Jan 23, 2008 at 10:51
I agree with nearly all of the above but...............even though I have an auto reset CB on a charge cct to the rear of my 4x4 I'm not sure they are the best way to go ???

Sure they're faster than a fuse, but if you had a short cct fault, they would continually try to reset onto the short!!

Not so with a fuse.........system isolated until you put another one in!

And if you blew one ( at most two ) you would get out the trusty digital multimeter and check why it blew ( everyone with dual batt systems should really carry one.....cheap $10 one is all you need )

Thats my thoughts anyway !!!!!!!
AnswerID: 283277

Follow Up By: Robbates - Wednesday, Jan 23, 2008 at 11:39

Wednesday, Jan 23, 2008 at 11:39
Yes I have thought of this too. My concern with fuses is that if the battery draws high current for a short time then drops down to a lower current draw for the remainder of the charge cycle then a fuse probably would have blown and therfore not charged the battery where a cb would have reset and continued charging when the current draw dropped ( I have heard that agm batteries can do this when discharged).

Am I correct wuth this thinking?
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FollowupID: 547897

Follow Up By: Mainey (wa) - Wednesday, Jan 23, 2008 at 13:15

Wednesday, Jan 23, 2008 at 13:15
Auto re-setting switches are not a problem in 12v systems -IF- the correct one is fitted.

AGM's charge an discharge just as wetcel batteries do - just charge heaps faster, hence their added value when time is short for a recharge as in Solar or generator charging.
Mainey...
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FollowupID: 547924

Follow Up By: Gronk - Wednesday, Jan 23, 2008 at 13:26

Wednesday, Jan 23, 2008 at 13:26
Only reason a fuse would blow on charging is the fuse is too small...

80A cabling........50A fuse !! Plenty of safety margin !!
If your charging is getting anywhere near 50A theres something wrong !!!

Auto resetting CB..........IF there is a fault ( say a dead short ) then the CB will be making and breaking a big load........how long do the contacts survive that sort of abuse ??

In most industrial machines the CB trips and has to be manually reset....which of course will result in it not latching on, so the cause has to be chased down ..
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FollowupID: 547926

Follow Up By: Robbates - Wednesday, Jan 23, 2008 at 16:11

Wednesday, Jan 23, 2008 at 16:11
Point taken re the auto reset cb.

Then which would be the best. I am now thinking either a manual reset 50 amp cb or a 50 amp maxi fuse.

I hear that cb have less voltage drop.

Rob
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FollowupID: 547954

Reply By: Thylacine - Wednesday, Jan 23, 2008 at 13:11

Wednesday, Jan 23, 2008 at 13:11
Fuse/breaker at each voltage source
Switchgear inside an enclosure with a battery is *really* asking for trouble.
Relocate your dual battery controller outside of box.


ed
AnswerID: 283312

Reply By: Von Helga - Wednesday, Jan 23, 2008 at 14:45

Wednesday, Jan 23, 2008 at 14:45
Rob,

I use the "Home of 12 Volt" in Adelaide to answer for my technical questions. Do the Google thing and give them a call.
I have nothing but praise for them.
AnswerID: 283336

Follow Up By: Robbates - Wednesday, Jan 23, 2008 at 16:09

Wednesday, Jan 23, 2008 at 16:09
Thanks for the tip. will do

Rob
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FollowupID: 547953

Follow Up By: Grassparrot - Wednesday, Jan 23, 2008 at 16:41

Wednesday, Jan 23, 2008 at 16:41
Second Von Helga's comments. Bought my system from them and get my technical info from them as well. Quite happy at this stage. He recommended auto 50 amp circuit breakers to me for same size batteries as you have. Your set up slightly different though.
Grassparrot
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FollowupID: 547960

Follow Up By: Robbates - Wednesday, Jan 23, 2008 at 17:42

Wednesday, Jan 23, 2008 at 17:42
Hi Grassparrot

Were these auto reset circuit breakers or manual reset?

Any particular brand?

Cheers
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FollowupID: 547976

Follow Up By: Grassparrot - Thursday, Jan 24, 2008 at 08:48

Thursday, Jan 24, 2008 at 08:48
Rob,

They are auto reset circuit breakers. The kit they did for me came with one near the isolator and after reading this forum I decided to put one near the AGM in the back of my triton. He supplied one and I bought one from my local auto pro, cost about $7 each from memory 6 months ago.
Grassparrot
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FollowupID: 548103

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