Parallel batteries

Submitted: Wednesday, Jul 27, 2005 at 00:27
ThreadID: 25087 Views:1915 Replies:8 FollowUps:5
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Hi all,
Advise required on to fuse or circuit brake... I'm currently running a redarc isolator
in my holden frontera with a trojan 100amp/h battery in the engine bay. This
has been working well for the past year, at maximum charge it's achieving 12.8v.
I'm wanting to connect a same battery to my c/trailer and run it in parallel with my secondary bat. Will run 6b&s (13.52mm2)to the trailer with a 50amp anderson plug. My question is, should I fit a fuse or circuit braker and especially what
amperage should they be? Please keep in mind that the trailer battery may be at 10 to 11v when connected to the car aux bat.
Cheers.
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Reply By: Well 55 - Wednesday, Jul 27, 2005 at 08:42

Wednesday, Jul 27, 2005 at 08:42
I run a 30amp blade fuze and holder in + line before the anderson plug, I have two 100amp battries in the camper and all seems ok to date.

After 3 days in one place, running fridge and lights, battery charge never fell below 12.8 volts. The fridge is a 90ltr Reefer DTV.

AnswerID: 122247

Follow Up By: Member - Norm C (QLD) - Wednesday, Jul 27, 2005 at 18:51

Wednesday, Jul 27, 2005 at 18:51
Great fridge Well 55. I've got the 70 lt DTV.
I've run 10mm squared cable from the aux in engine bay and another similar run from the CT batteries to my fridge. Both have 30 Amp circuit breakers at the battery. I have separate plugs in the vehicle so I can choose which battery I run the fridge off. For charging the CT batteries (2 x 120 AH connected in parallel), I have 80 Amp circuit breakers connected at each end to 25 mm squared cable.

Although I have not yet blown a circuit breaker (that I know of, as they automatically reset), I far prefer the circuit breakers over fuses. You would need to carry plenty of spare fuses. When one blows, the first thing you do is replace it. If there is a problem, that one will blow as well. That is 2 down and you don't know where the problem is yet!!!
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FollowupID: 377527

Follow Up By: Well 55 - Thursday, Jul 28, 2005 at 07:10

Thursday, Jul 28, 2005 at 07:10
Good point NormC about the circut breakers. I do have two of them one at each battery. The 30amp fuse is just on the charging wire.

Your right about the DTV fridge, a very good unit. Mine has the control seperate from the fridge as the fridge is built in. I like the way you can check the voltage and set temps at any time. I also have an internal temp readout.
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FollowupID: 377628

Reply By: Member - Collyn R (WA) - Wednesday, Jul 27, 2005 at 10:31

Wednesday, Jul 27, 2005 at 10:31
The function of a fuse or circuit breaker in this instance is to protect the cable against excess current in the event of a short circuit to earth.

The rating should thus be of the weakest link in that cable (in terms of current carrying capacity). 6 B&S can safely carry about 80 amps, so here the weak link is the 50-amp rated connector.

The fuse or circuit breaker should thus rated at about 50 amps and located as close as feasible to the battery.

Regarding fuses versus circuit breakers: historical US insurance records show that the former can be a fire hazard - and that circuit breakers are safer. This may have changed however since the widespread adoption of blade type fuses.

Fuses and cheap circuit breakers are temperature sensitive. They need locating in as cool a place as feasible (certainly not over an exhaust pipe etc). My own choice is a really good marine-type circuit breaker (Electric Boat Parts in Sydney is a good source). A good one will set you back about $50. Avoid the $15 cheapies - they work but are very temperature dependent.
Trust this helps
Collyn Rivers
AnswerID: 122256

Follow Up By: Member - Phil G (SA) - Wednesday, Jul 27, 2005 at 15:31

Wednesday, Jul 27, 2005 at 15:31
Collyn,

I've always preferred to use a fusible link at each battery, believing that voltage drop across the link to be minimal, and that its a very reliable connection (won't corrode like a fuseholder).

What are your thoughts?

Cheers
Phil
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FollowupID: 377490

Reply By: Vince NSW - Wednesday, Jul 27, 2005 at 10:33

Wednesday, Jul 27, 2005 at 10:33
No expert, but from everything I have read, you should not let your batt get below 11.5 V. When the experts put my system together they put a (Black box ??) that won't let me get to 11 V.

Vince
AnswerID: 122257

Reply By: Mainey (WA) - Wednesday, Jul 27, 2005 at 22:20

Wednesday, Jul 27, 2005 at 22:20
(QUOTE)Will run 6b&s (13.52mm2) to the trailer with a 50amp anderson plug.
Please keep in mind that the trailer BATTERY may be at 10 to 11v when connected to the car aux bat.(end quote)

IMO the 6 B&S lead is possibly too thin, remembering you are going back to the CT and you will be getting some voltage drop at the rear battery, I use 2 B&S cable with nil voltage drop back to my two 80A/h DC batteries.

Why would your CT battery be 10v to11v..?
Remember at 10 volt your battery is basically flat and soon will be on its way to the big battery graveyard. It would be advisable to use an electronic disconnection switch so the battery is automatically disconnected from the fridge etc, to save your batteries life, nothing worse than having a dead battery in the bush.

AnswerID: 122422

Reply By: Member - Collyn R (WA) - Thursday, Jul 28, 2005 at 13:59

Thursday, Jul 28, 2005 at 13:59
Re fusible links - agree they are just fine. I did not mention them as they tend to get confused with normal fuses.

6 B%S cable should be about right - assuming an 8 metre conductor run and 20 amp charge, volts drop is only 0.2 volt.

All in all though, if that trailer battery is run down regularly to '10-11 volts' the advice in this thread is irrelevant as the battery will have a half-life of only a month or two!
Collyn Rivers

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AnswerID: 122544

Follow Up By: Luc - Thursday, Jul 28, 2005 at 22:27

Thursday, Jul 28, 2005 at 22:27
Hi Collyn,

I agree cycling a battery 10 to 11 volt is not wise, my reason for noting this was to suggest a worse case scenario.
Can you give an opinion based on these assumptions:
1.Running 6bns from isolater to 100ah aux battery in eng bay with
an in line 40 amp c/braker is suitable.
2.Using 6bns lead over an 8meter length to connect another 100ah bat in
a c/t is suitable.
3.Having 2 40amp high quality circuit brakers located between these
batteries is suitable
4.Joining the vehicle and trailer lead with a 50ah anderson plug is
suitable.

This is based on daily running of an Engel 40ltr fridge .5 to 2ah draw which we disconnect of a night. Two fluro lights (small hand held type)for 3hrs, a 300w inverter primarily used to charge a camera battery/hand held uhf's5w1.5hrs and a lap top for approx 2hrs.

cheers luc.



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FollowupID: 377800

Reply By: Member - Collyn R (WA) - Sunday, Jul 31, 2005 at 19:08

Sunday, Jul 31, 2005 at 19:08
Luc

All is fine.

The 40 amp C/Bs are much larger than needed for the load current, but it is possible that charge current could reach that momentarily if the trailer battery is very low. The C/Bs are in any case fine for the cable capacity (6 B&S [about 16 sq mm] cable will carry close to 80 amps continuous current).
Collyn Rivers
AnswerID: 123039

Follow Up By: Luc - Sunday, Jul 31, 2005 at 21:47

Sunday, Jul 31, 2005 at 21:47
Collyn,

Thanks for your reply. My main concern all along has been the charge current on connection, thats why the 40 amp C/B. Do you think 30 will do? My Thoughts are to experiment with a 30 and 40 amp C/B.

Just as a matter of interest I took two volt readings, one from my aux battery and the other 5.5 meters down a 6 B&S line through a ciruit braker and anderson plug, there was no voltage drop. Both readings were 12.84v. Wiring up the trailer this week end, I'll keep you posted on the result.

Luc

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FollowupID: 378166

Reply By: Member - Collyn R (WA) - Monday, Aug 01, 2005 at 10:32

Monday, Aug 01, 2005 at 10:32
Luc

Bear in mind there will be zero voltage drop until you apply a load.

The purpose of the C/B is to protect the cable against overloading. The cable you are using will handle a continuous 70-80 amps and 150 plus amps on a cycling basis (if used for a starter motor cable).

If the trailer battery is low there is a real possibility of the alternator pumping well over 30 amps for a few minutes.

There is no point in using a C/B that is only just big enough to handle the proposed load where you have such a big cable safety margin anyway.

Go for the 40 amp unit - and if hard to find, pick a 50 amp unit. Do however buy a really good one (about $50). The cheap ones are simply too temperature dependent and may drop out at lower current.
Trust this helps
Collyn Rivers

AnswerID: 123111

Reply By: Luc - Monday, Aug 01, 2005 at 20:54

Monday, Aug 01, 2005 at 20:54
Collyn,

Thanks once again. Just recently heard of a lead earthing on a vehicle chassi but failed to trip the 30amp c/b. I'm unsure on the details because I've heard it 3rd hand, but could this be due to the aux battery on the trailer being earthed to the chassi rather than directly to the parallel battery?

Cheers,
Luc
AnswerID: 123230

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