Correct wiring for a Redarc BCDC 1240

Submitted: Saturday, Apr 19, 2014 at 11:36
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Im in the process of installing my Redarc BCDC 1240, I intend to have 2 aux batteries the 1st underneath my bonnet and the second ran into the rear of my Hilux ute, I will not not be installing a solar panel just the batteries. I have been told 2 different ways to install the Redarc the first is under the bonnet near the 1st aux battery and the second in the rear within 1 metre of the second battery which will be in a battery box which I will be able to take out when not needed. Can anyone please give me the correct way
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Reply By: Member - John - Saturday, Apr 19, 2014 at 12:48

Saturday, Apr 19, 2014 at 12:48
Brad, what you are proposing I think will negate the benefits of the DC-DC charger. One of the batteries is not going to be charged fully. I would put both auxiliary batteries in the tub, close proximity to each other and the DC-DC charger close by. If you have the aux battery bracket for the engine bay already, I would install a third battery in the engine bay and get a simpler battery isolator to allow that aux to charge. Or have the one in the tub charged by the DC-DC Charger and a simpler battery isolator looking after the one in the engine bay. Hope this makes sense? Good luck.
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Follow Up By: Sand Man (SA) - Saturday, Apr 19, 2014 at 13:50

Saturday, Apr 19, 2014 at 13:50
Good advice John.
You have it covered very well.

Perhaps the only thing to add to John's comment is the battery bank in the tub should be of AGM style and be as big as you need to run your high drain devices such as fridge, etc.

The battery in the engine bay is better off being of the standard wet cell deep cycle variety as they can withstand heat better and are much cheaper. But what would you be running off an auxiliary under the bonnet?

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Reply By: Member - John and Val - Saturday, Apr 19, 2014 at 14:04

Saturday, Apr 19, 2014 at 14:04
Hi Brad,

I agree with John's above comments, but would go a bit further.

The BCDC 1240 delivers 40 amps. That's a bit high for a single battery, so as John suggests, I'd go for a pair to be charged by the DC-DC charger. There will be little benefit using the charger with a battery mounted in the engine bay as losses can be kept very low there. In any case the BCDC would not enjoy the hot environment there, and the benefits it provides may be largely lost if it's mounted far away from the battery it's charging.

As John says, a pair of batteries mounted in the back, close coupled to the BCDC sounds good. To feed the 40 amp charger you will need a run of heavy cable (preferably both positive and negative cables) from engine bay to the rear of the vehicle. Worth giving some thought too to your alternator - can it handle the usual vehicle load and also deliver 45-50 amps for an extended period? (This is a real concern - I've had one die at an inconvenient time while working hard. They are not spec'd to deliver high output for long periods.)


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Reply By: Mick O - Saturday, Apr 19, 2014 at 17:00

Saturday, Apr 19, 2014 at 17:00
Here is a series of diagrams that were posted in a blog a week ago. They cover your situation.

DC Wiring diagrams

Also all the approriate wiring diagrams are available at the Redarc Website;

Redarc Wiring diagrams

Cheers Mick
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Reply By: Allan B (Member, SunCoast) - Saturday, Apr 19, 2014 at 18:39

Saturday, Apr 19, 2014 at 18:39
Hi Brad,

I would agree with most of what has been said above and add some further comment.

Under the bonnet is never a good place for a battery, especially deep cycle type, so I would endorse placing both batteries in the rear, connected directly in parallel.
The Redarc BCDC-1240 charging at up to 40 amps could be too much for most batteries (except certain models) so by placing the two auxiliary batteries in direct parallel will afford about 20A to each, a satisfactory arrangement.

Certainly do not place the Redarc BCDC in the engine compartment. Redarc's specification limits its ambient to 80c and that can be exceeded under the bonnet. In my own case, I have an aux. battery in the rear and another under the bonnet, partly due to space shortage in the rear and partly because it was all fitted up there on vehicle purchase. I have used two Redarc BCDC-1220's. one at the rear battery and the other on the cabin side of the firewall close to the under-bonnet battery. This arrangement is not shown in the diagrams of my blog (thanks Mick) as it is not a basic arrangement and I would not otherwise recommend it.

If you are placing two auxiliary batteries directly in parallel I would recommend that they are of the same specification and preferably the same age to avoid any balancing problems.

If you really must position one of the batteries under the bonnet it would be satisfactory if the BCDC is positioned somewhere in the cabin with its positive output cable routed to the positive terminal of aux. battery 'A' and its negative output routed to the negative terminal of aux. battery 'B'. Both aux batteries being connected in parallel using say 13mm2 (6 B&S) cable for both pos & neg. This will ensure that the batteries remain balanced during charging. Use 13mm2 cable to supply the BCDC from the alternator (cranking battery terminals). Let me know if this is not clear and I will sketch it for you.
Be sure to place suitable (60A) fuses or circuit breakers in each positive cable at the battery connection to protect against dramatic fault current and possible fire.


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Reply By: Member - Brad S7 - Saturday, Apr 19, 2014 at 19:47

Saturday, Apr 19, 2014 at 19:47
I would like to thank you all for your knowledge and help, I will reconsider my setup and more than likely set both in the rear, it's just sorting out room in the rear will be my next issue

Thank you again
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Reply By: The Bantam - Monday, Apr 21, 2014 at 09:40

Monday, Apr 21, 2014 at 09:40
Brad mate, while I will stop short of telling you there is no right and wrong, there is no single right way. There are however a string of issues that can make your installation better or worse to a greater or lesser degree.

ABOVE ALL, do not rely on generalisations, particularly in batteries there is a very wide range of specification and performance. There are plenty who will give you specific advice bassed on generalisations that simply do not hold.

Deal with specifics, look up the specs for the product in question and purchase or act bassed on those specifcations.

for has been posted that 40 amps seems a bit much......well if you have fitted an AGM with a maximum initial charge rated of 20 amps and you have a 40 amp dc to dc may well be.
But plenty of other batteries will cheerfully suck up that whole 40 amps and be very happy about it.

A couple of recuring personal view is that DC to DC chargers, AGM and deep cycle are all over sold.

DC to DC chargers in the right application are a wonderfull thing, but in many applications they will provide no advantage and may actually resut in slower charging than a simple voltage sensitive realy.

Most of the advantages of AGM can be had these days in fairly modest sealed wet cell batteries.

Most of the current technology sealed batteries also have improved deep cycle performance.

There is no substitute for enough battery capacity and speding the same amount of $$$ on twice the capacity in good marine cranking batteries can be a good choice.
Good AGM is typically twice the prive of a good marine cranking battery.
If you have enough battery and charging capacity you can avoid deep cycling for the most part.

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Follow Up By: The Bantam - Monday, Apr 21, 2014 at 10:03

Monday, Apr 21, 2014 at 10:03
Now as has been mentioned, heat is a major enemy of all batteries and they are better off out from under the bonnet.

If you are running a flat tray, the best place is under it...if you have a style side it gets harder especially if it is a dual cab.

If you want to run two batteries and you can mount them close all means charge & use them in paralell......but they need to be identical batteries..same size, type, brand, age and preferably from the same batch.

when you wire them run the incomming positive to one battery the incomming negative to the other and then paralell the positives and negatives with the same length and size link cables.....this has been proven to improve the evenness of the charging and loading over other wiring options.

If you cant do that don't get all bent out of shape.

I do have some questions over the sufficiency of 40 amps charging over 200 AH of batteries...if you run batteries with limited maximum initial charge rates, you have no choice.

But as long as you realise these things take time, it will work.

If you have to have a battery mounted under the bonnet....OK..but things will be less ideal and you have some choices.

Do you connect both batteries in paralell, and feed the one under the bonnet from the dc to dc charger and loop onto the second battery.

Or do you install a VSR to charge the underbonnet and run the dc to dc charger to charge the rear

or do you just install a VSR and not bother with the DC to DC charger.

All reasonable choices.

Now remember these DC to DC chargers like all multistage smart chargers prefeer a direct relationship with a single battery...or a pair of batteries that have the appaerance of a single battery.

That said, the Dc to Dc charger will charge the two batteries one located under bonnet and one in the rear...but don't be under the illusion that it will be even...good heavy wire front to back will help.

The rear battery will fully charge, but it will lag behind the front battery.

The load will also not be shared evenly.

As long as you realise this and independently charge the rear battery from time to time this should not be a problem.

If you are running these two batteries in different places there is no need for them to be identical...but better for them to be of similar type.

Choices Brad that is what you have.

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