AUX battery system

Submitted: Tuesday, Mar 07, 2017 at 21:07
ThreadID: 134429 Views:3152 Replies:2 FollowUps:12
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Hi everyone,

I have been reading this forum off and on now for quite some time and have noticed there are a lot of different perspectives on almost everything, especially in the electrical threads, so I thought id describe how I have set up the aux battery system in my Hilux, and see what the varying opinions and possible failure scenarios that I may have over looked might be.

I have a 98 AH AGM aux mounted under the bonnet at the rear where the ARB holder fits. Why 98 AH, maybe this battery company is more honest and doesn't round up, I don't know, but that's what is written on it. I just asked a battery dealer that I have known for a very long time for a good quality AGM that can withstand under bonnet temps, and that's what i got. I have not been doing any rough roads, but soon i will put the cranking battery in that holder as it is lighter and the AGM exceeds ARB's weight recommendations, and put the AGM in the original battery holder.

I chose a Redarc BCDC1225LV to limit the draw from the vehicles charging system as I am aware that the 90 amp denso alternators in these cars have a tendency to fry rectifiers if pushed too hard in hot weather. Also, being LV it can be used in whatever vehicle I might get next. No solar on the vehicle as yet but can be added later if it warrants it.

It is mounted in the under seat compartment under the rear seat on the RHS, believe it or not, it stays cooler in there than the under bonnet temps, but the reason i chose there is because it is mid way....sort of between the aux battery in the front and the Anderson plug on the rear bumper, which are connected in parallel from the output of the redarc through two 35 amp manual reset circuit breakers. The unit is earthed to the body at its mounting point and is supplied from a pair of 4.6mm2 cables(both positive so about 9mm2) through a good old fashioned black fuseible link wire at the alternator and the 40A manual reset circuit breaker at the unit which provides a good neat secure way of terminating the heavyish leads.

Running the output back to the aux up front through 13.5 MM2 cable and another fuseible link wire to protect it from that end. To the Anderson plug is a pair of 4.6 MM2 cables, yes both pos.

The fridge(CFX35 mounted permanently in the tub) circuit, which is the only thing I can think of that will be running off the aux when travelling, changes over from the aux battery to the start battery when the ignition is turned on leaving the full output of the redarc devoted to charging the battery(s).

I have given it only one good workout to test the system, haven't started on the trailer yet so with just the one aux. It was over Christmas, running the fridge for 3 days and 2 nights without starting the car, and a lot of use as it was the drinks fridge, volts were down to 11.0 with the compressor running. On start up i got 24.3 amps flowing into the battery which is higher than i was hoping for given the position of the redarc, have been using the fridge on and off since then for a day or two here and there and seems to go ok.

Only thing i need to add is a voltage display in the cab so i know if there is ever a fault, as the fridge remote display only tells me the main system supply voltage when travelling.

Eventually the trailer will have its own solar system with a decent size AGM as well and be used to power the camp, pumps, lights, stereo etc and will be plugged in to the Anderson plug on the car if we don't do enough daytime driving to keep the fridge battery in the car topped up.

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Reply By: Member - Phil G (SA) - Tuesday, Mar 07, 2017 at 21:37

Tuesday, Mar 07, 2017 at 21:37
Your Redarc BCDC and AGM battery should both be under the bonnet or both be in the cab.
If you install the Redarc in the cool of the cab and install the AGM in the heat under the bonnet, you have effectively disabled the temperature compensation that is built into the Redarc. So as the AGM gets hot, the cool Redarc will to continue to put out full voltage and overcharge the AGM and kill it.
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Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 08:06

Wednesday, Mar 08, 2017 at 08:06
Interesting point, i was under the impression that the redarc reduced the charge rate when it got hot to protect itself.
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Follow Up By: Member - Phil G (SA) - Wednesday, Mar 08, 2017 at 09:19

Wednesday, Mar 08, 2017 at 09:19
From your Redarc manual, page 13:
"As the temperature of the BCDC rises above a certain level, the current capacity of the output is decreased gradually in order to protect both the battery and the BC-DC unit."

Redarc emphasised to owners not to mount the device in too hot an environment so the device does not cut out and leave you with a flat battery and cold beer on the hottest of days. But more importantly, and what they don't emphasise is it reduces output to save the battery from being overcharged and gassing. This is especially important with an AGM because it loses capacity every time it is gassed and you eventually end up with a useless battery. Wet cell batteries can be topped up with water if they get overcharged.

So in your instance, if you mount the Redarc in the airconditioned cab, it will think ambient is 20 degrees and it will put out full voltage to maximise the current. While the poor old AGM is sitting in 70 degree heat gassing away from too many volts.

Every charger I know has temperature compensation - same as your car's alternator. But your alternator will reduce the charging voltage to a greater extent because it sees much higher temps than a 240V charger. Over the past 20 years or so, alternators have had temperature compensation to save the battery. The standard batteries that come with new vehicles often last beyond 6 years now.

On page 7 of the manual it also tells you what to do with the orange wire - this wire varies the maximum voltage of the unit according to what battery you have. You use lower settings if the battery is installed under the bonnet.

Link for BD-DC instruction manual.
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Follow Up By: Member - Phil G (SA) - Wednesday, Mar 08, 2017 at 09:27

Wednesday, Mar 08, 2017 at 09:27
Probably worth posting a table to illustrate how you must reduce the charging voltage with increasing battery temperatures. This table only goes up to 50 C but under the bonnet, temps will be higher than that:

Other DC-DC chargers like the Ctek have a temperature probe attached as part of the unit and you attach that probe to the side of the battery so the charging voltage is ways appropriate. I have a solar controller as well that can take a temp sensing probe.

Table courtesy of
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Follow Up By: HKB Electronics - Wednesday, Mar 08, 2017 at 09:59

Wednesday, Mar 08, 2017 at 09:59
Keep in mind gentlemen the batteries are located behind the headlights, there are ducts there to channel the cool air stream from in front of the radiator to cool the batteries, so in a moving vehicle the batteries should be cooler than the under bonnet temperatures.

There are also still many fixed voltage alternator out there that charge at 14.4V or there abouts, day in day out yet the cars batteries still survive.

Generally it is accepted you shouldn't charge a lead acid battery if it is over 50C so that means just about every battery in a car is doomed.

The Redarc has temperature compensation built in so the manufacturer states, it will also throttle back to protect itself from overheating in hot environments.

As for the power stream data even though the temperature seems reasonably accurate after searching the net it seems the originator of the table was Powerstream themselves, this data does not appear to have been validated by an independent source so would only treat it as Powerstreams take on things.

How old is the data, does it apply to AGM and VRL batteries, does it apply to modern batteries using calcium and super pure lead which gas very little even at very high voltages?

On checking their website I notice they also had a detailed study on how peak charge voltage affect on LifePo4 batteries, their basing their recommendations only on the 2A cells they tested! One would have thought they would have sourced a range of cells up to for instance 100Ah to see if the results were consistent. It would seem they didn't want to spend to much money on their research.

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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 11:21

Wednesday, Mar 08, 2017 at 11:21
There may well be airflow from vent areas around and near the headlights but that may not provide the cooling effect you expect.

Some time back I was concerned at the failure rate of the batteries (1 crank, 1 AGM) in my engine bay so carried out temperature tests. I measured 74 degrees (and rising) at the batteries on easy bitumen driving on a moderate day with an ambient less than 30 degrees. Clearly, heat was coming from hot air exiting from the radiator and radiant heat from the engine.

I arranged a heat shield (as below) and this brought the battery temperature down to ambient air temp. The battery life has improved to where one would expect of batteries located away from the engine. The shield sure is effective and poses no access problems.
Incidentally, there was no room for the Redarc BCDC that feeds the AGM within the 'protected' zone so it is located just aft of the firewall and behind the glovebox with short cabling to the battery. This seems to be entirely satisfactory.


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Follow Up By: Member - Phil G (SA) - Wednesday, Mar 08, 2017 at 11:29

Wednesday, Mar 08, 2017 at 11:29
Hi Leigh,

The original question stated:
"I have a 98 AH AGM aux mounted under the bonnet at the rear where the ARB holder fits."
So its irrelevant to him that other peoples batteries behind the headlights are cooler.

But its nice of Powerstream to share such data, which is no different to what can be found from many other sources

Do you have data to contradict it?
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 11:30

Wednesday, Mar 08, 2017 at 11:30
Phil, that Redarc Manual also says..... "pick a location for the BCDC that is close to the battery and away from any direct engine heat."

Some hope of avoiding "direct engine heat" as I referenced in my Followup above re heat shield. I cannot envisage avoiding engine heat anywhere in the engine bay. The hot air from the radiator swirls everywhere and the engine together with the exhaust manifold radiate a lot of heat.

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Follow Up By: Member - Phil G (SA) - Wednesday, Mar 08, 2017 at 12:09

Wednesday, Mar 08, 2017 at 12:09
Gday Allan, Yes, I also noted that statement - they want it close to avoid voltage drop in the cable between the device and AUX battery; but they don't want the charger to shut down in temps such as you experienced. 200 series owners are installing their Redarcs in front of the radiator. Hope they have fuse protection on both batteries!

The 200series has batteries behind the headlights and no ducting - all available air goes to the radiator and the many coolers in front of it (intercooler ducts, power steering cooler, trans fluid cooler, air cond. I don't think there is any air left for the batteries to cool! My temperatures are above 40 degrees after a 15 minute drive and in excess of 70 degrees on a hot summers day.

In your 70series you actually had the benefit of the exhaust being on the opposite side to the batteries. The V8s have exhaust on both sides, and covers all over underneath so there is no escaping under bonnet heat.
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 13:49

Wednesday, Mar 08, 2017 at 13:49
G'day Phil,

....................."Hope they have fuse protection on both batteries!"

Or top-level insurance! lol

Yes, not only was I fortunate to have the exhaust on the other side but I also had room for the heat shield. Not everyone may.

Most of my incoming air for the battery is from below the front edge of the mudguard. The inner-guard curves down to form an air scoop which is very effective. Don't know if it was by design or a previous owner. Would probably also be an effective water scoop during creek crossings. I'll have to get Roz to sit on the guard with her head under the bonnet and tell me!

Mind you, I am considering moving the aux battery from under the bonnet back to the cabin alongside #2 aux battery. Gotta find SOMETHING to fill my days!


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Follow Up By: Member - Phil G (SA) - Wednesday, Mar 08, 2017 at 19:41

Wednesday, Mar 08, 2017 at 19:41
Gday Allan,
Yes filling in the days is a great feeling!
I retired last year after 40 years of working 50-60 hour weeks including nights and weekends. But was able to take good periods of leave over the past 15 years to stay sane.
But I'm still 25 years younger than you - got the 60th coming up soon. I take my hat off to what you do - keep it up!
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Reply By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 11:44

Wednesday, Mar 08, 2017 at 11:44
G'day "qldcamper",

There are countless configurations that are acceptable and I would think that yours would be among them. The fridge "changeover" to bypass the dc-dc charger is a good idea and is as per my own arrangement.

I would have used somewhat heavier cabling. I suggest 16mm2 to be minimal. This will be important in the eventual cable run to the camper as the battery there may not charge well if subject to volt-drop following the charger in the vehicle. The alternative is to place another charger at the camper battery and feed it directly from the alternator output.

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Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 12:18

Wednesday, Mar 08, 2017 at 12:18
Hi Allan,

It does seem to be working well enough and the cable according to the data sheets is only slightly higher in resistance of the 1 metre of 8 B&S recommended by redarc, and that may serve as protection from overheating the battery, not going to worry too much about it till (if) the system lets me down.

The efficiency of the charge to the trailer battery is really irrelevant, anything that goes in while under way is a bonus, just as anything that goes into the front battery from the solar on the trailer is just a bonus, linking the 2 whilst parked up is just to extend the capacity for the fridge. The SOC of the batteries once leaving home will rarely if ever be 100%, anything that goes in is a bonus, always got the EU10i which i consider a must in any set up, takes away the stress of needing everything to be perfect, which as you know can only happen in theory.
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 13:34

Wednesday, Mar 08, 2017 at 13:34
Agree on all points Mate.
And if it works..... it works! lol

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