Advice on a simple dual battery system

Submitted: Thursday, Jan 07, 2021 at 14:56
ThreadID: 140949 Views:1349 Replies:12 FollowUps:43
I want to setup a simple dual battery system in my vehicle mainly to power a 12V fridge while I am travelling. I have an Anderson plug in the rear of my wagon connected to the starter battery which I connect the fridge to when I am driving. When I stop the car, I remove the plug and connect to an Anderson plug in an auxiliary AGM battery. I want to automate this process so that:

A) I don't have to change battery connections when I stop and start the vehicle
B) The auxiliary AGM battery remains at 100% state of charge via the alternator while the engine is running
C) There is no risk of draining the starter battery

I believe that what I need is a DC-DC charger but I am having trouble sifting through all of the available information to figure out which is the best product for my situation. It doesn't have to be mounted in the rear of my vehicle. I am happy with a portable option. My batteries and appliances all have Anderson connections so ideally I would like something off the shelf which is compatible with this. I don't use solar very often but would like a solar option in case I need it at some stage. I have looked at DC-DC chargers ranging from the Powertech product from Jaycar at $219 to Redarc products at > $500. Any assistance with choosing a suitable product would be appreciated.
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Reply By: Peter_n_Margaret - Thursday, Jan 07, 2021 at 16:11

Thursday, Jan 07, 2021 at 16:11
Leave the AGM and starter batteries permanently connected to each other.
Run the fridge (plus any other stuff) via an adjustable low voltage cut out (available from Ebay for under $10) set to leave enough power to start the vehicle (say 12.2V?) with some safety margin.
Cheers,
Peter
OKA196 motorhome
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Follow Up By: Matty72 - Thursday, Jan 07, 2021 at 17:20

Thursday, Jan 07, 2021 at 17:20
When you say leave the AGM and starter batteries permanently connected, do you mean just simply plug the Anderson cable from the AGM directly into the Anderson plug going to the starter battery without anything in between so they are connected in parallel? If so, aren't there issues with connecting batteries of different types and capacity in parallel and won't this risk draining my starter battery as the fridge draws charge from it when the engine is stopped? I have a low voltage disconnect device but I believe it's fixed at a very low voltage (something like 10.5 or 11V) at which point the battery could be damaged so I'm not sure how useful it actually is.
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Follow Up By: Peter_n_Margaret - Thursday, Jan 07, 2021 at 18:33

Thursday, Jan 07, 2021 at 18:33
The only thing you need in between the 2 batteries is a fuse at each battery. The bigger the cables the better.
Yes, connected in parallel.
Should not cause any problem to either battery.
The fridge will draw power from both batteries fairly evenly when stopped but you need to preserve enough power in the starter to crank, so the low voltage cut out needs to be higher than you have. Maybe as low as 12.0V would be OK? You can try a few spots when you have some jumpers available to see how it goes.
ps. there should be no damage caused to a LA battery at 10.5V.
Cheers,
Peter
OKA196 motorhome
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Follow Up By: Matty72 - Thursday, Jan 07, 2021 at 22:17

Thursday, Jan 07, 2021 at 22:17
I have a 30A fuse on the positive terminal of my AGM's. How many amps for the fuse on the starter battery? I am guessing a 30A fuse will blow as soon as the engine is started.

If I leave the fridge connected to the starter battery, it only continues to power it for a few minutes after the ignition is turned off. So I am guessing there is some kind of automatic built-in shut off mechanism which would cause the starter battery to be isolated when the ignition is turned off.
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Follow Up By: Peter_n_Margaret - Thursday, Jan 07, 2021 at 23:20

Thursday, Jan 07, 2021 at 23:20
Fuses are to protect the cables. It can be just under the level that the cable can carry without overheating.
Clearly you need to understand exactly what you have before solutions can be implemented.
Cheers,
Peter
OKA196 motorhome
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Reply By: Member - Triton man - Thursday, Jan 07, 2021 at 17:06

Thursday, Jan 07, 2021 at 17:06
Hi Matty 72,

A lot more info required starting with whether or not you have a smart alternator or not, how many days you drive and for how long etc. You have a very basic system by swapping plugs but that wont keep your aux at 100% either. I would never rely on connecting and disconnecting Anderson plugs. As a bare bones minimum you would need an electronic isolator (VSR) such as an Enerdrive or Redarc or KickAss VSR with overide but you will never get a 100% charge into your Auxilliary unless you had a DC-DC charger. If you want the bare bones, use a VSR but ONLY if you do not have a smart Alternator. A smart alternator may drop its output voltage so low it will not 'switch' the VSR on. Stick with the RedArc like the BCDC 1225D and sleep well. You may just save stuffing two batteries, drinking warm beer and not getting a start on your cranking battery. Depending on your driving habits (engine -on time) you may well need the solar, 240 volt to run the fridge and battery charger and/or a generator. Talk to a professional and dont be a tight ar$e in getting it right.
Triton Man

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Follow Up By: Matty72 - Thursday, Jan 07, 2021 at 17:37

Thursday, Jan 07, 2021 at 17:37
I have a 2018 Kluger with a smart alternator and my trips are generally short from day trips to 5 days sometimes with no driving in between arrival and departure. I use solar panels on longer trips. I understand that my current simple system is not ideal and that it won't keep my AGM topped up as it is not connected to the alternator. I aim to utilise as little of the AGM's capacity as possible and prioritise using the starter battery while the engine is running. I believe a VSR is not suitable as my vehicle has a smart alternator. I would prefer to go for a good DC-DC charger. I don't want to regret my choice either because I spent too much unnecessarily or because I spent too little when a RedArc is worth the extra investment. So thanks for the advice to lean towards quality. That's usually my preference.
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Follow Up By: Athol W1 - Friday, Jan 08, 2021 at 09:38

Friday, Jan 08, 2021 at 09:38
Matty.
I agree with Triton Man, and will add that Toyota typically fit Low Voltage alternators (different to Smart alternators), and as such will generally accept the VSR system, however a Low Voltage system NEVER fully charges either battery, and that applies more so to the auxiliary battery due to the losses in the cables.

For best results a DC-DC system, such as the Redarc BCDC1225d, is the only way to reliably fully charge the auxiliary battery, whilst charging the starter battery as per the vehicle manufacturers specs and preserving the ability to reliably start the vehicles engine.

Regards
Athol
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Follow Up By: Matty72 - Friday, Jan 08, 2021 at 11:14

Friday, Jan 08, 2021 at 11:14
Athol - Thanks for clarifying that. RedArc have a neat product selector on their website which states that post 2009 Toyota Klugers have a temperature compensating alternator which can drop the output voltage to as low as 13.2V as the temperature increases. I took this to mean "smart alternator".
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Follow Up By: Athol W1 - Saturday, Jan 09, 2021 at 08:46

Saturday, Jan 09, 2021 at 08:46
Matty
The difference between Low voltage and Smart alternators is that the low voltage just reduces the maximum output from the normally accepted 13.8/14.2 down to around 13.2 when certain conditions are met (generally temperature).

Smart alternators constantly change the output voltage depending on LOAD conditions. In the case of the Isuzu this can vary from as little as 12.1v when throttle opened (under drive load) to as high as 15.5v when braking or descending a hill, and when driving even on level road the voltage will be closer to the lower end of the scale. This is done in the name of fuel economy, reducing engine load. It is for this reason that the average VSR will not work with smart alternators.

Vehicles fitted with smart alternators generally disable the 'smart' function when the headlamps (or park lamps) are in operation, as the low voltage has a substantial effect on the light output.

Hope this helps.
Athol
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Reply By: Member -Pinko (NSW) - Thursday, Jan 07, 2021 at 17:25

Thursday, Jan 07, 2021 at 17:25
Matty 72
If you are going down the DCDC path which is probably the easiest way to go including starter battery isolation here is another DCDC device to consider.
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiljZ2Dl4nuAhVkxzgGHeC-Ao4QFjAAegQIBRAC&url=http%3A%2F%2Fwww.intervolt.com%2Fproduct%2Fin-vehicle-dc-dc-battery-charger%2F&usg=AOvVaw2kEmO65HXtFgSUlTGKzBeg
Living is a journey,it depends on where you go !
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Reply By: HKB Electronics - Thursday, Jan 07, 2021 at 18:49

Thursday, Jan 07, 2021 at 18:49
Kluger probably has a temperature compensated alternator but output will be on the low side so a DCDC is the way to go with that model.

If your going the path of a DC DC then you may as well get one that will look after the battery fully, for example the Redarc units don't have a proper temperature compensation function, some other brands don't either so check if you want this feature. A unit with a battery temperature sensor would be a good idea. If mounting under bonnet make sure the unit can handle the temperatures without derating. If it requires to be waterproof then that will cut your options too.

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Follow Up By: Matty72 - Thursday, Jan 07, 2021 at 19:01

Thursday, Jan 07, 2021 at 19:01
Is the temperature compensation function only required if the battery and DC-DC charger are housed in the engine bay? Is this necessary if they are housed in the rear cabin where the temperature is always modest?
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Follow Up By: HKB Electronics - Friday, Jan 08, 2021 at 10:46

Friday, Jan 08, 2021 at 10:46
The temperature compensation is to look after the battery no matter where it is fitted, charger should be located as close as possible to the battery, if charger is mounted away from the battery then a temperature sensor should be attached to the battery, a good quailty charger will have provision for a remote temperature sensor.

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Reply By: qldcamper - Thursday, Jan 07, 2021 at 19:03

Thursday, Jan 07, 2021 at 19:03
There is no such thing as a simple dual battery system on forums.
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Follow Up By: Member - silkwood - Friday, Jan 08, 2021 at 11:34

Friday, Jan 08, 2021 at 11:34
Not true... there is no such thing as a simple dual battery set-up DISCUSSION on forums (I refrain from using the term argument, you notice??)

Cheers,

Mark
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Follow Up By: Allan B (Sunshine Coast) - Friday, Jan 08, 2021 at 13:16

Friday, Jan 08, 2021 at 13:16
.
And you can't make a statement on this forum without someone challenging it! lol
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Follow Up By: Member - David M (SA) - Friday, Jan 08, 2021 at 16:10

Friday, Jan 08, 2021 at 16:10
"And you can't make a statement on this forum without someone challenging it"
Yes you can !
Dave.
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Follow Up By: Allan B (Sunshine Coast) - Friday, Jan 08, 2021 at 16:18

Friday, Jan 08, 2021 at 16:18
.
See what I mean?
Cheers
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 17:50

Friday, Jan 08, 2021 at 17:50
EXACTLY Allan, I have done the best I can with what I know, time to bail out now but Matty72, I will monitor on to hear how you finish up. Pleased I could contribute a bit.

My current set-up is a carbon copy from my last Triton (2007 ML GLX-R) to my current Triton (2017 MQ GLX+ dual cab. BTW, my Triton retains the constant voltage (non smart alternator). My old system worked perfectly so I reckoned, if it is not broken, no need to fix it. Stuck with that. P.S. I used to be an Electrical Mechanic years ago, have enjoyed my 4WD set-ups including drawers, shelves, canopys, extra lights + now a small van to tow along so multiple fridges, multiple battery stations and bugger me - it all works!!!

Cheers Matty, Allan and all the contributors to this Forum.
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 17:58

Friday, Jan 08, 2021 at 17:58
You are on the right track Matty but not sure about 'double regulation', i.e a regulated voltage as an input to another regulator like the RedArc. Best bet here is talk to RedArc OR go to a reputable solar or 12 volt shop. This point would be interesting as to whether it will work but with less efficiency as you say.

Triton Man (Denis)
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Follow Up By: Allan B (Sunshine Coast) - Friday, Jan 08, 2021 at 18:20

Friday, Jan 08, 2021 at 18:20
.
The function of a solar regulator is primarily to observe the battery voltage and regulate to bring the battery to the design "fully charged state". If the primary regulator is "looking" at a secondary regulator it does not see the battery voltage, in fact it sees no voltage and does not function correctly, in fact most likely to not pass current at all. Consequently, there should not be two regulators in series.
Refer to Redarc Installation drawing below.
Cheers
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Follow Up By: qldcamper - Saturday, Jan 09, 2021 at 18:14

Saturday, Jan 09, 2021 at 18:14
38 posts and still going, glad he didnt ask for a complex system with the ultimate efficiency.
Just once I would like to see that horse drink when you lead it to water.
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Reply By: Member - McLaren3030 - Friday, Jan 08, 2021 at 08:14

Friday, Jan 08, 2021 at 08:14
Hi Matty72,

My set up is that I have a Redarc DC/DC in the rear of my wagon that charges my aux. battery also in the rear of my wagon, the aux. battery then powers my 60Ltr. Engel fridge, and various 12 volt sockets also in the rear of the wagon. These sockets are used for various 12 volt items such as a Travel Buddy Oven, Makita Battery Charger for charging 18 volt batteries.

The Redarc is supplied from either the alternator when the engine is running, or via Solar through an Anderson plug. I did not want the aux. battery and the cranking batteries to be connected together, as I wanted to protect the cranking batteries from being drawn down by 12 volt appliances when the engine was not running. I explained what I wanted to my Auto Elec. and he worked his magic to achieve this.

Macca.
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 09:31

Friday, Jan 08, 2021 at 09:31
Spot on Macca, this was my advice also (see above) but meant to go back in a reply to Matty72, to say that the redarc accepts the solar also and if it was a vehicle mounted one (i.e. roof panels), you can leave them permanently connected as the redarc can handle both sources of input simultantaneously AND will prioritise solar first (free from the sun) before the alternator (an engine load). BTW, the built in (attached) regulator needs to be removed from your solar panels as the redarc regulates the higher voltage you will receive from solar panels. Even if you have fold-out panels, by-pass or eliminate the regulator on your solar (if it comes with one) and just plug it in. Another thing to consider is that Redarc is an Australian Company, (like GME is also) and you will be doing yourself as well as all of us, a favour. No problems with warranty, repairs, spares, technical advice or recalls (I had a re-call on my Tow-Pro Brake controller) . Have a look at their website, it is chock-a-block with some great stuff.

Matty72, I have a feeling you are going DC DC, good call, sleep well, travel safe. Triton Man
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Follow Up By: Matty72 - Friday, Jan 08, 2021 at 10:54

Friday, Jan 08, 2021 at 10:54
Macca - I might be confused about the wiring of the Anderson plug in the rear of my wagon which I had specially installed by an auto electrician. I assumed that 12V sockets in a vehicle connect to the starter battery which receives its charge from the alternator. Can 12V vehicle sockets connect directly to the alternator OR to the starter battery? When my fridge is connected to the vehicle's Anderson socket and is receiving power and I turn the vehicle's ignition off, the fridge continues to power for a minute or two before it stops receiving power and turns off. What does this suggest about the way the Anderson socket has been wired?

Triton Man - Yes I am leaning towards getting a RedArc DC-DC charger. The solar prioritisation feature sounds great. I have portable fold-out solar panels but I think the regulators are hard-wired into the units so there is no way of bypassing them. So when onsite, I would need to ensure they are charging the AGM directly and not via the RedArc DC-DC charger?
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 11:48

Friday, Jan 08, 2021 at 11:48
Hi Matty72, everyone on this forum is well intentioned and there is more than one way to skin a cat. Since you are leaning the correct way (RedArc DC DC), it is probably time you gave RedArc's technical people a call. They are professional and only too willing to help, there are also wiring diagrams for you to follow on their website. From my knowledge and experience, folding panels have wire connections from panels to panels (daisy chain) and the last wire(s) normally leads to a regulator (box) sometimes screwed or riveted to the frame (not ideal) and then from the box, leads going out to connect to your battery (usually with an Anderson plug. What is required is to go straight from the daisy chained panels direct to your anderson plug. Cut or disconnect the wires to re-direct them by not going into the box. Now on your new installation, this will be how it will happen with the RedArc being the regulator and close to the battery where it should be.
Always connect to the battery, Alternator connects to the battery to feed in, other wires feed out. The battery is the storage box to hold the volts and current required (electricity).

I will leave it to Macca to explain more as he has a similar set-up, suffice to stay if your fridge continues to run after a "minute or two", it is then NOT connected to an ignition circuit but it could be stopping via a too high voltage setting cutout on your fridge being set on the highest setting (most / some fridges have 3 levels of cut-out to save your battery), OR your batter(ies) may not be fully charged and therefore with the safety cut-out on the fridge, the compressor wont run. Check to see if there is any device that will work in your rear Anderson plug or use a volt meter if you have one. Good luck.
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Follow Up By: Matty72 - Friday, Jan 08, 2021 at 12:25

Friday, Jan 08, 2021 at 12:25
Triton Man - My panels each have their own built-in regulator cabled so that they unify into a single pair of lugs which connects to my AGM battery (bank). So the two panels aren't directly chained to each other. There would be quite a bit of rewiring required to provide the option to use the built-in regulator or a separate regulator. What would be the consequence of routing charge via both the panels' built-in regulator and also the DC-DC charger's regulator?

I don't think the fridge is causing the disconnect. I have connected a volt / watt meter to the Anderson plug at the rear of the vehicle and it too cuts off a minute or two after the ignition is turned off so it appears to have something to do with the connection to the battery rather than the fridge. Not sure whether the auto electrician has installed some kind of disconnect device but I think I'll contact him to explain and setup a RedArc DC-DC charger for me.
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 13:36

Friday, Jan 08, 2021 at 13:36
Oh I see !! - thats a game changer. I knew house panels for domestic solar can have individual Regulators in it but this is the first ?I have head of these 12V (nominal) integrated panels being on the market. Best I bail out now. My soar panels 3 x 40 Watt are about 9 years old but still work and it had a regulator mounted on one of the fold legs. I drilled out the rivets and removed the regulator and went direct. I would say your auto electrician has put in a VSR as your cranking battery will stay high > 12.8 volts for a few minutes before it disconnects. They generally connect at 13.2 and disconnect at 12.80 and this could cause the delay. It would appear your Anderson plug is not connected to your auxillary battery at all, or a fuse is blown as a VSR either connects batteries in parralel (to charge) or seperates to isolate as two distinct batteries. Your fridge should run off your auxilliary battery when you are stopped, or both batteries when you are motoring. Best get you get that BCDC charger as soon as you can. Ring those techs at RedArc and see what they say about those solar panels with the integrated regulators Matty, you have two problems now.
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Follow Up By: Matty72 - Friday, Jan 08, 2021 at 13:56

Friday, Jan 08, 2021 at 13:56
Triton Man - Your theory makes sense because it looks like the Anderson connection provides power only while the ignition is on and for a short time only after it is switched off. The auto electrician is on leave but I looked up the invoice and it looks like he installed an ignition relay. Not sure whether that's the same thing as a VSR? I'll find out and get him to install a RedArc DC-DC charger next week. The solar panel issue is lower priority as I can just disconnect from the DC-DC charger when onsite. I can always reconnect to the DC-DC charger and run the engine if top up charge is needed when there is low solar output. I have never connected the AGM and starter batteries together. Until now I have run them separately and manually swap cables when the ignition is switched off or back on again.
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 15:22

Friday, Jan 08, 2021 at 15:22
Yes Matty, the solar panel set-up is not a priority, just a messy work-around. It is a bummer to do it the manual way but oh-well, thats what you've got. Having a higher unregulated raw voltage going into a DC-DC charger close to the aux. battery is much more efficient than a regulated lower voltage going direct into your aux. battery.

I am not full-bottle (or even half-bottle) on Ignition relays vs. VSR's so best wait till your auto-electrician gets back on deck and install the BCDC charger which will
1) Eliminate your fridge turning off when you stop
2) Provide a good higher voltage and better charging for your aux battery
3) Join and seperate your batteries as what a VSR and ignition relay would do and
4) Should you ever change or upgrade / downgrade your solar panels down the line, get the unregulated type and run those through your charger as well.

You WON'T need to discard your Anderson plugs, they are the plugs of choice and most things can connect that way. If you need lots more, go to Connector-Tech ALS in BNE (ctals.com.au), best price by far and a genuine brand "Anderson", not a knockoff. As Allan mentioned, connections to your DC-DC charger terminals can be turned into Anderson Plugs, BTW, there are several colours available (black, grey, Blue, Red and Yellow), not just the gray we see everywhere so you can colour code them if you wish.
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Follow Up By: Matty72 - Friday, Jan 08, 2021 at 16:46

Friday, Jan 08, 2021 at 16:46
Triton man - It's good to know that the BCDC charger doubles up as a solar regulator. Maybe I'll look at modifying the panel wiring to give the option of built-in or BCDC charger regulation. So there is no problem having the solar charge current "double regulated" other than loss of the charge efficiency that would otherwise be gained by using the BCDC charger as the sole regulator?
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Follow Up By: Member - McLaren3030 - Saturday, Jan 09, 2021 at 08:43

Saturday, Jan 09, 2021 at 08:43
Hi Matty,

The 12 volt sockets I was referring to are in addition to the OEM sockets. See photos attached.



Since the photos were taken, I have replaced to two AGM batteries (total of 150 AmpHr) with a single 100 AmpHr LiFePo4 battery. Located in the same position. The Redarc DCDC is in the left hand wing of the drawer system.

Macca.
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Reply By: Allan B (Sunshine Coast) - Friday, Jan 08, 2021 at 11:42

Friday, Jan 08, 2021 at 11:42
.
Matty, for your needs, I endorse fitting a dc-dc charger in the rear, close to the auxiliary battery, with the fridge connected to that battery. The fridge will then be fed from the alternator when the engine is running and from the aux battery when the engine is stopped. At the same time, the charger will charge the aux battery when the engine is running. If you choose a charger with the feature of solar input a solar panel can supply power to charge the battery and the fridge when stationary.


The Redarc BCDC1225D would satisfy these needs. This charger can be found on eBay for $494 complete with the 40A Midi fuse kit that should be used.

Temperature compensation is not necessary for a battery located away from the engine bay and charged at a limit of 25Amps.

Your existing cabling and Anderson plugs could still be used by fitting Anderson plugs to the charger.

Be aware that this system will perform automatically but the time that your fridge will be supported is still dependant on battery charge-time from driving or solar input. A solar panel of 120W to 200W would probably be appropriate for average needs.
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Follow Up By: Member - Triton man - Friday, Jan 08, 2021 at 11:56

Friday, Jan 08, 2021 at 11:56
Hi Allan and Matty72,

Allan is 100% on this, and $494 for a quality system is well spent. Won't go over old ground here, looks like we all go for the DC DC charger with the model Allan (and others) suggests being your best option.

A big "Hi" to Allan here.

Triton Man
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Follow Up By: Allan B (Sunshine Coast) - Friday, Jan 08, 2021 at 12:14

Friday, Jan 08, 2021 at 12:14
I should add that the Redarc BCDC incorporates solar regulation so do not use an external regulator on the solar panel. Connect the panel directly to the BCDC solar input.
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Reply By: Steve in Kakadu - Friday, Jan 08, 2021 at 12:41

Friday, Jan 08, 2021 at 12:41
My set up in the 200 series is.

A Protecta isolator switch under the bonnet next to the car battery, the a 10 mm cable through to the rear draw system and connected to a DC DC charger from Kings and then connected to the 105 AH AGM. The rear battery is earthed to the car body at one of the removed rear seat bolt holes, I also have a 160 watt solar panel on the roof rack connected to the DC DC charge and an Anderson plug I can plug my other stand alone 120 watt solar panel.

This runs 2 x 38 ltr Engels, a 1500 watt inverter and a few other 12 volt items you take on your travels, I haven't worked out how long I could park up without giving the car a good run but I think it should be good for a while. I've had the system running for about 18 months now with no issue.

Regards
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Reply By: Iza B - Friday, Jan 08, 2021 at 19:50

Friday, Jan 08, 2021 at 19:50
I have used a Powertech brand dual input from Jaycar for nearly two years now. I use it to charge both an AGM and a DIY LiFePo4 battery; not at the same time though. Third the price of a RedArc and a lot cheaper than the D250S I used before I went down the LiFePO4 route. Hard (Fused) to a smart Alternator and flying XT90 plugs to whatever Solar array I am using at the time. The setup is the simplest arrangement I can think of.

Iza
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Reply By: Batt's - Saturday, Jan 09, 2021 at 01:11

Saturday, Jan 09, 2021 at 01:11
I would just fit the Powertech from Jaycar with some 8 gauge wire it will do what you need, recharge the agm while driving and isolate it from the starter when the engine is off. I fitted the same charger to my wife's 85ah agm a couple of months ago it works well. It can also recharge a lithium battery if you get one later on.

It has solar imput so that can be wired up with an anderson plug ready to use with a solar panel for extended trips.
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Reply By: Frank P (NSW) - Thursday, Jan 14, 2021 at 00:27

Thursday, Jan 14, 2021 at 00:27
Yet another way to skin the cat ...

I use a VSR in the engine bay to switch on a 6AWG supply (an electrical bus) to the ute's canopy where all the "bits" are located, including the second battery. You could use a simple ignition controlled isolator if you wish.) The bus also supplies the Anderson on the drawbar to service the Karavan when it's there. The Karavan has its own, independent charging system.

A DC-DC charger in the canopy is powered from the bus to charge the ute's second battery.

I use a relay to switch the fridge supply. When the bus is powered by the VSR, the fridge runs off the crank battery/alternator. When the bus is unpowered, the fridge runs off the second battery. I did that because I was advised that it's better for the DC-DC charger to charge an unloaded battery. I now think that's a bit theoretical, so you could simplify things by leaving the relay out and running the fridge full time from the second battery.

I prefer not to have all my eggs in one basket, so my DC-DC charger has no solar input. Therefore I have a separate solar reg permanently connected to the second battery. When camped up I plug my panels into that.

I've had this system on two vehicles over 12 years. It works flawlessly.
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Follow Up By: qldcamper - Thursday, Jan 14, 2021 at 11:43

Thursday, Jan 14, 2021 at 11:43
Similar system to mine Frank, only I use a delay timer to prevent the fridge dropping out to low voltage while the engine is cranking.
I used the C/O relay to take thw load of the accessory system off of the DC/DC unit so 100% of its capacity is devoted to recharging my batteries.
However the OP asked for examples of the simplest system which Alan posted. Not the most efficient but definatly the simplest.
Its nice to have an efficient system but if you are not electrically inclined it becomes an expensive nightmare if something goes wrong.
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Follow Up By: Member - Triton man - Thursday, Jan 14, 2021 at 11:55

Thursday, Jan 14, 2021 at 11:55
Hi Frank P, another interesting scenario with swapping the fridge power supply via the relay. Could be a bit 'theoretical' as you say. A VSR will not switch on immediately as we know so there will be ~15 second delay till volts come through. As all things (load) is connected to the battery, the battery acts (and is a storage device) so there is no cause for alarm with 'the fridge dropping out to low voltage' as qldcamper suggests.

Read your earlier 'blog' you put up many years ago - good stuff.

My money is on Allan B's setup as qldcamper admits, use the "KISS" principle
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Follow Up By: qldcamper - Thursday, Jan 14, 2021 at 13:16

Thursday, Jan 14, 2021 at 13:16
Just shows how easy it is to mis understand a statement.
My comment on using a timer was simply implying i used a different method of preventing the same problem, i was not saying thats Franks use of a VSR was leaving his system open to failure.
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Follow Up By: Frank P (NSW) - Thursday, Jan 14, 2021 at 21:16

Thursday, Jan 14, 2021 at 21:16
Hi Triton Man and qldcamper,

Just to clarify, the fridge won't be subject to a voltage sag during cranking because at that time it will be running off the second battery. By the time the VSR lets the volts come through everything is stabilised in the vehicle's charging system and then fridge changes over to the alternator/crank battery. The DC-DC charger has a soft start habit so gradually brings itself on line once the power is supplied via the VSR.

Qldcamper, I have the same preference as you - to let the full output of the DC-DC charger go to the target battery. There has been some discussion about the need for that here and elsewhere. I'm not sure it is as important as I thought it was when I built the system, but it remains my preference.

Triton Man, thanks for that re the blog. That was in my Prado. I've basically done the same thing in my BT50, but simplified it with the same result. I have a schematic which I can post here if you're interested. But it's not the simple system the OP was asking for.

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Reply By: Allan B (Sunshine Coast) - Thursday, Jan 14, 2021 at 13:51

Thursday, Jan 14, 2021 at 13:51
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My reply above to Matty was for a minimal change to his existing system. But, my preferred basic system is as per the diagram below. In this, the aux battery is charged by a dc-dc charger which overcomes line volt-drop and eliminates the need for an isolation solenoid or relay. However, the inclusion of an 100A 'isolation' relay in the circuit performs to totally isolate the feed to the trailer when the engine stops. In turn, the 30A relay in the trailer now de-energises so that the compressor fridge feed transfers from the alternator to the aux battery.
The ignition-activated relay has an additional benefit for those drivers who are concerned about possible tampering with a 'live' Anderson plug when the vehicle is unattended.

If a 3-way fridge is being used then dispense with the 30A relay and connect the fridge 12v directly to the line feeding the input to the dc-dc charger. In this way the fridge draws 12v power only from the alternator and cannot discharge the aux battery.


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Follow Up By: qldcamper - Thursday, Jan 14, 2021 at 14:15

Thursday, Jan 14, 2021 at 14:15
Similar to my original set up which worked well till the start battery aged a little and weather cooled. The fridge randomly (it seemed at first) would be found in fault with the lo voltage code flashing.

Eventually after tripple checking everything I realised that changing the fridge to the crank battery with ignition it would dip below the preset lo voltage setting of the fridge momentarily during cranking so if the compressor was running during a cold morning engine start the fridge would fault.

Hence the reason i use a 30 second delay on the relay and as Frank has used a VSR to avoid the same set of circumstances.
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Follow Up By: Allan B (Sunshine Coast) - Thursday, Jan 14, 2021 at 15:54

Thursday, Jan 14, 2021 at 15:54
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There is a crafty way of overcoming such problem without adding further devices. Simply connect one side of the isolator relay coil to an 'ignition' source and the other side to the starter motor activation circuit. During cranking both these connections will be 12v positive and the isolation relay does not energise. Upon release of the cranking function, the relay finds an 'earth return' via the starter and activates.

Yes, I know it is a bit 'Joseph Lucas' and may confuse an auto electrician, but I grew up with 'relay logic' and it will work.
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Follow Up By: qldcamper - Thursday, Jan 14, 2021 at 16:21

Thursday, Jan 14, 2021 at 16:21
Or just supply the relay with an accessory feed that drops out by itself when the key is turned to crank, I know it might seem a bit confusing to an old sparky that thinks he is the smartest guy ever,but all cars have them. I chose not to because i wasnt sure if the fridge would take kindly to so much switching back and forth.
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Follow Up By: Allan B (Sunshine Coast) - Thursday, Jan 14, 2021 at 16:28

Thursday, Jan 14, 2021 at 16:28
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Yes, I'm aware of the accessory feed de-energise during cranking*, but the risk with that is that the relay circuit will be energised when the key is in 'accessory' mode and thus placing a constant drain on the cranking battery. That may not be what you wish to have happen!

* Not a function of all vehicles.
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Follow Up By: qldcamper - Thursday, Jan 14, 2021 at 17:31

Thursday, Jan 14, 2021 at 17:31
Well the simplest solution would be to replace the ign relay in your design with a VSR as Frank has done.
But I know how hard it is for some people to admit they are not perfect at everything and can not tollerate being corrected.
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Follow Up By: Allan B (Sunshine Coast) - Thursday, Jan 14, 2021 at 21:03

Thursday, Jan 14, 2021 at 21:03
.

"Tolerate" is perfect with only one "l".


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Follow Up By: Gramps - Thursday, Jan 14, 2021 at 21:10

Thursday, Jan 14, 2021 at 21:10
Don't you just love 'electrotrickery' threads LOL

Regards
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Follow Up By: Allan B (Sunshine Coast) - Thursday, Jan 14, 2021 at 21:51

Thursday, Jan 14, 2021 at 21:51
.
Even better than tyre threads Gramps. lol
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Follow Up By: Frank P (NSW) - Thursday, Jan 14, 2021 at 21:56

Thursday, Jan 14, 2021 at 21:56
"However, the inclusion of an 100A 'isolation' relay in the circuit performs to totally isolate the feed to the trailer when the engine stops."

That is another reason for my VSR. It is not a conventional isolator. It is a dumb 300 amp solenoid driven by a small, adjustable Redarc VSR, which makes it a high capacity VSR. Like Allan, I did not want a live Anderson at the back of my unattended vehicle.

All my accessories are powered off the cold side of that big VSR - driving lights, transmission cooler fan, Karavan charging system, second battery DC-DC charger, daytime running lights, etc. It serves as a readily accessible, high capacity IGN source so If I forget to switch anything off before leaving the vehicle it will get switched off anyway when the crank battery voltage drops to 13.2 or whatever I've set it to.

It's 300 amp because in my setup I can parallel the aux and crank batteries via that solenoid for winching or starting in the event of a flat crank battery (not uncommon in Rangers and BT50s until owners became aware of the vagaries of the electrical system)

Drifting further away from the OP's request for a simple system :-(.

Sorry.
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Follow Up By: Allan B (Sunshine Coast) - Friday, Jan 15, 2021 at 10:30

Friday, Jan 15, 2021 at 10:30
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Sounds good Frank, but in my (under construction) MB Sprinter motorhome I have dispensed with the ignition-controlled changeover relay for the sake of simplicity. My only auxiliary load during motoring is 2A average for the fridge and I figure that it is such little extra for the 40A dc-dc charger that is was not worth worrying about.

I thought about parallel/jump-starting but figured that the need was so unlikely that either a set of jumper leads or a portable lithium jump starter would suffice. Useful to help others too!

Incidentally and very off-topic, have you visited Redarc website lately? They have totally revised it to be very posh and commercial. You need to wade through pages of mindless promotion and I gave up trying to find the specs for a particular dc-dc charger. Never mind, I have moved to Enerdrive anyway!
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