TIP # 131 New “KISS Engineering” Charging System for Tow Vehicle Fridge Battery

Submitted: Wednesday, Nov 15, 2006 at 22:34
ThreadID: 123065 Views:4981 Replies:8 FollowUps:10
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Well, I am up against it and have reviewed all the choices myself, regarding charging a third battery in the tow vehicle to run fridges and such for my new F-350. Obviously the first choice is to supplement with solar for when you are parked up, but what I am dealing with here is how to charge off the truck alternator automatically whenever you drive it, and I am reporting my new invention.

While most of my Tips are exclusive to Bushtracker Owners, and kept on the Owners Forum, this is an item of general interest. If there are any Electrical Engineering sorts out there that want to pick it apart, feel free to jump in and stomp all over it, but I do think it works in the following simplified way.

I will digress for a moment to review the old and new options, particularly for you Newbies that do not have the history and experience: I have used an ARB dual battery system since the 1980’s. I personally like the simple Solenoid system to run the batteries. You can make it up yourself, or have any Auto Electrician do it... It is basic non exotic KISS Engineering for the basic Landcruiser dual battery system, just a Ford Starter Solenoid, with both Positive leads of the Batteries joined to it.... Ignition on and it mates them together with an energized coil sucking in the electrical contact, ignition off and the batteries are separated as the spring loaded contact separates. It works this way so you cannot flatten both as only one battery is on the starting system while the other second battery is used for a fridge or something... This used to cost about $130 from ARB plus installation. Now there is a problem with this: In reality, you are charging batteries unevenly, as it reads the second battery and the first battery as one average voltage, so it slightly overcharges the first battery as it struggles to bring up the second battery that is running the fridge. Yes it would shorten battery life. And the same would apply to a third battery running hooked up to the first or second main battery. HOWEVER… Will it shorten battery life enough to pay for the more exotic systems? This question is somewhat debatable. I personally like this old simple system, as I wonder if the $500 and up, more exotic systems, really last long enough in the Bulldust and heat of the engine compartment to pay for themselves in extended battery life...

Look, there are also a variety of popular Electronic Units that do a better job of this, with varying degrees of charge control, like Pirahna, ARB Smart Solenoid, and others... The way they work is: As well as separating the batteries with ignition off, they also direct the charge to the lowest battery first and then bring them both up together. The Pirahna I have had myself on a Ford, and yes they do work well, but with these exotic units up in the $500 range; it gets quite expensive to run these with a third battery added in on board. Again I wonder if they are going to last long enough to pay for themselves with extended battery life?

So now the invention I dreamed up overnight. Why leave the imbalance of one of the two main batteries hooked to the third battery in the back. Yes the voltage regulator will read them as one bank, as one average voltage potential: so to charge up them all together the first battery would be overcharged a bit, while the other two were being brought up to full potential as it would read all three as an average. This imbalance of charge distribution is the problem that can shorten the battery life. Now, in Electrical Engineering Theory, this imbalance situation should not be true with big enough cables: If they all are in parallel they should be receiving charge equally. In practice this does not work with large battery banks, each down the line receives a slightly lower potential charge. That coupled with the fact that the two batteries powering the fridge would be lower in charge when fist fired up, while the one remaining battery would be higher in charge; which is the MAIN problem as it reads them all as one average voltage. As the alternator brings up the two lower batteries, it will be slightly overcharging the one solo battery that did not need as much, as it reads them all as on average potential and charges accordingly as one unit… This is less of a problem, with the Ford that runs both main batteries in parallel at the same time, and more of a problem with the Landcruiser type system where they are disconnected from each other when the vehicle is turned off, and the fridge runs just on one of the two main batteries. My solution is quite simple: Run a charging circuit from EACH of the main batteries, back to the third battery independently!! (Note: Do it correctly with Fuses or Circuit Breakers on each end, or like I did with automatic circuit breakers on each end of each charging line, so any short circuit along the length and you are protected from the batteries on both ends from an electrical fire.)

My KISS Engineering (Keep it Simple, Stupid) is to circumvent both problems: Run a parallel line from EACH of the two main batteries to the third battery. They should all carry the load evenly, and both sides charge evenly. The only caveat is to assure the fridge low cut out voltage is still high enough to assure that your engine will still start. If this is true, then you have an inexpensive fool proof system to charge the third battery, with little or no disadvantages, and without the big ticket electronic toy on board. If you drive long enough, all three will “Equalize” just like the individual cells of a much larger battery, which in effect the three of them would electrically act like; as if they were one big bank. Even if the cables you run back are not large (mine are not) the higher potential batteries will continue to charge the lower potential batteries even when the vehicle is turned off. It works.. I am only running a twin 6mm from each battery to the one battery in the back. Not driving much the past three days, this morning my third battery was sitting a 12.38, in 15 minutes of getting to work, it was up in the 13.9 area. I think this system works well...

Yes, you can do it with a marine “Make Before Break” switch, but then you have to pay attention to it and manually divert the charge. And if the switch gets a bit of dust or something and you break the contact with the engine running, even for 1/100th of second, you can blow the diodes in your alternator.

The simple idea, is a robust solution. Test to see if your fridge cuts out before the batteries are run down too far to start the engine. But this is a simple system, that will work really well with solar addition, of a panel on the truck, as even with a malfunction in a few hours of daylight you would have enough charge to start the engine anyway. I dunno, I like it, I am going to do it on mine, KISS Engineering wins again…

P.S. Having done it and monitoring results on mine, I'd say it works a treat!! It looks like it will be good for two or three days of bad weather, and if I do not supplement with solar after that long I will go for a drive, plug into solar, plug into the van, or unplug the fridge! It looks to work well.

Regards to all, the lone Ranger is on duty 24/7, scouting for the Bushtracker Wagon Train….
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Reply By: Noosa Fox - Wednesday, Nov 15, 2006 at 23:47

Wednesday, Nov 15, 2006 at 23:47
In our Ford we have 2 starting batteries and I have an auxilary battery set up in the tub to run the fridge etc off. Our Engel fridge is a few years old and does NOT have a low voltage cutout.
As both Ford batteries are permanently joined together by battery cables of about 8mm diameter there is not going to be much of a voltage difference between the 2 batteries. 8mm diameter cable should be about 50 sq mm cable.
I then used a solenoid switched through the ignition and joined the auxilary battery to one of the starter batteries, again using heavy 8mm diameter (50 sq mm) battery cables, and the voltage reads the same at all 3 batteries when tested.

From the Auxilary battery I then ran 4mm dia cables (approx 12 sq mm) to an Anderson plug on rear of vehicle and on to the caravan batteries.

When the vehicle is parked I have 2 portable 80 watt solar panels that can be plugged into the Anderson plug to charge the auxilary battery and or the caravan batteries. This system works very well and the auxilary deap cycle battery is now over 5 years old.

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Follow Up By: Noosa Fox - Thursday, Nov 16, 2006 at 00:16

Thursday, Nov 16, 2006 at 00:16
The reason I have the portable solar panels is so that I can have ample power when bush camping with the vehicle without the Bushtracker in tow.

When camping for extended periods in Bushtracker the extra portable solar panels can be placed in the open while the van's solar panels are being sheltered by a tree from the hot afternoon sun.

Works for me.
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Follow Up By: Burt & Mary - Saturday, Nov 25, 2006 at 02:51

Saturday, Nov 25, 2006 at 02:51
Steve Lone aRanger hope others don't take what you write as the right or proper way of doing things electrically.
Strongly suggest owners get proper and correct advice on electrical matters.
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Reply By: Bushtracker - Thursday, Nov 16, 2006 at 00:08

Thursday, Nov 16, 2006 at 00:08
A bit of OverKill does not hurt, other than your pocket book. This method is more than most would need. If you want to throw a couple thousand at it, there are a number of ways to do it.. The newer fridges and gear are also self sufficient with less solar than this. My last one, an 80 litre Trailblaze on the F-250 ran with surplus power with 120 watts of solar on the canopy. On my Landcruiser it limped along OK with 60 watts on a rack on the roof..

The point of my Tip #131 may make it so I might not even bother with much solar input, and the cost is waaay down to near nil. Most people with the modern set up we use have surplus power with four panels on the van, you may not need to carry extra solar, 98% do not..... A lead from the van to the vehicle, charges the vehicle when you are plugged into 240v with the vans battery charger, or in the case when you have excess solar on the van. Both systems will charge the tow vehicle through an Anderson Plug.

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Follow Up By: Noosa Fox - Friday, Nov 17, 2006 at 03:17

Friday, Nov 17, 2006 at 03:17
You commented that my wires were an overkill and you said "I am only running a twin 6mm from each battery to the one battery in the back."

Could you please tell us if the 6mm wires that you use are 6 sq mm, 6 mm dia or what? It is my understanding that the standard way to measure electrical cables is in sq mm, but many people are often sold something enirely different by Auto shops when they ask for 6mm wire. Often what they get as 6mm wire is the outside diameter of wire and insulation that will fit through a 6 mm dia hole.

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Reply By: Turist - Thursday, Nov 16, 2006 at 01:09

Thursday, Nov 16, 2006 at 01:09
Engel fridges do not have a low voltage cut off, early models or current models.
They will run on very low voltage and pull a battery right down if not monitored.

Engel do however have as a seperate item a low voltage cut out that will save your batteries.

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Follow Up By: Bushtracker - Friday, Nov 17, 2006 at 00:28

Friday, Nov 17, 2006 at 00:28
I have had an LVD made up for such a situation. Any of you Boggers that want can have such a thing made or probably buy it at Radio Shack.. It is just a relay controller, that will cut out the power on the relay, that holds the contact in; in the line that goes to power the fridge. Hits a certain voltage, cuts the power that holds in the relay, and no juice to the fridge. You just use a N>O relay (Normally Open) and most relays you can hook up either way anyways, N.O. or N.C.

Pretty simple stuff really.. I would be surprised if you could not buy it off the shelf at Tandy, or Dick Smiths,
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Follow Up By: NIK `N` OFF - Saturday, Nov 18, 2006 at 02:51

Saturday, Nov 18, 2006 at 02:51
Bob / Turist,

Engel did in fact have a low volt cut out on the 60lt at least ours did [10.5v] our 40lt anniversary models do not have a low volt cut out and will just keep dragging a battery further down.

I too have the decision of what to do for 12v acc in the F250. In the Cruiser i removed the factory batteries and fitted larger capacity batteries, a Trojan 115a/hr DC for aux and Overlander 700cc for the starter, i used a REDARC smart relay and I went with the trojan as that was what was fitted to the van and keeping the batteries the same meant the solar panels kept them all charged up when i used a lead from car to van, KISS & everything worked.

But after reading much information on the F250's mainly USA based, i am wary about altering the F250 system from the way it is, i'm thinking along the lines of Steve [Ranger] a low voltage switch and possibly a sealed 3rd battery fitted behind the rear seat of the dual cab all connected in parrallel using large battery cable.

We will have 4 panels 4 batteries in the BT

Interested in hearing all thoughts regards the way people have done this,


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Follow Up By: Bushtracker - Saturday, Nov 18, 2006 at 03:03

Saturday, Nov 18, 2006 at 03:03
Nick, Redarc, Good!

I do this stuff full time... Example: I have a custom made LVD to run on the control wire, and a big Redarc Solenoid sitting right on my desk... But why not use it? I have not found a need for it just yet..

The purpose of this Thread, is that we may be getting too complicated, just to suit the people that are flogging the electronic gear... Read my latest comment on the bottom of this Thread, as the latest is I am going to feed the third battery with a 125 Watt solar panel, bracket sika-flexed down on the roof of the cab, and without all the fancy guff, it will feed back to front on solar, and front to back on Alternator.

With a 125 on the cab, I can't see running low on power at any time regardless... If I am parked up for a long time, I will turn off the fridge. In normal travel the solar and simple system will supplement the van..

K.I.S.S Engineering... Ranger, out there......
I get a new van and new truck every now and then just to re-invent the wheel and look for the current "best way"... I live continuous R&D as a Lifestyle..
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Follow Up By: Bushtracker - Saturday, Nov 18, 2006 at 03:05

Saturday, Nov 18, 2006 at 03:05
Sorry Mick, typo again, I could use the Edit button.... Ha!
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Reply By: TRB60 - Saturday, Nov 18, 2006 at 02:20

Saturday, Nov 18, 2006 at 02:20
Have been using the solenoid system for some 30 years with no problems, batteries last 3/4 years (STANDARD WET CELL) have tried deep cycle batteries, do not last as you need to run the same batteries together.

Regards Terry Bridges

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Follow Up By: Bushtracker - Saturday, Nov 18, 2006 at 02:40

Saturday, Nov 18, 2006 at 02:40
Yes, Terry, agreed and same length of tests...

As I said above: The alternator reads them as a unit, cells in a larger battery, and so to bring up the lowest Potential (voltage) it overcharges the other... Anyway, the latest:

What I am going to do now, is add a 125 watt solar panel on the roof of the cab, and feed the back battery. Because I do not have any fancy electronic charge diverting controller in line, my system will charge in both directions. Alternator flow through to back, solar panel flow through to front.

I still say K.I.S.S. Rules!! Best Regards Ranger "Out there doing it 24/7"....
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Reply By: Maximus - Saturday, Nov 18, 2006 at 04:06

Saturday, Nov 18, 2006 at 04:06
I did not want to change anything with the F250 batteries so purchased a Thumper battery ( sits in the back of the truck with the Engel )which plugs into the van when I want to give it a charge or for a short time straight from the truck whilst driving. I run the Engel from a Hella plug that comes off the Effie batteries, whilst travelling. If I stop for an hour or so I plug the Engel into the Thumper ( has four Hella outlets ). If stopped over night I plug the Engel into the van where I have a couple of Hella outlets. Hope it does not sound complicated but for about $500 3 years ago it works very well.
I can also charge up the Thumper with a charger when on 240V.
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Follow Up By: NIK `N` OFF - Saturday, Nov 18, 2006 at 19:15

Saturday, Nov 18, 2006 at 19:15
To Barrie Hopefully i will get to check out your system Barrie when Stewart is back from Iraq and we can have a mini BT gathering & swap ideas, afraid it will have to be around our backyard pool though as im not gathering in a tent without my BT !

To Brian, I have scanned some info regards wire sizes from Collyn Rivers book 'Guide to solar that really works', i will try and post it up.

Im sure i'm not showing anything new or that Steve [Ranger] doesnt know about, i certainly do not want to appear an expert on these matters because that is far from the truth but i know enough from reading that not using cable large enough to carry the required voltage can cause problems with insulation melting and or the appliance not operating as it should, just understanding the different ways of measuring cable sizes is confusing enough.

BUT i will leave all this to those in the know and place our trust that the BT wiring is well and truly up to and above the standards required.


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Reply By: Maximus - Saturday, Nov 18, 2006 at 23:41

Saturday, Nov 18, 2006 at 23:41

Yes we will have to have a meeting of the local BT clan when Stewart returns. You are in Bunbury I think? We have the BT at the farm in Margaret River.

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Follow Up By: NIK `N` OFF - Sunday, Nov 19, 2006 at 09:40

Sunday, Nov 19, 2006 at 09:40
Hi Barrie,

No Safety Bay is our base. But Margaret River sounds ideal, you supply the wine :-)


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Reply By: Andy1 - Wednesday, Nov 22, 2006 at 15:58

Wednesday, Nov 22, 2006 at 15:58

As you say KISS is the way to go but in reality in buying a BT we have all moved a little off this path. It seems that there are as many solutions to the additional battery circuit as there are BT owners. We have two additional batteries in the back of the F250 which are charged using a Rotronics unit - never a moments problem although it is probably pointless going down this, very expensive, path now with the advent of the smart solonoids which are truely KISS.

Why two batteries? This was a decision made before BT started to use AGM & Gel batteries & we found the ACDelco's a bit on the minimalist side so we have the facility, as do many BT owners, to join the BT batteries with the auxs in the F250 & charge all with the solar system. Also like most F250 owners we have an Engel in the cabin for "on the road" refreshments. As an eight Pepsi Max a day man this is a necessity of life.

Just a side question Steve, what is your latest battery recommendation? Are Gel now standard or do some still upgrade to the AGM?

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Reply By: Bushtracker - Wednesday, Nov 29, 2006 at 04:14

Wednesday, Nov 29, 2006 at 04:14
Well, the final bit is going on now...
I put an Anderson Plug on the back, and that way the Van can charge the third battery as well.. I don't really prefer automatic resetting automotive circuit breakers, as you can drop a link and not even know about it. I replaced all of them with plug in spade fuses which came in last week, on both ends of each line... The system works brilliantly! I hooked up a charger to the Anderson Plug and it charged up the third battery and the two under the bonnet. The Twin 6mm to each battery from the third, is ample, just fine on that distance. Twin 8mm would have been better, a little overkill but better, we just did not have any at the time, so the 6mm will do. The amperage of the 25 amp charger splits through twin 6mm to each battery, it works just fine. The same goes for the alternator charge, which is reduced as the two main batteries come up in charge so it is substantially throtled back to the third. I actually want a little voltage drop as I have one of our sealed batteries in the back right now. The minor voltage drop through two sets of fuses and 6mm drops it just enough so the alternator will not overcharge the sealed one in the back. My alternator is running about 14.8, and the sealed battery will take 14.4 tops. This all works. If I get voltages up too high on a long drive, I will have to change out the battery to an AC Delco type that will take the higher voltages. But that is what I am running now...

And it is protected on both ends at each battery. Boggers, this is important to fuse the power line on each end, and it is hooked up to a battery on each end, and a fray / wear spot dead short needs to be fused at both ends at the hook up point on each battery. My system can feed both ways...

I am also adding in an 85 watt Kyocera Panel, frame sikaflexed down to the roof of the cab, cross ways to the cab.. A 125 did not fit. This feeds down to a Prostar Regulator. The whole thing works really well without the charge diverter. I like it.

Kiss Engineering, and one less thing to go wrong. Ranger
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