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….
