It's actually not that different if you look at ambulances for example. It is very different from anything I've seen in these forums. I researched and considered the different pros and cons until I finally found the perfect solution. I don't mean to knock any of the other setups. They each have their pros and cons. As long as you understand the implications, by all means, choose the one right for you. I've summed up all my findings and wrote the following for the beginner but think most anyone will find something useful.
Many, many times, people dealing with batteries for a living have documented that interconnected batteries should be the same model, purchased and installed together, remain interconnected, etc. Doing otherwise can cause conditions where current flows from one to the other. This means extra cycling, extra wear and shortened lifetime. Larger battery voltage differences induce larger currents, causing them to overheat and become dangerous.
Relays, solenoids, contactors, have moving parts. The contacts can arc and weld together. This seems unlikely but it wasn't fun when it happened on my hoist while the load was moving upward. Then there are all the considerations for when to connect and when to isolate, and the controllers to automate this are not cheap.
Most setups have the "starting" battery with the factory circuits, and a deep cycle "auxiliary" battery with the extra stuff like refrigerators. This creates the potential to drain both, leaving the vehicle unable to start. Sounds like a huge pitfall to me.
Antman took another approach. Now all the loads are on one battery, and the other is always ready as a backup starting battery. It's not a backup as in "I hope I never need it" but "I will need it to start the motor after I drain the main battery." I suppose you could throw it to power a winch or something too. I could have used a relay, but got cheap and lazy and simply put a
battery selector switch under the hood. I like the simplicity and the 4 switch positions.
Relays, solenoids, contactors, are basically the same, except for the use of the terms. It usually depends on the amount of current involved. Another option is solid state isolators. The alternator always charges both batteries but they are always isolated, unless you add a switch or relay or jumper cables of course. Unlike a relay, these cause a voltage drop. Diodes have a voltage drop usually as high as 0.5V and the final result is the batteries never reach a full charge. Half a volt doesn't sound bad but it could mean a 50% drop in
state of charge because the two are nowhere near proportional. This means less performance and less lifetime for the battery. There are diodes with less voltage drop like Schottky diodes. They of course cost more. It's not so much diodes themselves, but the assemblies, and the few makers of each. Better still are FET isolators.
Aside from efficiency, some solid state isolators have a separate feedback line, usually identified as B+, in the case of our alternators. This allows the alternator output control to compensate for any loss before it reaches the battery. It is actually required for this type of output control and ignition or the vehicle will not start. I was worried about the LE push button ignition but got the expected results when I ran the line from the isolator, through the passenger side of the fuse box, and onto the terminal where the alternator line used to be. It makes sense. The product does what it is supposed to do. But honestly the theory of operation behind the altenator output control and exactly what how the isolator does it are a little mysterious to me, and I've taken their design for granted. Same goes for the 37.5 airbags with explosive charges aimed at me and my family so whatever.
I used the
Argo FET 200-2 isolator.
Speaking of alternators, compared to older vehicle designs ours has more output, but the electrical system load is even greater, and in that respect it is pretty marginal. This can make it difficult to keep a larger battery charged on short drives. Just something else to consider. You might want a
Battery Minder or
Battery Minder for AGMs on the garage wall. They're kind of nice to have. I saved the battery in my jump box with the desulfation feature. Of course this is just a workaround to
upgrading the alternator. I hardly want to consider dropping that kind of coin as long as this one is working and until other options have been battle tested. I also decided against the linked alternator when selecting the isolator rated for 200A. Isolators don't get much bigger anyways.
I moved the OEM Panasonic battery to the passenger side and used the
Shrockworks kit. It is a lot of money for what it is. But considering what it would take just to acquire the means to design, cut, and fold something that might hold a candle to their product, it is a bargain. It's also a great design with excellent directions. I never thought I would find packaging to be as impressive as it was.
For the new main battery I needed a
deep cycle, or dual purpose, and went with the X2Power
SLI31AGMDPM. It was locally available and it turns out packing and shipping 84 lb of lead and acid aint cheap. All these batteries are rebrands of the same few makers. This is a rebrand of the Swiss owned, American made Northstar battery with a goofy label and seems to be exclusive to Batteries Plus. It should be up there with the Odyssey and now discontinued Diehard Platinum both made by Enersys. These American made AGMs are hands down the best but I won't get into that here. Unfortunately prices have risen, almost doubled, due to the price of lead I'm told for whatever reason.
When it comes to selecting parts, if it is marine rated, it is probably good enough to go under the hood, or better. This usually comes for a price but it's better then getting stranded on a boat. Automotive grade should be minimum for anything you stick in there and I constantly see people miss this.
Believe it or not I found a lot of use of the wrong type of wire, so let's talk about that too. Welding cable is more flexible and cheaper. The insulation is not always oil resistant or rated for under hood temperatures, 105 C. I cut up a long 4 AWG marine battery lead. For other runs I went a little overkill and used this
2/0 AWG. It is rated SAE J1127.
Do buy pure copper wire. Do not buy copper-clad aluminium or all aluminium. As with everything else in this post, I'm highlighting what you need to know. Google can lead you to all the safety and performance reasons.
There's a lot of cheap wire loom out there too. Again, oil and heat require
better stuff from
Del City.
Tinned wire is expensive and I highly doubt Toyota uses it in anything. Do use tinned cable lugs.
The battery negatives are connected with 2/0 along the cross member above the radiator. I did not want to create another ground point or allow all the current to go through something like a single point in the body. The positive side goes back along the firewall. Auto makers would not run something like this across the front of the vehicle should there be a front end collision.
I don't think it is required but I crimped, soldered, and used some
good heatshrink. I went big on diameter because it has a good shrink ratio. The adhesive should seal things up nicely.
While you're at it, throw dielectric grease on all connections.
Pictures! I just got it in and despite all the research, some of the mounting is absolutely embarrassing. I will go back and clean it up. I might borrow
ideas from Keith_PDX. I need to properly mount the isolator as soon as I find or fab a battery box, as well as the switch. Red tape was all I had in the garage.
Main battery
Backup battery
Air pump relocated
Air hose with new coupling
Linear Mode
