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Twin battery systems


daveturnbull
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The previous owner of my Land Rover installed a twin battery, but without any kind of split charge system. Each battery has it's own cut off switch.

I asked about his thinking behind doing this and was told this:

When we were doing recovery we were always risking a flat battery especially with winching but also with the lights and heaters on all night in a welsh mountain. So by fitting a second battery (battery 2). we could charge it up by switching it into service when traveling to an event and then isolating it by switching it out of service during the event. then if we had any sort of flat battery problem we would switch it in start the car and let both batteries would charge. The alternator is capable of charging both at the same time. BUT there is an important point to remember. NEVER SWITCH BOTH OFF WITH THE ENGINE RUNNING this will blow the alternator.

Ideally the first battery should always be connected and the second regarded as keeping a charged replacement in the car without the hassle of connecting it up via jump leads.
I did consider a split charger but if you try and start the car with one flat battery the full starting power runs through the split relay. The way its currently wired is very simple and works very well without any complex wiring. i.e. as i said it's just a second battery wired for emergency use only.
It does not matter which battery you regard as battery 1 and battery 2 and it does not hurt to swap from time to time i.e. leave battery 2 on permanently and switch in 1 as required.

Does that seem like a good plan? Just interested in peoples thoughts.

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I personally don't really agree with the concept of the basic split charge relay for a lot of the applications they are used for. I think they're as misunderstood as a hybrid car.

I understand the concept he is working with, my problem would be that I would always forget to turn the relevant battery off or back on again.

What do you intend to do with the car? He says that it doesn't matter which battery you consider to be 1 and which you consider to be 2. Well are they both engine batteries that are only used for starting the car and occasional winching or the odd worklight? If so fine, but if one is a domestic battery that your planning to flatten regularly when camping then it surely does matter which is 1 and which is 2?

My personal choice would be to have 2 batteries and 2 alternators with a link button like you have on boats (or just a set of jump leads) for the infrequent occasions that you might not be able to start the engine and depending what you intend to do with the truck denotes the type of batteries you have and which battery you connect the winch to.

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I also understand his concept and idea but went with the fit and forget solution of a T-max split charge relay and controller.

I have my second battery permanently wired for the winch and a relay/fuse board for aux wiring applications, spot lamps, work lamps and aux power sockets, I want that the primary starter battery is exactly that and for the running of the car and the second battery does everything else. I too would forget which battery is which and most likely end up stuck with two flat batteries where my forgetfulness would prevail.

I prefer my own way, and that of many others, to keep systems separate and wire them in accordingly. For the most part it is easier to maintain the vehicles own electrical system and to build up your aux system how you want but with peace of mind knowing one does not interract on the other without your input.

just my 2c

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I don't think there's much wrong with that set up Dave, the only weakness will be remembering to switch on/off batteries as Al has said. It has the advantage of being simple and, if decent isolators have been used, robust.

HTH

Mo

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I get his thinking, but I would install a split charge system, and then fit a heavy cable between the two positives with a big isolater. Should you need to use a jump start from one to the other, switch in the isolater to take the load off the split charge. Once running, remove the key.

To do it though, they do both really need to be start batteries, and not one start one leisure

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My approach has been to feed the secondary battery/batteries via an ignition-controlled relay and a "diode box" - basically a 100-amp 'Schottky diode' on a heatsink - from the +ve terminal of the primary battery.

The primary battery always gets charge priority - only when it's up to voltage does the secondary battery get charge. You never get a situation where a prolonged discharge load on the secondary battery with the engine not-running can discharge the primary battery.

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Some very differing views here. It's not my daily driver, and I'm quite happy to remember switching things on / off so I think I'll keep the setup. I do want to move and combine the isolators tho, as I plan to make the back more 'family friendly' and a big red switch is going to be far too tempting for little fingers.

I have two starter batteries with an X-charge relay, fitted with a blue sea systems isolator/selector to give the option of combining the batteries if need be.

I have been looking at the blue sea ones. Which one did you go for and where did you mount it?

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I quite like it, it's simple, fairly idiot proof and not much to go wrong.

You could do it without the big switches by putting a decent chunky relay in (like the X-eng style one) and using a regular toggle switch conveniently placed on the dash, possibly with a warning light. Saves having to reach around to engage / disengage.

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This is what I have on mine, with the dual monitor as an add on. The additional mod i have is a battery isolator switch on each battery positive to completely disconnect the two batteries and allow the solar top up charger to keep the main battery topped off.

http://www.nationalluna.com/Datasheets/Intel%20Solenoid%20Instructions.pdf

The solenoid relay is chunky, and i like the idea of the timer which puts the charge back to main batt first 5 mins after starting. Or you can see the batt states on the monitor. or if you want to manual overide for 5 mins to e.g winch or jump start.

Once the charge voltage drops off (engine off) the aux battery seperates from the main.

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I like the keep it simple approach 2 batteries and 2 alternators all connected together, less components to fail and in many years of winch challenge competitions with up to 5 winch motors to run the only failure I had is a failure I had was an isolator failure in a 2 motor winch when it melted a bit so the contact stopped contacting (was rated at 2500A for 5seconds or 250A continuous)
I've never run out of battery power or had the engine stop because of a flat battery.

With a split charge type system or isolating a battery when winching you are just adding components that can fail because the are not rated high enough for winch loads or removing capacity from the system.

The likelihood of ever draining a 2 batteries and 2 alternators to a point you could not restart a hot engine is so low that it is not worth the extra complexity and capacity reduction.

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Yup totally agree with no spilt charge and I run my two batteries the same way. I thought about this long and hard and liked the simplicity, Yes you need to change them over manually but that's no biggy. I've got just one 3 way marine selector switch which is perfect. I've been running both mine in parallel just recently with the cold mornings. I'd say you can damage the alternator by isolating both while running, but I've done it and it's not harmed my alternator... it's not something I do regularly mind. It's all really personal preference.

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Some very differing views here. It's not my daily driver, and I'm quite happy to remember switching things on / off so I think I'll keep the setup. I do want to move and combine the isolators tho, as I plan to make the back more 'family friendly' and a big red switch is going to be far too tempting for little fingers.

I have been looking at the blue sea ones. Which one did you go for and where did you mount it?

It's mounted to the front of the battery box into the footwell...

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It's a 3002 HD switch. Rated at 500a continuous, 700a for 5min, 1150a for 1min and 2750a for 5sec. I'm imagining things will melt/catch fire long before the switch gives up! I used a 92mm holesaw for the cutout.

https://www.bluesea.com/products/category/11/35/Manual_Battery_Switches/HD-Series

It was the highest continuous rated I could find at the time.

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It's mounted to the front of the battery box into the footwell...

It's a 3002 HD switch. Rated at 500a continuous, 700a for 5min, 1150a for 1min and 2750a for 5sec. I'm imagining things will melt/catch fire long before the switch gives up! I used a 92mm holesaw for the cutout.

https://www.bluesea.com/products/category/11/35/Manual_Battery_Switches/HD-Series

It was the highest continuous rated I could find at the time.

I like that, it's a nice clean install, and difficult to snag on.

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It's great except it could do with some grommets around he positive wire.

I run a t max and have a isolatora for both batts. Main battery cuts all but split charge on an FIA cut off. The other battery for aux items stereo, winch, amp, etc runs off a massive marine silly amp rating cut off switch.

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