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Ideas And Help With (electrics)

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Moving into my least fav area on LRs electrics,, .............


.................otherwise known a F Electrics ;)

Problem is that I have the 2 red top optimas I have had for some years on the hybrid,

I had 2 as I ran a mechnaical front winch and an electic rear winch.

The batteries were (eventually) wired so 1 ran the engine and one was for the winch, with the ability to link the 2 together for 'real problems' of self starting if needed....

Now I have 2 x Hydraulic winches, and have no real need for the huge power output,

but the idea of twin batteries has always appealled with the peace of mind of having a spare to self jump start the 90 etc......... should I say leave the interior light on for a week without knowing - not that I would ever do such a thing :(

So, what I have now is 1 optima running the engine etc, with a single "Kill" switch, and the other sits next to it, not connected to anything, and basically I just keep swapping them around and charging (optimate) them both when I remember....

What I want to do is to wire up the batteries in the best way and I am looking for ideas....

I would like

1. Both batteries connected in some way

2. A way that they both retain full charge

3. A way I could isoltae 1 until called on for its "Umph"

4. I have a huge plug I want to add which has a Male Female connector - useful for jump satrting dead 4x4s at events

5. Some sort of guage(s) which show battery condition(s) all of the time

I have looked at the National Luna Split charge unit, and others, but I'm open to suggestions ?


Nige :ph34r:

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A cheap and simple method is to use a solenoid (you can get them in various ratings) to connect both batteries. Basically you need to connect both batteries to the solenoid. A relay that is activated by the charge warning light is used to activate the main solenoid (there is not enough power from the charge warning light to activate the main solenoid). So when the charging light goes out both batteries are connected and being charged by the alternator. If the ignition is switched off or there is a rpoblem with the alternator then the batteries are disconnected. I use this in my Discovery and run all my spare electrics (fridge, work lamp etc) off the second battery. This way I can flatten the second battery and still have a good starting battery. You can also fit a toggle switch to temporarily link the batteries in an emergency.

It's not as good as a dedicated split charge/conditioning system but it is cheap and easily made.


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Ivan is pretty much right. The issue with solenoid switches is generally the coils are not continuously rated and let the smoke out when you least want it to.

You really need a very heavy duty relay - as big as you can find. Lighting relays up to 90A are available and this is plenty for your application.

Both car batteries and alternators are designed to work together and there is limited merit in having a managed charger / conditioning unit in there as well. I think most are sold because they are a bit bling - blue LED's and the like. The most I would do is fit a volt meter on the dash for each battery.

The normal split charge arrangement connects both batteries together whilst charging/discharging - which is fine for them.

If you just connect a pair of batteries in parallel, it is likely that one may have a little more charge than the other. It will pump charge into the other battery - but over-shoots a bit and the other battery needs to pump charge back - and it over-shoots too. This way, a small amount of charge is continuously cycled back and forth, flattening the batteries over time.

Give me a shout if you want a diagram, help or even a suitable relay!


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Ta for the thoughts and feedback.

I'll probably go after some thought and a chat with JW the "Low Tech Route"

Thinking of having the 2 batteries joined up togehter with a kill switch between them, and then hard wire to each battery a "Optimate" plug so I can charge either one by plugging into the Optimate.

May well drive around sometimes connected so they both charge and sometimes disconnected, via the kill switch, seems simple cheap, and see little to no downsides ?

Duiscuss ?...point out why not ?

E;ectrics are not my strong point (understatement of the year No 1)

Nige :ph34r:

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i used a Voltage Sensitive Relay From Merlin equipment, it basically connects the batteries in parellel when the charging voltage reaches 13.5v, then disconects them once the engine-start battery drops to 12.7v.

its a brilliant, fit and forget device, i added a 'red-key' switch to bypass the relay and join them together in 'emergency parallel' in case of engine-start battery failure.

check-out merlins website or give them a call, ive got a wiring diagram i drew-up that i'll email you if you let me have your address....

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Nigel, you can fit a split charge relay, use a normal relay, vehicle wiring products sell a 180amp one which I use.

Connect the coil of the relay to earth and to the battery light terminal on your altermator.

Connect the switch contacts to the live pole on each battery via an in-line fuse (100amp).

Connect the live terminals via a kill switch for self starting only and use the second battery to run you auxhiliary equipment.

Some people will disagree with me but this is how I do it and how I have done it for others and I finished ahead of most of them. :P:P:P:lol:

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sounds interesting

E-mail PM'd



I've fitted a VSR (Voltage Sensitive Relay) too, a great bit of kit, no mucking around behind the dash or anywhere else for alternator light wires etc.

Just the live from the Engine Battery, a live to the Aux Battery, and a little earth wire for it to all work.

Really is Fit and Forget. the only thing it doesn't do is emergency self jump starts. I wouldn't just stick a switch in to overide the VSR, as this will parallel both batteries, and if the Engine Battery is proper dead, paralleling it with the Aux Battery will kill that too.

Personally, if/when i end up in this position, i'll just swap the wires over.... ;)



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Nige I have a two way switch from a boat chandlers in Portsmouth it is four position Batt 1, Batt 2 Batts 1& 2 or Off. t also has aterminal for the charge light to tell you if whatever you chose is charging Good quality and I've had it underwater (at Minstead last year) with no adverse effects. You're welcome to pop in and have a look anytime.

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When we had the caravan in the UK and went to a sheep farm without hookups for 2 weeks in the summer, I decided to fit a second battery to my Discovery 200TDi. Installed it where the jack normally goes, moved said jack to a location next to the brake servo, and wired batteries (identical layouts for interchangeability) with the live connections permanently hard-wired with the normal thick red battery cable. I fitted one of those DISCARNECT (I think they're called units) to the negative terminal of the second battery and earthed the other end to the body nearby.

Unscrew & remove the connector screw and the second battery is isolated from the normal battery. Replace the screw and both are connected in parallel, so you can then charge both together. I also fitted an "S" socket under the front bumper so that I can use a 3-pin flat chocolate block plug/socket (from Maplins, on a strip) such that if I connect it one way it charges one battery and when I reverse it it charges the other (assuming the Discarnect screw is removed) without even raising the bonnet. Still use it here to this day. Simple and effective, no electrickery, relays or any of that carp.

- Richard

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The way i have always wanted to do it but never got around to is

Go to a Yatch chandlers and they have heavy duty battery cut of switchs (which all boats use)

They have 4 positions, Off, battery 1 only, Battery 2 only, Or Both batterys

They are used on boats for you have them off when not on the boat, and both to start and charge, and 1or2 onlyfor when in port and using all the elctrics but with no engine running

Easy to wire up and nice switch as well,

Give them a look at and i think you will be impressed

Cheaper and better in my mind than some arty farty spilt charge system

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That 4 position switch looks like a good idea. Nice to have the backup battery for those cold mornings!

Would it not be better to combine this with a simple split charge relay though, so both of your batteries are being charged when required automatically but you have the ability to choose your starting battery(s) in the event one dies.

If you were using the switch only, wouldn't you have to make sure it was set to both batteries in order to charge both? and unless the switch positions are make before break you would have to select your battery before starting and running the engine otherwise switching between batteries would break the connection to the batteries and stop the engine (dependant on where you wire it in of couse).

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