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Wiring Up twin Batteries (12V)


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Hello Electrical Gurus. :)

Ok.............I have at the moment 2x 'Red Top Optimas'.

These are really a "left Over" from my days with electric winches, but even now I do like the idea of having a second battery as a back up etc. At the moment I have a National Luna Split Charge system on it - and its coming off for all sorts of reasons.

So............. I want to keep both batteries, and am just having a ponder as the best way to wire them up after I take the split charger out ?...........

My thoughts are :

Battery 1 - basically keeping as it is :

HUGE 50mm earth to chassis from Neg Terminal.

Large cable* from the starter to a Master Cut out Switch

From this a large cable* then goes to the positive Post

(*Large cable is 35mm welding cable)

So

Best way to go with Battery 2 ????...................

Use the post on the main cut out add another cable and then go across to Battery 2 Postive terminal ?

Add a Cable from Postive terminal Bat 1 to a second Cut out No 2 and then to Battery 2 Postive terminal ?

Or just a big cable from Battery 1 Positive to to Battery 2 positive and have both battery 1 through master cut out ??

(Battery 2 does have is own sepearte HUGE Earth)

Alternator is 90amp

So really what or which is better / and why ?

Best option of battery 2 ...........and the pros and cons please ?

What would you do ? / Or have done ? and why ?

Nige :huh:

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Good point Chris, but I no longer have Leccy Winch :lol:

I was wondering

Cut out postive to Battery 1 then battery 1 positive to Battery 2 postive, and 2x huge earths seperately.

If I then connected my accumate charger to Battery 1 positive and battery 2 negative will it charge up BOTH Batteries, or as now do I need to do each in tern ?

The Accumate is the bigger version of the Optimate "trickle Charger"

??

Nige

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use a BEP Marine VSR (Voltage sensitive relay) to connect the two batteries.

Available from Merlin equipment.

You are right not to muck about with toytown battery management systems, total waste of time, the VSR basically watches your start-battery voltage, and when it reaches a certain level, (ie when you start the engine, and the alternator is charging) joins the second battery together in parrallel, when the engine start batteries voltage drops below 12.5V (engine off) it disconnects the second battery from the system.

Most fire engines, police cars and ambulances use this very item, as do most small boats to disconnect the engine start batt from the 'house batts'

if you wire a 'red key' switch in parallel with the VSR you can manually over-ride it and connect the second battery to start the engine, if you manage to kill the engine start batt. this is common practice on boats, and shown on the BEP wiring diagram.

once fitted you should run all non-essential items off the second battery via its own fuse box (CB, extra lights, radio, fridge) that way they can never run your engine start battery flat.

hope this helps.

M@tt.

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Good point Chris, but I no longer have Leccy Winch :lol:

It is more about what the batteries do to one another when connected but neither charging nor discharging than what you run off them, as I understand it. Si would be able to give chapter and verse on it.

Chris

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It's nice to see other people mentioning a VSR. I've had one fitted to the 90 for about 3 years now, and it works faultlessly. I don't reckon they'd like a winch, or work well with one, but that's no bother here :).

Like Matt said, you could wire it with an 'overide' switch for jump starting yourself, if you wanted that ability at the flick of a switch, the few times i've needed to do that i've just swapped the leads over on the batteries. Other than that, a VSR just gets on and does it job.

Unlike a 'dumb' relay, a VSR won't close until the engine battery is charged, which is a good thing.

no stupid LED bar graphs

no current drain when the engine is off (ok, mA, just enough to run a chip)

easy wiring - a high current wire to each battery +ve, and a small earth. that's it.

the BEP marine unit is potted, so 100% waterproof, mine's been waded and is fine.

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Guest WALFY

I have twin batts in my 90. Although not wired in by me, I'm a complete wiring mong. If the main goes flat there is a started solinoid in there and a small button on the dash to join the 2 batts together when needed.

You could ask me questions but you prob won't get a sensible answer. Best bet to ask Weeble as he wired it all up for me.

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The setup I use is made with two cut-off switches. Both located under drivers seat, and both the batteries (110Ah each) are also located under drivers seat.

Battery 1 has negative post connected to a common vehicle earth point with 50¤ cable. From this common vehicle earth point there is a 50¤ connection to both chassis and engine. The positive post on the battery is connected - by 50¤ cable and via cut-off switch one - to a common positive point.

Battery 2 is wired similar. Negative to common vehicle earth point. Positive via cut-off switch two to common positive point.

From the common positive point goes one 50¤ cable to winch solenoid box. Another 35¤ goes to aux. fuse panel. Another 35¤ goes to positive point on starter solenoid. Everything electrical in the car is wired throug this common positive point.

By turning cut-off switch one ON the common point receives power from battery one. The alternator will charge battery one. Similar with cut-off switch two. By turning both cut-off switches to ON the batteries are connected as one large battery - usefull for winching. (Yes, I still use an electric winch - but slowly gathering the components for a hydraulic setup...)

For providing information about the battery and charging status a voltmeter is fitted. This is connected to the common positive point, and will as such be able to tell the voltage for each of the batteries, providing the relevant cut-off switch is switched on.

There are no automatically switching between the two batteries, althoug it would be rather simple to make some kind of split charge relay.

For a simple - and quite robust - split charge systen use a standard 40amp automotive relay: terminal 85 goes to D+ on alternator, terminal 86 goes to vehicle earth, terminal 30 goes to positive point on main battery (or common positive point), and terminal 87 goes to positive point on aux battery. When the alternator is delivering current the relay will connect "30" and "87", thereby charging the aux battery. A similar setup can be made using a winch solenoid - thereby enabling the split charge system to cope with rather large currents. Remember to fuse the cable which goes to "30" and the cable which goes to "87". Both of the fuses should be as close to the positive battery point as possible.

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  • 7 months later...

I have seen the answer to this somewhere - but way do some folks connect the master Cut-off switch to the Positive....while others connect it to the Earth?

Also MY FIA one shows a connection to the 'coil'....but its not connected in reality? is this because its a Diesel? If not what should I connect it to?

Thanks all.

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Your FIA switch should be connected to the stop solenoid wire. this will kill the engine just like the connection in the coil circuit would on a petrol.

The other connection on the FIA switch should have a resistor to ground, and be connected to the alternator side of the main terminal.

There's not much in the earth/live debate really. If you put a switch in the earth you can undo the postive terminal without the fear of the spanner shorting on the battery box.

A lot of it will depend on how the vehicle is wired. Some people put the switch in the earth as they see that as the simplest way to switch off 'everything'.

post-84-1206370877_thumb.jpg

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If you just wire them in parallel, you lose the ability to jump start yourself if you leave your lights on etc.

There's also the matter of low current oscillations between the two batteries - who knows if that's a real-world problem?

I've got a VSR fitted, I like that i can leave the stereo/PC/worklights on with no worry of flattening the cranking battery.

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If you just wire them in parallel, you lose the ability to jump start yourself if you leave your lights on etc.

There's also the matter of low current oscillations between the two batteries - who knows if that's a real-world problem?

I've got a VSR fitted, I like that i can leave the stereo/PC/worklights on with no worry of flattening the cranking battery.

Thanks - thats my thought.

I have twin batteries, with a split charge Voltage sensing relay setup. My auxilliary stuff is all wired up to the second battery.

I also have either..... a DUD Optima (that is not holding its charge)....OR some leakage current. My battery flattens in about 3 days :(

I really need to get around to checking it out....as my emergency starting procedure involves the 2 batteries, a Spanner....and some dexterity to hold it in place while I turn on the ignition....

I also need to replace my starter motor...as 50% of the time it just whirrrrrrs and does not engage :(

It never rains but it snows Eh :blink:

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Guest diesel_jim
If you just wire them in parallel, you lose the ability to jump start yourself if you leave your lights on etc.

But.... if the two batteries are connected in parallel (ie, one big battery) it'll take longer to drain flat, should he leave his lights on etc.

depends on what the final "use" for the vehicle is. if it's just to install 2 batteries because he's got them, i'd just wire them together. If it's to run work lights/PC's/etc then get the fangly VSR/blocking diode etc.

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If more than one battery is to be wired as one then this is how I do it on my camper. Wire them + to + and - to - but then use the pos of battery one and the neg of battery two as the main engine battery leads this then stops one being a master and the other a slave. All cabling to be done with thick welding cable, I went a bit OTT as I was given some 70mm welding cable, now thats thick.

BTW I don't think its a good idea to wire dirrerent batteris up like this or even ones of not a similar age.

Gaza

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But.... if the two batteries are connected in parallel (ie, one big battery) it'll take longer to drain flat, should he leave his lights on etc.

depends on what the final "use" for the vehicle is. if it's just to install 2 batteries because he's got them, i'd just wire them together. If it's to run work lights/PC's/etc then get the fangly VSR/blocking diode etc.

I agree, to a point. BUT blocking diodes are a 100% no-no, the 0.7v voltage drop across them is not acceptable, IMHO.

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Guest diesel_jim
If more than one battery is to be wired as one then this is how I do it on my camper. Wire them + to + and - to - but then use the pos of battery one and the neg of battery two as the main engine battery leads this then stops one being a master and the other a slave. All cabling to be done with thick welding cable, I went a bit OTT as I was given some 70mm welding cable, now thats thick.

BTW I don't think its a good idea to wire dirrerent batteris up like this or even ones of not a similar age.

Gaza

It doesn't make any difference where you take the +ve and -ve from, anywhere on the +ve side and anywhere on the -ve side will give the same power and draw the same amount of current from either battery...using a terminal on one battery won't "draw" more current from that particular one.

I agree, to a point. BUT blocking diodes are a 100% no-no, the 0.7v voltage drop across them is not acceptable, IMHO.

why? it's only 0.7v from 14.4~14.7v, prior to the electronic split charge units, this was the only way people did it. my dad's got this setup on his boat (24v, 12x 6v batteries) and it works perfectly, has done for years.

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Has the boat got a remoted sensed alternator regulator? that's the proper way to do it with split charging diodes.

The charging voltage of a battery is critical, for proper performance. The 0.7v voltage drops means the batteries aren't properly charged and won't reach their full potential, in terms of output, or longevity.

Yuh split charge diodes work, like you say, loads of people have and do use them. They're just a bit of a bodge in my opinion, especially when used without a remote sense alternator.

I agree with your first point - being a pedant, if the connection resistances were particularly high, moving the connection point could have an effect, but they shouldn't be, so connection position is, or at least should be, irrelevant.

:)

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Guest diesel_jim
Has the boat got a remoted sensed alternator regulator? that's the proper way to do it with split charging diodes.

Actually, it's so old it's got dynamo's on the engines! :blink::blink::blink: (Marinised Mercedes OM321 6 cyl's x2)

I think it was one of Noah's cast offs! :lol::lol:

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Right - my take on this!

Lead Acid batteries do not like being trickle charged. It causes electro-chemical changes to the cells whose effects are similar to over-discharging the battery. The effect has been likened to putting rocks in the cistern of your WC. It still flushes and you still get the same pressue - but not for as long.

The reason this is an issue when connecting a pair of batteries in parallel is that, unless the batteries are well matched to one another, you get minute currents flowing randomly between the two batteries. The currents are too small for your average multi meter to pick up, but can be large enough to have an effect.

If the two batteries are well matched to one another, this does not occur. Hawker Cyclone batteries for example are designed to be connected in parallel. Two batteries of the same type bought at the same time are likely to be good too - as Tony amongst others on here have found.

Trickle chargers (in particular the solar ones) IMHO are a really bad idea! They will fix the immediate proiblem of your batteries going flat over a few days/weeks - but over a year or two, they will wreck the battery. A better solution is to use a regular battery charger on a time switch so it delivers a decent current for a short duration every 24h. This works really well. Many of the managed chargers do pretty much this, but by sensing the voltage. When the voltage drops below a threshold, it gives it a decent charge to take it above - then switches off until next time.

I agree that blocking diode split charges are not ideal for many applications due to the voltage drop of up to 0.7v. It is also difficult to arrange these to work with battery sensed alternators (300tdi, Td5, Puma).

Relay based split charges do not have the same voltage drop, but only isolate the batteries when the alternator stops charging. In most applications this is not a problem, but there is still the possibility of drawing down both batteries while winching to the point that you cannot re-start the engine if you stall it. While the engine is running, it gives you the equivalent of one big battery.

The biggest advantage of this type is it is easy to diagnose problems and easy to bypass temporarily to get you out of a hole. They tend to be water proof and mechanically robust.

The more expensive split charges use MOSFETs as a sort of solid-state relay. The MOSFET only drops about 0.005v which is of no consequence. These are feature rich, often with LED Bar-graph indicators of interesting things. As a split charge, they have few shortcomings other than cost and occasionally water ingress. Sometimes problems are not so easy to diagnose - particularly if the split charge itself is faulty, you cannot trust its diagnosis of a fault.

VSS (Voltage sensitive switches) are half way between a Relay type and a fully managed LED's abound type. They will not let your main battery be drawn down by your winch. Most of the units in use are marine - so pretty good in water. They do still contain some electronics which has been known to suffer with voltage spikes from jump starting and even winches!

Personally, I think a split charge is worthwhile even if you think you have two matched batteries (just for peace of mind). The split charge is likely to be a lot cheaper than a new battery (or pair of batteries). If you need the type of monitoring provided by the techy types, fit a volt meter to each battery. That will tell you as much.

Hope this is un-biased enough!

Si

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Don't have Si's understanding of the electrics but have experienced coming out to find both (twin exides, bought as a matched pair and wired in parallel) batteries so flat they couldn't raise a glow on the ignition light :(

For now replaced by a single exide while I consider the options but I won't be simply wiring them in parallel in the future again.

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That is one of the issues with just connecting them in parallel. If one battery goes faulty (leaky, reversed cell etc), it will draw down the other and ruinate it! At least with a split charge, the two are isolated most of the time.

Si

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