Linking 2 leisure batteries??

When I did my self build it used 16mm welding cable from the engine battery to the two leisure battery’s for decent charging and the same cable from the batteries to electric bus bars, worked a treat.
 
I linked my two 125ah lbs and my 125ah vb together, I took advise from Peter, (Listerdiesel) he knocked me up a 25mm2 cable and sent a fuse, 25a i think, it worked fine til I sold the van.
 
It really depends on how much current you take out of the batteries.
For instance, do you use an electric kettle or a microwave, or an electric grill?
If not, a 10 or 12 mm2 should do fine.
 
Not to argue, but ask a professional, most of what you read on a moho forum will be hearsay.


Admin knows his stuff
 
Pudsey Bear said:
Not to argue, but ask a professional, most of what you read on a moho forum will be hearsay.


Admin knows his stuff
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Although this forum is not made up of members doing there first VW conversion, many members have decades of experience and knowledge to share. I believe a combined effort of members here could build a moho from scratch including engine, transmission and chassis

Many could tend the bbq, drink beer and offer moral support as well, myself included
 
Pudsey Bear said:
I linked my two 125ah lbs and my 125ah vb together, I took advise from Peter, (Listerdiesel) he knocked me up a 25mm2 cable and sent a fuse, 25a i think, it worked fine til I sold the van.
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I think you may have posted up a wiring diagram from Peter as well? That diagram is a good resource for folk to look at as well.

There are two aspects to doing wiring.
1) doing it safely. That is a minimum and people need to remember that the way a motorhome is built and the wiring used is suitable only for how it was supplied. As soon as upgrades occur (bigger batteries, more powerful chargers, inverter additions, etc), that original wiring is likely to no longer to fit for purpose and should not be used as a template on how to install
2) doing it well. There is a difference between fitting something and fitting something so it works in the optimum way. someone can install a second battery with 1mm wiring if they want and as long as they have got suitable fusing in place it will be safe enough, but they will have wasted a lot of their money as they will not get close to the best out of their system.
The cost of doing it right is rarely much more then doing it badly.
 
I don't think I ever did a diagram nor Peter, it was just connecting 3 batteries together in reality so just + to+ & - to-, but fusing was mega important as it is in any wiring, one thing I always to without fail is to have a fuse next to each battery regardless if it is linking two batteries in the same place double failsafe, fuse sizing has always confused me, all I recall is fuse the cable not what it is powering in a 12/24v system.
 
Pudsey Bear said:
I don't think I ever did a diagram nor Peter, it was just connecting 3 batteries together in reality so just + to+ & - to-, but fusing was mega important as it is in any wiring, one thing I always to without fail is to have a fuse next to each battery regardless if it is linking two batteries in the same place double failsafe, fuse sizing has always confused me, all I recall is fuse the cable not what it is powering in a 12/24v system.
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I think the diagram was posted on Motorhomebuilder in fact and from memory was by Peter although not identified directly as such (the description of the creator being a great help to many would certainly match the description of Peter).

Basically, the fuse rating MUST be lower than the ampacity of the cable, and it should be greater than the peak constant current draw of the circuit.
If you have multiple batteries in parallel, then the current from each battery will proportionally lower (so say you are drawing 600W via an Inverter from the battery - that would be 50A from the bank. So if two batteries, 25A each; if three batteries, 17A each, so you could fuse each individual battery on that basis.
 
ChrisHunter. Is your name Jose' because there is no way to do it like that.
 
Yes, that is a possibility, but I would say that unless there is a chance that the wires between the batteries would shortened to the metal mass, a single fuse between the batteries and the cable will suffice.
(By the way, being an electrical engineer, I think I know a little about this subject).
 
What sort of electrical engineer?

I would also use grommets when going through any metal, and a fuse at each end anyway they are so cheap..
 
Jack vdl said:
Yes, that is a possibility, but I would say that unless there is a chance that the wires between the batteries would shortened to the metal mass, a single fuse between the batteries and the cable will suffice.
(By the way, being an electrical engineer, I think I know a little about this subject).
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Qor you....
A single fuse between the batteries?
which side are you protecting? the battery side or the battery side?
And which side are you leaving unprotected? the battery side or the battery side?
 
What I meant to say is that you can connect the batteries together without a fuse when there is no danger of the connecting wires coming in contact with the metal of the van.
For instance when the batteries are placed next to each other.
When this is not the case yo need to fuse the connecting wire at both ends next to the battery poles. And you have to fuse the cable that runs from the batteries of course also next to the battery pole.
The fuses are there to protect the cables against overheating or, in case of a short, getting so hot that they set the van on fire. If leading the cable trough a metal sheet/wall a grommet or other form of protection of the cable is a very good idea.
What kind of electrical engineer am I? A retired one and a general one. I have worked on installations in ships as well as in campervans.
 
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I'm an electrician by trade and i see alot of amateur jobs that haven't come to harm by sheer luck. There's quite alot that can go wrong with a battery, even though 12v is not enough to drive current through skin, a battery is quite capable of releasing in excess of 500 amps if shorted, enough to send a spanner or the base of a van seat red hot in a very short space of time, let alone a thin piece of cable. All cables should be protected against overload and short circuit by at least one method, ideally 2. (IE insulation AND fuses /max demand calc etc) I'd size a cable according to ohms law and power equations, looking at how much it may reasonably draw in either fault or operational conditions to avoid overheating/melting. A 12v 2400w inverter can pull approx 200 (2400w/12v=200a)amps continuously so your looking at cables at least twice the thickness of those coming in your house to supply such equipment. So about 50mm2 between primary and supply, 35mm between 1st & 2nd battery if a 3 battery setup, and 25mm between 2&3 or on a 2 battery setup if all batterys are equal size and type. Welding cable is often pure copper so better that than jump leads which are often mixed with aluminium and can carry far less current for size. I believe regulations state the need for protection against vibration work hardening and subsequent failure of cables in vehicles, another reason to pick fine strand flexible copper cable over house type cable or aluminium. I'd size a fuse to not pop on max demand, and cables larger than that so they are protected. Personally, as well as sc/oc fuses in the positives as close as possible to the battery terrminals and on every cable size change/final circuit, i also place a fuse inline on the negative between the main and aux systems, that way if ever there's a bridge between chassis and aux live before the fuse (say in an accident or when tightening the live with a metal spanner) it'll disconnect it. Serves as an isolator between the van primary system and expensive secondary system equipment if having welding done etc also.
As battery cables are 2 way(charge and discharge) I'd also look at how much i want it to charge bearing in mind the the thinner the cable, the higher the resistance, the higher the volt drop between supply(alternator etc) and load/battery, and the less the percentage of charge the battery will finally obtain. A battery will require the full 14.4 v to charge to 100%, so you are looking at obtaining as close to zero volt drop as possible in order to utilise the full capacity of the battery and maximise its life. Volt drop calcs can be found online and is far more of an issue in 12v systems than 240v systems.
I'd also mention hydrogen build up, its released when charging and is highly explosive so make sure you vent large or powerful lead acid battery systems.
Hope this helps, I'd agree with the majority of posters here, the more i learn about electricity the more I realise how dangerous it can be and how much of an art it can be to get the most out of a battery system, so if in doubt consult an expert.
 
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