Correct cable specification for leisure battery

Chris

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Hi chaps, I could use some help from all you sparky types out there. I have to confess that when they did the volts x amps = whats? I was in the pub drinking Mansfield Bitter and listening to Blinded by the Light on the jukebox :dance: .

I want to wire my leisure battery in which will be a cable run of about 5m in total. It really is a one way deal here. I will not be expecting to 'back boost' my main batteries (80 series). I have a split charge voltage sensing relay to go under the bonnet already. I do have an inverter that can run off the battery if I need 240v but mostly my fridge, lights etc are all 12v :idea: directly from the terminals. What sort of gauge cable should I be using? I can go the whole hog and wire it with 50mm CSA but at about £7.00 / m I would like to save a bit if I can. All this will be doing is carrying the charge from the alternator via the main battery terminals to the leisure battery. I don't actually know how many amps the alternator kicks out. It's a standard one. Going to be what, 80 to 100 amps? If I need a boost at any point, I can either take the main battery out and drop the leisure battery in its place or at least use jumper cables from it sitting on the ground. I don't envisage having that sort of emergency in mainland UK really. The engine will always get a good run, so it's unlikely to ever have to be charging a dead battery. Some of my kit has low voltage protection too. So, best cable size for charging a leisure battery would be.....
I'd like to see the maths on this if I can. :thumbup: I know it's not exactly rockett salad but I can't remember. :roll:

Chris
 

Steve Wright

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Chris said:
Hi chaps, I could use some help from all you sparky types out there. I have to confess that when they did the volts x amps = whats? I was in the pub drinking Mansfield Bitter and listening to Blinded by the Light on the jukebox :dance: .

I want to wire my leisure battery in which will be a cable run of about 5m in total. It really is a one way deal here. I will not be expecting to 'back boost' my main batteries (80 series). I have a split charge voltage sensing relay to go under the bonnet already. I do have an inverter that can run off the battery if I need 240v but mostly my fridge, lights etc are all 12v :idea: directly from the terminals. What sort of gauge cable should I be using? I can go the whole hog and wire it with 50mm CSA but at about £7.00 / m I would like to save a bit if I can. All this will be doing is carrying the charge from the alternator via the main battery terminals to the leisure battery. I don't actually know how many amps the alternator kicks out. It's a standard one. Going to be what, 80 to 100 amps? If I need a boost at any point, I can either take the main battery out and drop the leisure battery in its place or at least use jumper cables from it sitting on the ground. I don't envisage having that sort of emergency in mainland UK really. The engine will always get a good run, so it's unlikely to ever have to be charging a dead battery. Some of my kit has low voltage protection too. So, best cable size for charging a leisure battery would be.....
I'd like to see the maths on this if I can. :thumbup: I know it's not exactly rockett salad but I can't remember. :roll:

Chris
Hi Chris
It depends on what you really want to draw through it, and its not just that, but also the length, as the longer the cable the more the volts drop at the other end, and there might be a possibility to draw a lot of amps.

The thicker the better ! I run 50mm" but I do run a winch off it for the trailer.

Unfortunannly if you want to do the job properly you need to pay the price, or (beg borrow or steal(poss not) as I did from a mate.

But if you want to draw 170 Amps you will need 25 mm2 and 40mm for 300 Amps

Steve
 

Jon Wildsmith

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When your split charge relay has the aux battery connected to the vehicle electrics you can no longer think of them as the aux battery powers the inverter, main bats power the side lights for example, they more or less share the loads, so you could still see a significant current flow along your 'charging' wire when you run a 2kw inverter flat out or say a portable winch ;)
 

Chris

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Ok, but I am not running anything like that at all. I don't have a 2k inverter to start with. All that will ever be running when I drive is a 12v fridge. The inverter would be used when static, with no engine on. I wouldn't need 240v when I am driving. I certainly won't be running a winch off this. OK, I am NOT an electrical engineer, so forgive me, but I would have considered that I was 'drawing' anything down the cable. Am I not 'pushing' current down it? It's just a passive battery sitting there waiting to be charged isn't it? So, there is resistance in the cable (which I DID say would be about 7m in total) and we don't want that to get hot do we., but 50mm to charge a battery seems a little excessive. It's not about being cheap, it's about not needing to over engineer something, pointlessly and expensively. I do take the point about being connected they are all one big battery, sort of, but other than 12v for a fridge that is connected to a pretty massive battery there isn't a lot going on. The main battery has to reach it's optimum charge before the relay kicks in so the alternator shouldn't be struggling to top this up, I would hope. If the main battery is struggling, say headlights, fans, demisters etc then it will cut off the relay and divert everything to the original set up.

Does that clarify the picture a bit? For my simply wired bran, the battery can't 'pull' amps from the alternator. It gets what it's given and that would presumably be just what the max of the alternator can provide. So I couldn't get 400 amps going down the cable could I? Not without, as you say Jon, a winch on there.

Chris
 
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Rob

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I see where you are coming from but even if your electric winch is connected to the main battery then you will draw current from the leisure battery when it is connected via the relay, and trouble is there is no turn off override on the cryx relays we all bought. This could lead to burnt out cables (the ones going to the 3rd battery) if they are not thick enough.

BTW I may be wrong...
 

Chris

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Ahh, now that may well be a real world problem. I see where you are coming from Rob. Are we sure that when the main battery drops, it doesn't cut off the relay? I suppose if it was already engaged and you started to winch, then it could draw for a while until it shuts off. Now, this might not be such an issue as the LB will be on an Anderson connector so I could easily pull it if I was winching.


Chris
 

Rob

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Disconnecting the battery when winching should work, but what about starting. There were plenty of times where I must of started the truck using all 3 batteries unintentional as it takes a while for the relay to kick in, and when it does it may turn on and off couple of times so disconnecting the LB seems less practical. You would need to fit a LED to indicate when the relay is engaged or not.

If you can live with waiting occasionally to start the truck after a stall or similar then you need to work out how much current the main battery could draw in worse case scenario and then work out how thick your cables need to be.

This is why I fitted mine under the bonnet, fitting it in the back would cost £££
 

Jon Wildsmith

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Too much voltage drop doesn't just make the cable hot, it also stops your aux battery charging properly and yes 0.5v does make a big difference to your battery ;)

If you're really not going to fit a winch then 25mm2 might work but I would just use 40mm2 and forget it.
 

sae70

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Mmmm.....tricky one this :think: I can see what your saying Chris you only want to use the LB for running the items that you mention from the rear & the cables are just for charging so only need to be rated for the max output of the alternator :| From what I remember of my reading on this subject over the years is that while the engine is running & the batteries are charging they all act as one large battery even to the extent that the oldest weakest battery will pull all of the others down to its level :!: This is why people talk of having a complete set of brand new matched batteries so theres no discrepancies between them. For my money you have to @ least match the size of the existing main cct wiring with maybe a single step up in CSA to allow for volt drop :D

Hope this helps :?
 

Graham

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35mm welding cable.
One cable for pos, and 1 cable for neg.

More then sufficient.
Just ensure good clean, maximum surface contacts between the battery output terminals.
 

Chris

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Hmm, never had this problem on Lil Blue as it was just a 4 amp feed to the battery on a manual switch. Very tempted to just do the same thing again!

Rob, I think I would struggle to get this battery under the bonnet. It's a marine cranking battery. 1000MCA and 110 AH so it's quite large. I would still need to run the cabling to the rear where the inverter etc will sit. I have a load of 50mm in the garage, but I was saving that for my rear winch project (eventually). Bit of a nightmare all round really. I shall have to look in the piggy bank again. If all it was doing was charging (which actually it is) I would go thinner but it's the damn relay factor that screws things up. I can see the potential now for drawing upon the LB when I don't want to. I think that in terms of starting, this is pretty momentary, but if you had to crank and crank for some reason, they would all be connected together at once.
You see, this is why I ask you guys.

Hmm.

Chris
 

AndyCook

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with the cyrix relay system i have setup on the LC90, sometimes when i have started the engine both batteries have been joined (I have LED lights to tell me status of linking) - this has only happened if I have stopped for a minute or so, and there hasnt been enough voltage drop (interior lights, radio etc) for the voltage to drop enough for the cyrix relay to disengage.
when i have had the engine running and use the winch, i havent seen the system disconnect yet - as I think the alternator keeps the voltage up enough to keep them engaged - i will pay attention next time i use the winch - but i havent used the winch under a real heavy load yet

I wired my LB with 50mm2 cable - however, I have the LB in the near-side battery bay (My "farmers" version FX only came with one starter battery!) - so only a short run of cable - not 7m!
 

Steve Wright

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Graham said:
35mm welding cable.
One cable for pos, and 1 cable for neg.

More then sufficient.
Just ensure good clean, maximum surface contacts between the battery output terminals.
Whats wrong with using the chassis as the Negative ?

Why use double length of cable when the chassis will do just as well !
 

Steve Wright

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Chris said:
Hmm, never had this problem on Lil Blue as it was just a 4 amp feed to the battery on a manual switch. Very tempted to just do the same thing again!

Rob, I think I would struggle to get this battery under the bonnet. It's a marine cranking battery. 1000MCA and 110 AH so it's quite large. I would still need to run the cabling to the rear where the inverter etc will sit. I have a load of 50mm in the garage, but I was saving that for my rear winch project (eventually). Bit of a nightmare all round really. I shall have to look in the piggy bank again. If all it was doing was charging (which actually it is) I would go thinner but it's the damn relay factor that screws things up. I can see the potential now for drawing upon the LB when I don't want to. I think that in terms of starting, this is pretty momentary, but if you had to crank and crank for some reason, they would all be connected together at once.
You see, this is why I ask you guys.

Hmm.

Chris
Chris

You say you have a load of 50 mm cable for your winch project ! you have just solved your problem !!

Run the 50 mm cable down from the front battery, along the chassis leg to the back, put some form of termination at the back, then run a slightly thinner cable from there to the battery via a split relay, and you have sorted the battery charge problem, and then when you fit the winch either use a Anderson plug, or just connect direct to the 50 mm cable

All sorted !
Steve
 

Gary Stockton

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I dunno - size counts, doesn't it?? From a previous post elsewhere:

Charging the batteries:

We have learnt that current is taken, not given. We have also learnt that current is a factor of Voltage and Resistance - the higher the voltage and/or the lower the resistance, the higher the current.

With a battery charging circuit, we have a power source (the alternator) and a resistor (the battery). These devices are, in effect, in series.

batt_chrg.jpg


The alternator provides a constant voltage output which, for now, we will assume to be 13.6v. The battery also has voltage - let's say the battery is at 12v. The effective voltage, or Potential Difference, is 1.6v (13.6-12).

The battery's internal resistance changes with charge state, but let's assume it is around 0.1?

Charge current is therefore 16A (1.6v/0.1?) (I=V/R)

What does this mean and how does it help you. Well, if your 105A/h deep cycle battery is sitting at 12v, it has been discharged to 25% of it's capacity (any more discharge and you risk damaging it). 25% charge means you have taken out 75A of current over time. If you put it back at 16A, it will take over 4½ hours to recharge.

Not so simple; As the battery charges, the potential difference drops, the internal resistance drops slightly and, therefore, the current drops. To bring a battery up to 80 or 90% of charge state is quick (a couple of hours). The remaining 10-20% can take up to 48 hours. That's a lot of petrol in a Cruiser.

So what can you do about this? Well, you can't change the batteries internal resistance and you can't push current down the batteries throat. That leaves one option - increase the potential difference. I.e. increase the alternator voltage.

But be warned, do not exceed a charge current of 30% of the batteries A/h rating - 10% is recommended. So, for a 105A/h battery do not go over 30A charge current. If you do, the battery will boil, overheat and die a horrible death.

Also, remember that any voltage drop over the wire between the alternator and the battery will result in a lower potential difference at the battery and a subsequent lower charge current.

Time to design a dual battery system. For this exercise, we are not going to add our gadgets (fridge, etc) just yet.

Basic rules:
KISS - although some prefer bells and whistles. If it floats your boat and it works, do it.
Keep the cables as short as possible/practical
Build in a margin of "over-engineering".

But the primary rule is safety/protection/prevention.

On with the design:
1) Cable thickness
A fully revved up alternator can charge a very flat battery at 25-30A at 14.1v. We also know that a deep cycle battery needs to be charged at 14.1v or higher. So, our cable needs to a) be able to handle 30A and, b) have minimal volt drop over it.

2) Cable length
Obviously we want to keep the cable as short as possible (volt drop, cost, etc). On most of our Cruisers, we have the two batteries on either side of the radiator. The shortest route is to feed the cable between the grille and the radiator. I personally prefer to route the cable via the firewall for reasons which will become apparent later.

3) Isolator
If you are a subscriber to the KISS principle, use a solenoid. There are many other devices on the market; some work well, some don’t. If you take the solenoid option, it should be triggered only once the engine is running and not from an ignition source. More on this later

4) Safety
Fuses MUST be fitted to the +ive cable on both batteries as close to the battery as possible. The fuse rating should be higher than the maximum current expected, but well within the cable’s smoke limit.
Cable should be covered with split sleeving, securely tied and routed away from sources of abrasion. If it is routed through a hole of any material, use a grommet or suitable rubber U-channel.

5) Connections
All connections should be made with suitable crimps and the correct crimping tool. A bad connection/crimp will result in resistance which creates heat, which creates a bad connection, which creates more resistance..............And the wire corrodes.

and see here http://www.the12volt.com/ohm/page2.asp#12

and

Cable does have resistance. The thicker it is, the lower the resistance. Length also plays a role: 1m of 1mm2 wire has the same resistance as 100m of 100mm2

and

Power is the product of Voltage and Current (P = V x I) and is measured in Watts. Power is not usually measured, it is calculated.

It is important to know the power of a device as we need to know how much current it will consume. Not many devices are labelled with the current consumption, but most do list the power rating.

As an example, SWAMBO wants to take her hairdryer in the back of your Cruiser. The hairdryer is rated at 1500W @220v.

From the formula, P=VxI; I=P/V
1500W/220v = 6.8A

OK, so the camp site you're at doesn't have 220v. You knew this before you left which is why you installed a 1500W inverter (we're living in a perfect world for the moment). How much current will flow on the primary, 12v, side of the inverter?

I=P/V
I=1500W/12v
I=125A

and

Time for a practical - the much talked about and dreaded voltage drop.

pump.jpg


In this simple circuit, we have a battery, in the front of the vehicle, a cable running to the back and a tyre pump. The tyre pump is a Finni, of course, which is rated at 60A current draw (full load).

Our cable length is 12m (6m +ive & 6m -ive).

For now, let's assume the cable is 2mm2. Resistance per metre is 0.009?, so total cable resistance 0.108?

Using Ohm's law, we can calculate the voltage drop over the cable:
V=IxR
V=60A x 0.108
V=6.48v.

We can see that the pump is actually only going to get 5.52v (12v battery minus 6.48v lost on the cable). That pump ain't going to pump.

This excersise is actually a silly demonstration, because the cable would have melted long ago. :ncool:

As a rule of thumb, cable thickness should be specified at a minimum of 1mm2 per 10A of current required.

If we use this rule, then we should have specified 6mm2 cable. Let's reculculate voltage drop. 6mm2 Cable resistance is 0.0034? therefore total cable resistance is 0.0408?

V=IxR
V=60 x 0.0408
V=2.45v

So our pump is going to get 9.5v instead of the full 12v. It will pomp, but not at full speed.

OK, let's get silly; go big or go home: 25mm2 cable. Resistance per metre is 0.0008?. Total R is 0.0096?

V=IxR
V=60 x 0.0096
V=0.57v

Now the pomp will pomp :cheers:

Pop quiz:
Replace the pump above with a fridge. The fridge draws 3.5A when running. Calculate the voltage drop for 2mm2, 6mm2 and 25mm2 cable.

with many thanks to Jos at LCCSA ... Some folk take exception to this, and to the over-engineering / safety aspect. I don't want my fridge to not fridge, or to have a fire thanks. I go with the suggestions for 35mm and 40mm cable (BTW - I used 25mm for mine and it works fine, with a big 200amp relay and big mega-fuses either end of the +ve cable ...)

EDIT: Oh yes - I ran a cable for both +ve and -ve sides - no earthing to chassis ....
 
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Chris

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Genuinely, thank you Gary. Nice to hear from you in any capacity. Thought you'd been deported. I am right up there with the physical wiring stuff. Fuse all sorted, crimper all sorted etc. The 50mm cable I have isn't long enough and I already have some connectors on there. It's not going to be used in this project. End of. Gary I am with you on the no chassis return route. When I put the system in Lil Blue, I went with some good advice on that in terms of potential leakage. Keep the system completely closed loop. I don't want to end up draining the main batteries thinking that I am using my LB!!

I don't have a problem with the whole circuitry thing that's easy. It's the energy going down it that always has me foxed. The relay needs to be sited near the main battery or it will be affected by the voltage drop in the main cable I think, Steve. The instructions say that this is where it needs to go, anyway.

Cheers guys.

Sorry what sized cable should I use? :doh:

Oh stop it!

Chris
 

Rob

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Gary thanks for posting that amazing explanation now I remember almost everything. One thing, to work out the resistance of a copper cable you can use the following formula

R=rho*l/A

Where R is resistance in ohms
Rho is resistivity in Ohm meters
l is length in meters (use 1 if you want to work out resistance per unit length (in meters in this case))
and A is area in m^2 (to convert mm^2 to m^2, divide by 10^6)

Resistivity of copper at 20 deg c is 1.68×10^?8 ohm meters (I would check this for temperature extremes).

Remeber to check your units!!! Also ^ means to the power of.

Source: http://en.wikipedia.org/wiki/Resistivity
 
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