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hensoni
Member Since: 01 Oct 2007
Location: Sleepy Somerset
Posts: 576
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You cannot (safely) use a thin gauge cable to limit current flow. Electrickery does not behave like water! The current rating for cable is mainly dependent on its cross sectional area and type of conductor (aluminium, steel, copper, etc).
If a 65Ah depleted battery is connected to a fully charged 110Ah battery with a thin cable, the cable will melt. Period. The low internal resistance of both batteries will allow a huge amount of current to flow. To safely carry the currents required, you should be considering a minimum of 25mm2 cable. Even 25mm2 is only rated at 60A continuous, so this is marginal for a completely depleted battery.
The other consideration is the voltage drop in the cable. This is dependant on the length of the circuit (including any return path) and the resistance of the cable. Voltage drop is dependent on current flow. While resistance is constant and it makes no difference to the current flow, the heat dissipated is Volts (dropped) * Amps and a long cable and a lot of current can make for a hot installation. A cable run to the back of the vehicle would therefore need a larger gauge cable than an identically performing circuit residing totally in the engine bay. This is why thin jump cables are useless, it's because the VOLTAGE at the receiving end is too low and the receiving battery does not receive sufficient voltage.
Edited - typos! Club missing my D3
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 11th Jun 2012 9:29 pm |
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ebygum32
Member Since: 03 Oct 2010
Location: yorkshire - London
Posts: 1590
 
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Last one I did on my camper van 5 years ago just used 30amp make break relay max cable size was 40 ish amps fused each side with 15 amp fuses then cable coming off switched live from behind ignition to small 15amp circuit breaker switch mounted on the dash, oh and a negative running to body.
No problems at all ran it for five years even when leisure battery was dead just pushed the switch on the dash energised the relay circuit made and all protected by 15amp fused, no burnt cable,
This was my own doing and I accept no responsibility if someone copies it and has an accident with it  Goodyear Duratracs
Pirelli ATR gone
GG AT Gone
RLD Spare wheel protector
Leeds - London weekly return
Club ctek mxs10
Yorkshire 4x4 Response
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| 11th Jun 2012 11:10 pm |
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NoDo$h
Member Since: 02 May 2006
Location: Finding new and exciting ways to milk badgers.
Posts: 19689
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Don't confuse luck with wisdom I know it's not considered "kind" to say no these days, but no. Just no, ok? And if it's not ok, still no.
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| 11th Jun 2012 11:19 pm |
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Moo
D3 Decade
Member Since: 13 Aug 2010
Location: Wiltshire
Posts: 14236
 
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Make your life easy - buy the TMax. i did and it's brilliant. I even installed the battery monitor to a Brodit phone cradle inside the car so that I could link the batteries with the push of a button.
PM me an email address and I'll send some photos. Easy installation with a great piece of kit.
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| 11th Jun 2012 11:30 pm |
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nighthawk
Member Since: 24 Jul 2010
Location: Malta
Posts: 1163
 
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Yep, almost all split charge systems a few years back in the Defender days were made from 30 or 40amp relays, even when competition winches and winch challenges were involved (some manual intervention was required before and after the actual winching to prevent the winch load melting the relay). There were times when it wasn't ideal and the system could really have put more current too good use, but nothing ever burned down or melted. Maybe there were other forms of protection involved (I never designed one), but anyway I can't see the owner changing fuses every time one battery gets depleted more than the other 
The X-Eng charger uses a 180A relay, but ONLY because it's designed to support having the winch connected to both batteries, hence some winch current will flow through the relay. But not everyone does it that way. Some prefer having a dedicated winch battery as mentioned above.
I will stand back from this discussion due to not being qualified enough . I'm no sparky, just someone who does a lot of research before deciding what's best for ME to do. I suggest everyone does the same. Go around and ask the professionals before spending cash on 2/0 cable and and 200A relays and fuses Dennis
2006 Discovery 3 TDV6 Commercial Manual Buckingham Blue
Last edited by nighthawk on 12th Jun 2012 6:26 am. Edited 1 time in total
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| 12th Jun 2012 5:35 am |
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ids
Member Since: 12 May 2009
Location: Herefordshire
Posts: 384
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I used the kit available from http://www.rawcomponents.co.uk/intelligent...ystem.html which I found out about on the dark side
It's a great kit, and fitted the D3 perfectly. I am using it with a Yellow Top to power my aux kit including Carputer.
Great service from the supplier as well. Did take some pictures but not got round to uploading them yet. There relay fits perfectly in the second battery box. All the other bits are high quality.
I topped it off with a good length of cable in plastic tubing for the run to the back, terminating at a 4 way fuse box in the jack compartment.
Ids
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| 12th Jun 2012 6:20 am |
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nighthawk
Member Since: 24 Jul 2010
Location: Malta
Posts: 1163
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Nice kit ids. Yet another relay with 80amp fuses... And nobody can deny there is no electrickery limiting current in this one... it all shows clearly. One 100amp relay with 80 amp fuses. The only thing that could be limiting current, the AWG6 cable, just like the one used in the traxide kits.
I bet my money that if you replace the AWG6 cable in that kit with thicker gauge cables, the fuses will blow when the state of charge between batteries is not the same... Dennis
2006 Discovery 3 TDV6 Commercial Manual Buckingham Blue
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| 12th Jun 2012 6:58 am |
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ids
Member Since: 12 May 2009
Location: Herefordshire
Posts: 384
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@nighthawk. Yep you are right as the kit used to come with 70a fuses and I did have one pop when I ran the battery too low. They now supply 80a fuses and claim this should cover it. In fact I had a long conversation with their MD about how and why they went. I'm happy with it although would be interested in a dash view of both batteries
Ids
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| 12th Jun 2012 11:30 am |
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methley
a.k.a. Big Hairy Ski Monster
Member Since: 07 Oct 2008
Location: Marshfield, South Glos
Posts: 2433
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I just blew my split charge relay, connected with 16mm cabled and a 200A mega fuse. It was the 100A/180A relay shown in the kit above. It's been absolutely fine for several years but what caused it to go was that I had jumped the car from the spare battery a few times after the alternator went so it must have been very depleted.
Once alternator replaced and everything connected back together everything worked fine for a few seconds until the relay gave out with a puff of smoke.
I've now put some charge back into the spare battery using a jumper cable and I await delivery of a replacement relay!
Weird thing is the spare battery has been flat before after a few days of running an Engel on full freeze. The relay coped fine with that. Andy
Fridge Slide for sale https://www.disco3.co.uk/forum/post2064622.html#2064622
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| 20th Aug 2012 10:07 am |
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drivesafe
Member Since: 23 Feb 2006
Location: Gold Coast, Australia
Posts: 867
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Hi mzpicg and nighthawk, thanks for the thumbs up and you both got very close to how my set up works.
The biggest risk anyone faces with a dual battery system ( split system ) is not a short circuit but current overload and a number of you have already found this out by the fact you have blown fuses and worst still, one was advised to use a larger fuse.
This is absolutely nuttso!
I use auto resetting circuit breakers to protect the circuit from both short circuits and current overloads.
You CAN NOT use fuses to get overload protection yet companies still provide fuses in dual battery systems.
If you deal with people who actually work in the industry and are not just sellers, you are more likely to get the correct info on how to set up properly and safely in the first place.
For example, the SC80-LR has a continuos current rating of 90 amps but the circuit is protected by a 50 amp auto resetting circuit breaker.
50 amp circuit breakers do not trip at 50 or 60 or even 70 amps straightaway so you can exploit this function in the way that a system is designed to be able to safely handle short high current situations but if the load exceeds the design limits, the system goes open circuit, whether because of current overload or a short.
It then resets and continues to charge, unless the high current overload or short persists.
With a fused system, you either continually replace fuses, if you even know they have blown, or you continually increase the fuse size, which results in other problems arising as posted above, such as stuffed relays/solenoids.
There is a hell of a lot more to putting together a safe and effective dual battery system than just throwing a few mismatched parts together.
Nasher, I know you are trying to get an economic split system set up, but you need to do a lot more research, particularly on the dangers of getting it wrong, before you will have what you are after. 2008 TDV8 RR Lux + 2009 D4 2.7
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| 20th Aug 2012 9:29 pm |
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Nasher
Member Since: 07 Mar 2009
Location: Clanfield, North of Pompey - UK
Posts: 2772
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drivesafe wrote:
Nasher, I know you are trying to get an economic split system set up, but you need to do a lot more research, particularly on the dangers of getting it wrong, before you will have what you are after.
Other things have got in the way of me doing this since my first post back at the beginning of June, and I must admit that after reading everything that was posted I decided I'd probably save up for a bit and go down the Traxide route, it will just have to wait until I've a couple of hundred quid about spare.
Nasher. Heaven doesn't want me, and hell is afraid I'll take over.
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| 21st Aug 2012 5:00 pm |
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countrywide
Member Since: 16 Sep 2007
Location: UK
Posts: 6019
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hensoni wrote:If a 65Ah depleted battery is connected to a fully charged 110Ah battery with a thin cable, the cable will melt.
It won't, as they are connected in parallel and not series. It could melt if you try to connect a load to it though. Also the AH has not bearing on what happens at all as that is the capacity of the battery not the ability to supply instant current. That's the difference between cranking batteries and leisure batteries.
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| 21st Aug 2012 5:13 pm |
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stevep
Member Since: 16 Apr 2013
Location: London
Posts: 91
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| Warning for future readers! |
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I realise this is an old thread, but I was browsing for a recommendation for a split-charge setup and came across it. I was a rather disturbed to read some of the ideas put forward here regarding the use of a thin cable as a current-limiting device. Of course, every conductor has a resistance, and thus causes a voltage drop across its length (which involves various factors such as length, voltage, current flowing, CSA of the conductor and so on.) The power 'lost' within it is dissipated as heat. In low power electronic circuits, resistors are indeed used as current-limiting devices, but the powers involved are small and thus little heat is generated. But in this context, with currents of tens of amperes (or more) involved, the power 'lost', (ie. heat generated,) could be huge.. Since P=I^2R, you could be looking at hundreds of watts- equivalent to a toaster element, say.
Thin wires have indeed been used for years as current-limiting devices... They're called fuses. They're designed (by the use of a suitably-thin conductor) to melt, and thus prevent any more current flowing.
Other uses for thin wires conducting high currents are heating elements. Not what we're after in a split battery setup!
I don't mean to disrespect anyone here, I just think dangerous misinformation needs correcting for future readers.
Please don't try to limit current by under-specifying cable cross-section!
PS, On the two-battery point, if two batteries are connected in parallel, one fully-charged and the other drained, current will flow until they equalise. Thus one battery is a current source, the other is a 'load'. If you use a wire so thin and/or long that it's resistance causes a power dissipation high enough to raise its temperature to the melting point of the conductor, it will melt (ie. act as a fuse.) Even if it doesn't get that hot, it can get hot enough to cause problems. Such as fires.
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| 26th Jun 2016 9:08 pm |
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