Lotus Elan

Main battery cable size and earth return size

PostPost by: pauljones » Tue Feb 19, 2013 7:25 pm

Hi all,

Im now replacing the main black power cable that runs from the battery to the solonoid, and then from there to the starter. Then starter back to the battery.

Can anyone tell me what size I will require,my earth from the boot stud to battery terminal is 16mm2. Is that OK,or do I need bigger?

At some stage I will put in a battery cut off but dont know where yet, Has anyone fitted one recently.

Many thanks,Paul
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PostPost by: oldelanman » Tue Feb 19, 2013 8:16 pm

pauljones915 wrote:At some stage I will put in a battery cut off but dont know where yet, Has anyone fitted one recently.


Hi Paul,
Some discussion here which might help....elan-mods-f31/battery-cut-off-switch-t27407.html
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PostPost by: Galwaylotus » Tue Feb 19, 2013 8:45 pm

The recommended maximum voltage drop for a starter motor main power cable is 0.5v in a 12v system. The maximum continuous current for a 16 sq mm cable is 80A. Maximum resistance per meter at 20 deg C for that cable is 1.16 milli-ohms. As V=IR, the calculated voltage drop at the max rated 80A for a 2m cable would be less than 0.2v so you should be ok. I always reckon bigger is better when it comes to cabling! :lol: All data courtesy of Bosch Motor-Vehicle Batteries and Electrical Systems 2003 Edition.
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PostPost by: Higs » Tue Feb 19, 2013 9:21 pm

The calculation above is slightly wrong. The max rated current of 80amps is the safe continuous current that the cable can handle. When starting the car., the current drawn will be 200amps or so. This will only be for a few seconds but suggests the cable is too small. Therefore get a bigger cable rated at at least double the 80amps.

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PostPost by: pauljones » Tue Feb 19, 2013 9:55 pm

Guys,

Thanks for the answers, This is how im reading it, 25mm2 will give me 200amps capacity while 35mm2 will give me 240 amps capacity. To be honest i looked on ebay at cables for this info. So i'll assume any thing between the two is ok.Now this is for the main power to the starter,via the solonoid.
Does the earth return have to be the same size or not?? On the understanding that it will go from starter to battery by a cable and not the chassis.

Paul
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PostPost by: Galwaylotus » Tue Feb 19, 2013 10:00 pm

2.4kW, 3.2hp! That's a heck of a big starter for a 1.6 litre Twink!!! The key point is "continuous" current. There will be an instantaneous current spike until the starter gets spinning but I can't see 200A for any reasonable period of time! A continuous current rating of 160A (twice 80) calls for a 50 sq mm cable.
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PostPost by: Galwaylotus » Tue Feb 19, 2013 10:02 pm

pauljones915 wrote:Guys,

Thanks for the answers, This is how im reading it, 25mm2 will give me 200amps capacity while 35mm2 will give me 240 amps capacity. To be honest i looked on ebay at cables for this info. So i'll assume any thing between the two is ok.Now this is for the main power to the starter,via the solonoid.
Does the earth return have to be the same size or not?? On the understanding that it will go from starter to battery by a cable and not the chassis.

Paul

If the earthing is by cable the same rules apply as current is constant throughout the circuit. The voltage varies with the resistance.
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PostPost by: pauljones » Tue Feb 19, 2013 10:08 pm

Galawaylotus

2.4kW, 3.2hp! That's a heck of a big starter for a 1.6 litre Twink!!!

Im afraid I dont understand the figures, not an electrical guru at all.

Can you explain why 50mm2 and not 25mm2 or even 35mm2 please. Also does your last mean thesame size for both out and into the battery.

Paul
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PostPost by: TroonSprint » Wed Feb 20, 2013 12:24 pm

I agree with Paul that this is a confusing subject to those of us without much clue about electricity. I looked on the Vehicle Wiring Products web site (http://www.vehicle-wiring-products.eu/VWP-onlinestore/cable/batterycable.php) and they offer four sizes of battery cable, viz:
16 sq.mm, 7.5mm OD, 110 amps
25 sq.mm, 9.8mm OD, 170 amps
40 sq.mm, 11.8mm OD, 300 amps
60 sq.mm, 14.5mm OD, 415 amps

You can also use welding cable, the big advantage being that it is very flexible. On the same web page, VWP offer:
16 sq.mm, 110 amps and 35 sq.mm, 240 amps. Trouble is it is only in black. I bet red is available elsewhere.

So what would one use for an Elan?
Mike
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PostPost by: types26/36 » Wed Feb 20, 2013 4:27 pm

No connection .....(excuse the pun) but I used this when I replaced my S2's main cable:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... OC:GB:1123
(not sure if this should be posted in "items on other websites :roll: )
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PostPost by: RichardHawkins » Wed Feb 20, 2013 7:57 pm

I measured the cross sectional area of my main supply cable (S4 Elan) from the battery. I came up with an answer of 42 square mm (seems a funny size, probably 40). Brian Buckland advises that you do not use anything smaller, so I bought 50 square mm flexible (welding cable) from Automtive Electrical Supplies. The car is still in bits so I don't know whether this was necessary, or if I am just being cautious.

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PostPost by: elansprint » Thu Feb 21, 2013 9:09 am

I upped the size of the cable as the twink usually takes a bit of cranking to start & the volt drop does not help due to the long run to the boot. The touring car boys tend to use welding cable as it is flexible you can put red heatshrink on each end to identify it.

regards

Ian
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PostPost by: markcs » Thu Feb 21, 2013 10:38 pm

Hi, I hope the following helps:

There are three key aspects of sizing a cable:

1) The continuous current flowing through the cable.
2) The acceptable voltage drop
3) Whether or not the cable forms part of a wiring loom or is in free air

Taking each in turn:

1) More current through a cable equals more heat (due to its resistance). Many of the tables that you see on the internet are simply sizing the cable according to this limitation. However this is only the first step.
2) oldelanman is absolutely spot on with his description of allowable voltage drop. This is typically 0.5v. However I would argue that his calculaltion is perhaps slightly too optimistic as he uses 2m length of cable in his calculation (I think), I would double the volt drop he found (on my Plus 2 anyway!) as the length of cable you should use in the calculation is the length of the entire circuit (NOT the length of the wire with the chassis acting as the return).
3) If the wire is in free air, more can air can circulate around the wire and therefore keep it cool. If the wire is enclosed in a loom then you have to take this into account when sizing a wire as it will tend to heat up more.

If you take all this into account then the battery cable size needs to take into account the steady state current (don't forget that this is NOT necessarily the current that your car is drawing, as the alternator is providing this current in normal circumstances, but the maximum charging current that the battery will normally draw) and be able to ensure that the voltage drop will not exceed 0.5v along the length of the circuit at the increased current flow when starting the engine. A cable over around 32 sq mm should be absolutely fine, but there is no harm in going larger (apart from weight and cost!).

You do not need to worry about the absolute value of the starter current in any way if you use this recommended cable size, as it is a transient current and will not be applied long enough to cause any significant cable heating. Your limitation in this particular case is actually the duty cycle of the starter motor rather than the size of the cable.

If I could make one other point, the resistance of the battery cable is actually relatively small compared to the likely resistances of the cable terminations to the battery, the solenoid, the starter motor and the return wire terminations etc. If you consider ohms law V=I/R, the volt drop across a resistance increases proportionately with an increase in current. If you therefore have poor quality connections, then these can result in a significant additional volt drop at high starter currents. You can gain some idea of the implication of this if you consider that your battery cable (as explained by oldelanman) can have a resistance of around a thousandth of an ohm per metre, whilst one poor connection could be half an ohm or so! At low currents this is not quite as important as the resulting volt drop will be rather less and explains the symptoms of electrics working fine when driving but having very poor starter performance.

So for starting purposes, what is really important is the critical quality of all your connections rather than worrying about a few extra sq mm of cable size!! Do bear in mind as well, that batteries can have significant internal resistance. This is why I would always recommend you consider fitting an AGM battery rather than the traditional wet battery as they do have a significantly smaller internal resistance (in addition to having a very small self discharge rate), something like an Odyssey PC680 would be ideal for a Lotus.

Welding cable makes excellent battery leads as it is highly flexible, being made up of hundreds of small strands and its insulation is also incredibly tough.

Apologies for the long winded explanation, I hope someone finds it vaguely useful!

Mark
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PostPost by: oldelanman » Fri Feb 22, 2013 7:35 am

markcs wrote:2) oldelanman is absolutely spot on with his description of allowable voltage drop
markcs wrote:if you consider that your battery cable (as explained by oldelanman)

Hi Mark,
I would love to take the credit but it was actually Galwaylotus :lol:
Roger
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PostPost by: pauljones » Fri Feb 22, 2013 4:50 pm

Mark,

Many thanks for your informative reply, I have taken the points on board from all the posts. I will be off to shops tomorrow for a look at cables and terminals. Hopefuly get what i need. Thinking welding cable as the first choice but ill see whats available. Ill be using battery terminals with a wing nut fitting to hold them still. As for a new battery,Not sure the funds will allow it.

Thanks for allyour help guys,

Paul
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