Lotus Elan

maximum negative cable length?

PostPost by: PAW50 » Fri Mar 15, 2013 10:51 am

Does anyone know if there is a maximum length the cable should be from the negative battery terminal to the earthing point on the chassis? I want to fit a battery isolation switch and I'm trying to decide the best place on a plus 2. I've read somewhere that if it's too long it may cause starting problems.
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PostPost by: oldelanman » Fri Mar 15, 2013 11:39 am

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PostPost by: markcs » Fri Mar 15, 2013 1:05 pm


In practical terms, adding a short length of additional cable (say around 0.5m) will not cause any issues at all. However please make sure that all your connections in your power and return cables are really well made.

If you would like some technical detail to support the above statement:

Assumptions. You are using copper multistrand cable of approx 40 sq mm in cross sectional area, additional length is 0.5m, resistance of this length of wire is approx 0.0002 ohms, starter current is approx 150amps (not sure this is right, but close enough - must get the scope and clamp meter out to measure it one day!) and your steady state current draw is around 20amps (only really relevant if your alternator has failed and the battery is providing your backup up power, but include for completeness).

Starting the engine. Voltage drop due to additional cable will be around 0.03v. No problem here as long as all your cable connections are in fine fettle (a poor connection can be several times the resistance of your additional 0.5m of cable). Also worth bearing in mind that your 12v starter motor is not designed to work at 12v! The designer will have assumed a circuit voltage drop and designed the motor to work at around 10 - 10.5v.

Steady State. Voltage drop due to additional cable will be around 0.004v. Again no issue here.

Hope this helps.

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PostPost by: andyhodg » Fri Mar 15, 2013 3:55 pm

Hi All

I know these electrical discussion can get really pedantic and I don't want to start a long discussion chain.

The only observation I would make is that if for instance a substantial cable of around 40 mm2 was used to directly connect the battery earth to the starter motor the overall resistance across the earth path would probably be less than that across the original earth path to the starter motor given at least two flexible leads, 4 connections and the chassis itself.


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PostPost by: MickG » Fri Mar 15, 2013 4:18 pm

Why not isolate the positive supply?

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PostPost by: john.p.clegg » Fri Mar 15, 2013 5:18 pm

Go on then I'll have a go,there's been plenty of chat about this recently...here's what I did,the cable from the battery runs behind the panel,just cut into it and fit the switch....

John :wink:
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PostPost by: PAW50 » Fri Mar 15, 2013 8:36 pm

Thanks everyone for the info. I had been thinking of using the rear foot well and from the picture it seems ideal
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PostPost by: bobm3142 » Tue Mar 26, 2013 7:57 pm

My S3 has an isolator fitted (not by me) and it is perfect. It is in the positive from the battery and located in the passerenger side just under the door. It has a removable key (huge chunky thing. I guess specifically made for this purpose) which accesses the isolator through a hole in the side wall (through the carpet). It is virtually invisible and would never be found by someone who didn't know it was there. I think that I would rather put the isolator in the positive supply since it is not so easy to bypass and leaves the earthing as standard.

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