My lead from the battery to the starter is over 3 metres long, should I expect any cranking problems with this length? Would it be worth running a second lead?

It is all about perceived voltage drop.

What size is your cable?

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.4066&voltage=12&phase=dc&noofconductor=1&distan ce=1&distanceunit=meters&amperes=200&x=38&y=8

The starter cable in an MX5 is probably longer than that and in terms of rear mounted batteries, they're probably both considered short.

What size cable are you planning? 16mm² should be plenty but if you're worried, I can't imagine the price difference to 25mm² would be night and day, just harder to route and terminate.

don't forget the 'earth' return path is just as long, and a steel chassis isn't the best conductor. maybe worth putting in an explicit cable for that too.

Depends what gauge cable you used. There will also be a volt drop accross the master switch so use a good quality one .
You can measure the cranking voltage drops with a multimeter or better an oscilloscope by measuring while cranking at battery then at the starter.

For normal cranking the voltage at starter while cranking with a good battery should be about 10.5v
Normally with long run of cable use one up in cable size from what you would normally use.

[Edited on 8/3/21 by britishtrident]

[Edited on 8/3/21 by britishtrident]

One lead of sufficient cross section should suffice.

Ensure that the connector is crimped on tightly. You want to emulate something like the tool below but it can be achieved with a block of steel and a rod. Unfortunately I cannot find photo of me doing it so a description has to suffice.

1. Saw a V in the block and put in a vice.
2. Insert the crimp into the V and on the opposite side place a small rod (can be a bolt cut to length).
3. Tighten up the vice. The rod will be pushed into the crimp and reduce its diameter with the V preventing the crimp from flattening it out.

You can buy proper battery/welding cable crimping tool for about £18 to £30 on ebay I use the type you hit with a hammer but there are also cheap hydraulic ones

[Edited on 8/3/21 by britishtrident]

quote:

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Originally posted by britishtrident
You can buy proper battery/welding cable crimping tool for about £18 to £30 on ebay I use the type you hit with a hammer but there are also cheap hydraulic ones

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I got one of these for doing the landrovers battery and winch leads, it cost about £20, worked perfectly and seems bomb proof. I'd sell you it but I need it for making leads for the robin hood. I bought the connectors on ebay. Just get a nice beefy cable and run it in the tunnel. Maybe put a fuse on the battery terminal just in case the cable got cut in a crash and shorted.

Battery cable on the Furore is around 3.5m long & I use the chassis for return path, never caused any problems with cranking with either car engine or bike. I also run the electric reverse on the bike engine variants from that same cable & it operates the "reverse gear" starter motor without problem

If using chassis for return path it would be wise to have the same earth strap diameter as the +VE cable

quote:

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Originally posted by snapper
If using chassis for return path it would be wise to have the same earth strap diameter as the +VE cable

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Starter ground to block.
Block connected to battery by cable the same size as positive feed.
Sensors grounded to block.
Oversize cable by 50% on max current.
Remember each connection is going to have a few tens of milliohms resistance and at 100A that's a volt dropped per connection.