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Author: Subject: Battery cable sizes
omega0684

posted on 25/4/15 at 08:31 AM Reply With Quote
Battery cable sizes

Hi guys,

I haven't posted for a while as life has been a bit manic but I have finally managed to get back in the garage and I'm hoping to get the car on the road again this year. As part of the upgrades I have had to move the battery into the bottom of the passenger footwell as so my battery cables no longer fit. I have measured the length of the new cables needed which measure 52 inches for the positive cable (battery to starter) and 38inches for earth cable ( battrey to chassis). I'm running the standard pinto starter motor and the the odyssey pc545 battery.

My old battery cables were 25mm2 but with the extension in length of the new leads how do I now know what sizes I need. After quick google search and uses of "battery cable calculators" say that I should be using 40mm2 but I don't know if this is correct. Could do with some help understanding the electric calculations please?

All the best

PintoBoy





I love Pinto's, even if i did get mine from P&O!

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britishtrident

posted on 25/4/15 at 11:08 AM Reply With Quote
You would probably get away with 25mm^2 starters on 2 litre engine generally draw less than 100 amps continuous but as the Pinto can suffer from slow cranking due to static ignition advance 35mm^2 welding cable might be a better choice. Welding cable is more flexible than starter cable .so is easier to use in bigger diameter sizes. Welding cable is black only.
Use a short length of red sleeving at each end to differentiate the cables. No

[Edited on 25/4/15 by britishtrident]





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wilkingj

posted on 25/4/15 at 11:30 AM Reply With Quote
Welding cable is good. Have done my Motorhome batteries in it.

I got Red and Black in welding cable. Have even seen it in Yellow!
If you can only get Black, then put some Red tape or a heat shring sleve over the ends, and at odd intervals on the cable before installing it.
Welding cable is more flexible and easier to run.

Can't think that 25mm wont be big enough for a 2 litre car. Its what I used on my V8 Viento.
35mm would leave less room for errors!

Get the correct sized lugs and crimp the cable ends.
I put my cable in split convoluted tubing and secured with stainless rubber covered P clips.


Geoff

[Edited on 25/4/2015 by wilkingj]





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coozer

posted on 25/4/15 at 12:47 PM Reply With Quote
What about the boost cables you see in services?

Two flavours if I'm correct with the most expensive being 400amps.

They any good as a source of cheap cable?





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Smoking Frog

posted on 25/4/15 at 04:00 PM Reply With Quote
quote:

Could do with some help understanding the electric calculations please?


If you want to understand the calculations this may help.

Calculate the minimum CSA (cross sectional area) for automotive thin wall cable.
You need to know four values to work this. Length (Metre), Volts, Acceptable Voltage Loss (%), Amps

Resistivity of copper is a constant value and here it's expressed as Ohms per mm²
Resistivity = 0.0185
Length = 1.5
Volts = 13.85
Acceptable Voltage Loss = 2%
Amps = 45

First convert acceptable voltage loss percent to voltage.
AVL = Volts * Acceptable Voltage Loss / 100
Plug in our numbers
AVL = 13.85 * 2 / 100 = 0.277

Calculate minumum CSA
CSA = Length * Amps * Resistivity / AVL
Plug in our numbers
CSA = 1.5 * 45 * 0.0185 / 0.277 = 4.5
Minimum CSA = 4.5mm²

Always select a cable using CSA and amp rating.
Looking through manufactures list of thin wall cable there is one with a CSA of 4.5 but it is only rated as 42A. So the one to get would be
the 6.0mm² which is rated at 50A.

Now you may say why go to the trouble of calculating the CSA when common sense would suggest a cable greater than 42A. Well maybe another
example with a cable length of 2.5m will make it clear. The only value that has changed is the length.

CSA = 2.5 * 45 * 0.0185 / 0.277 = 7.5
Minimum CSA = 7.5mm²
Looking through manufactures list of thin wall cable the one to get would be the 8.5mm² rated at 63A.

Can't say I'm a expect on cable selection, but I needed to write a program based on above calculations. Please let me know if you find a mistake.

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britishtrident

posted on 26/4/15 at 06:50 AM Reply With Quote
A couple of years back I measured the cranking current for a couple of engines with a DC clamp Meter.

Ambient temperature 15c
Both engines had been run that but had been that day but had cooled off for at least 4hrs

1.4 k series. Current 64 amps. Cranking Voltage 10.5. Type 045 battery 380cca
1.8 k series Current 85 amps. Cranking Voltage 11.0 Type 096 battery 680cca

It would be interesting to get more results from different engines.





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owelly

posted on 26/4/15 at 08:45 AM Reply With Quote
I found a set of good quality jump leads at my local motor factors for £20. They were made from good quality fine strand copper with two coverings of supple rubber. Red and black. And 3m long!





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coyoteboy

posted on 26/4/15 at 10:12 AM Reply With Quote
The only two I've measured were a 600cc bike engine (peak 100, constant 40) and my 3SGTE which peaked at 220 and fell to 120 constant.

Average quality clamp meter on unknown condition batteries but no starting issues so assume nominal.





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steve m

posted on 26/4/15 at 12:45 PM Reply With Quote
It may be a silly question but why is the neg lead so long at 38"
surely that's what the chassis is for, so two short leads one from battery to chassis, and one from chassis to engine

steve





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