Discussion
Looking at changing the lead from Battery to Starter Motor on my S3, but what size (Amps) do I need?
Found one rated at 170 amps, but no idea if that is suitable.
Ring terminal at Starter is 8mm diameter and my attempt at measuring the length with a piece of string is 1600mm, though it was a struggle reaching inside engine bay whilst garaged.
Any prior knowledge welcome.
Found one rated at 170 amps, but no idea if that is suitable.
Ring terminal at Starter is 8mm diameter and my attempt at measuring the length with a piece of string is 1600mm, though it was a struggle reaching inside engine bay whilst garaged.
Any prior knowledge welcome.
50mm cable is needed
0.25 volts volt-drop is acceptable
calculation 1 shows 1.6 meters of 50mm cable at 250 amps current draw (doubtful your cars starter will draw that much current but it might) and the volt-drop would be 0.27 volts
calculation 2 shows 1.6 meters of 35mm cable at 200 amps current draw (there's a good chance that your cars starter will draw that much current) and the volt-drop would be 0.31 volts
0.25 volts volt-drop is acceptable
calculation 1 shows 1.6 meters of 50mm cable at 250 amps current draw (doubtful your cars starter will draw that much current but it might) and the volt-drop would be 0.27 volts
calculation 2 shows 1.6 meters of 35mm cable at 200 amps current draw (there's a good chance that your cars starter will draw that much current) and the volt-drop would be 0.31 volts
Voltage drop calculator never lies, it does show how far off the mark manufacturer cost cutting engineers are prepared to go
There is often a problem in finding a correct sized eyelet, it needs to snugly fit over the conductors before being crimped (can be a touch bigger if soldering) and also of the correct hole size to be a good fit over the starter motor solenoid terminal
Being a perfectionist, have often found a terminal of correct conductor size with a too smaller hole for the terminal, have often drilled an eyelet hole out for a perfect fit
Suffering the problems of adapting a terminal to fit has always proven to be worthwhile
There can be nothing worse than a burnt terminal and solenoid main positive stud that are difficult to get at
There is often a problem in finding a correct sized eyelet, it needs to snugly fit over the conductors before being crimped (can be a touch bigger if soldering) and also of the correct hole size to be a good fit over the starter motor solenoid terminal
Being a perfectionist, have often found a terminal of correct conductor size with a too smaller hole for the terminal, have often drilled an eyelet hole out for a perfect fit
Suffering the problems of adapting a terminal to fit has always proven to be worthwhile
There can be nothing worse than a burnt terminal and solenoid main positive stud that are difficult to get at
Anyone wanting to use a calculator. Please read the instructions first.
https://www.12voltplanet.co.uk/cable-sizing-select...
Voltage Drop Calculator
Select a supply Voltage (V): 12
Enter the current draw in amps:
300
Select the cable size in mm2 : 35
Enter the one-way cable length to the load in m:
0.8
Click this button to calculate the voltage drop:
Drop in volts is:
0.23
Drop as a % of supply Voltage is:
1.92
Generally acceptable max. voltage drop is around 3-4%
It is important to note that voltage drop occurs not only along the positive cable to the load but also along the negative return cable. When you enter the cable length as the 'one-way' distance to the load the calculator assumes (for simplification) that the return distance is identical, giving you a total cable length which is twice that of your entered value. In practice your return cable length might be much shorter as it will may be grounded to a nearby point on the chassis (at least in vehicles) , so the remainder of the distance back to the battery negative should have an extremely low resistance relative to a cable. In this case the actual voltage drop would probably be less than calculated, but it provides a 'worst case' figure to work with.
https://www.12voltplanet.co.uk/cable-sizing-select...
Voltage Drop Calculator
Select a supply Voltage (V): 12
Enter the current draw in amps:
300
Select the cable size in mm2 : 35
Enter the one-way cable length to the load in m:
0.8
Click this button to calculate the voltage drop:
Drop in volts is:
0.23
Drop as a % of supply Voltage is:
1.92
Generally acceptable max. voltage drop is around 3-4%
It is important to note that voltage drop occurs not only along the positive cable to the load but also along the negative return cable. When you enter the cable length as the 'one-way' distance to the load the calculator assumes (for simplification) that the return distance is identical, giving you a total cable length which is twice that of your entered value. In practice your return cable length might be much shorter as it will may be grounded to a nearby point on the chassis (at least in vehicles) , so the remainder of the distance back to the battery negative should have an extremely low resistance relative to a cable. In this case the actual voltage drop would probably be less than calculated, but it provides a 'worst case' figure to work with.
Edited by O mage on Sunday 7th June 10:09
Edited by O mage on Sunday 7th June 10:10
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