Why cheap jumper cables won’t start your car

Why your car won’t start with cheap jumper cables

Cheap jumper cables can’t start your car because they have too much voltage drop

Wire thickness, wire metal type, and cable length all have a direct bearing

Here’s a set of inexpensive jumper cables—$10.49. Now scroll down to the chart and see what voltage they deliver to the dead vehicle.

on how much power the cable will transmit to the dead battery and starter. Wire gauge numbering is inverse, so the smaller the number, the thicker the wire. Thicker wire has less internal resistance so it carries more current. But current carrying capacity is also affected by jumper cable length. The longer the cable, the more internal resistance and the lower the amp carrying capacity. We’ll come back to that in the chart below. But first, let’s take a look at how much power a starter motor really needs.

How many amps does a starter motor need?

A new starter draws 60 to 150-amps WITH NO LOAD. That’s free-wheeling, not connected to the engine. Most car batteries can handle that without breaking a sweat. Now let’s connect it to a real engine.

You may think a 4-cylinder engine doesn’t need as much cranking power as a large 8-cylinder since the starter in the smaller engine doesn’t have to as much mass. However, since a 4-cylinder engine doesn’t have as many opportunities to spark or ignite the air/fuel mixture per revolution as a 6-cyl or 8-cylinder, it must rotate FASTER. So really, it’s very possible for a 4-cylinder engine to draw as much current as a larger engine.

At around 80°F a typical starter motor draws 150 to 200-amps

That same engine at -4°F draws 250 to 350-amps

Cheap jumper cables let you down when you need them most

Nobody wants to spend a lot of money on jumper cables they’ll only use once in a while. But what good are cheap jumper cables that don’t work when you need them?

Here’s the bottom line: The amount of power jumper cables can carry is directly related to the wire gauge and cable length. If you buy cheap cables and they don’t carry enough current, you can’t start your car.

Here’s how much current jumper cables can carry

A 2 gauge, 20 feet jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 11.036 volts or an 8.03% drop
A 2 gauge, 20 feet jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 10.394 volts or a 13.38% drop
A 2 gauge, 20 feet jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 9.751 volts or an 18.74% drop=GOOD LUCK!
A 4 gauge, 20 feet jumper cable set connected to a starter that’s drawing 150-amps  will drop voltage to 10.468 volts or a 12.77% drop
A 4 gauge, 20 feet jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 9.446 volts or a 21.28% drop= NO START
A 4 gauge, 20 feet jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 8.425 volts or a 29.79% drop= NO START
A 6 gauge, 20 feet jumper cable set connected to a starter that’s drawing 150-ampa will drop voltage to 9.562 volts or a 20.32% drop= NO START
A 6 gauge, 20 feet jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 7.937 volts or a 33.86% drop= NO START
A 6 gauge, 20 feet jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 6.312 volts or a 47.4% drop= NO STARTn

An 8 gauge, 20 feet jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 8.126 volts or a 32.28% drop= NO START
An 8 gauge, 20 feet jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 5.543 volts or a 53.81% drop= NO START
An 8 gauge, 20 feet jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 2.96 volts or a 75.33% drop= NO START
A 10 gauge, 20 feet jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 5.83 volts or a 51.35% drop= NO START
A 10 gauge, 20 feet jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 1.73 volts or a 85.581% drop= NO START
A 10 gauge, 20 feet jumper cable set connected to a starter that’s drawing 350-ampa will drop voltage to -2.37 volts or a 119.82% drop= NO START

What about 30-ft jumper cables?

A 2 gauge, 30-ft jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 10.55 volts or a 12.05% drop
A2 gauge, 30-ft jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 9.59 volts or a 20.08% drop= NO START
A 2 gauge, 30-ft jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 8.62 volts or an 28.11% drop= NO START
A 4 gauge, 30-ft jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 9.70 volts or a 19.16% drop=GOOD LUCK!
A 4 gauge, 30-ft jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 8.169 volts or a 31.93% drop= NO START
A 4 gauge, 30-ft jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 6.637 volts or a A 44.69% drop= NO START
A 6 gauge, 30-ft jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 8.34 volts or a 30.47% drop= NO START
A 6 gauge, 30-ft jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 5.90 volts or a 50.78% drop= NO START
A 6 gauge, 30-ft jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 3.46 volts or a 71.09% drop= NO START
An 8 gauge, 30-ft jumper cable set connected to a starter that’s drawing 150-amps will drop voltage to 8.126 volts or a 32.28% drop= NO START
An 8 gauge, 30-ft jumper cable set connected to a starter that’s drawing 250-amps will drop voltage to 5.543 volts or a 53.81% drop= NO START
An 8 gauge, 30-ft jumper cable set connected to a starter that’s drawing 350-amps will drop voltage to 2.96 volts or a 75.33% drop= NO START

Get the point? Cheap small gauge jumper cables will have a very hard time starting a car with a dead battery.

ANSWER from photo above: A 10 gauge, 12-ft jumper cable set connected to a starter that draws 150-amps will drop voltage to 8.30 volts or a 30.81% drop in power= NO START.

©, 2019 Rick Muscoplat