Rick's Free Auto Repair Advice

Car won’t start even with a jump start

Why your car won’t start even with a jump start

I see this question all the time on auto forums. Their car has a dead battery and uses another vehicle to jump start the dead battery. But it doesn’t work and the want to know why their car won’t start with a jump start. Here are the most common reasons.

Corroded battery terminals prevent starting, even with a jump start

Those corroded battery terminals are most likely the reason your

severly corroded battery posts and terminals

These corroded battery terminals are killing this battery

battery is dead in the first place. Corrosion at the terminals/post causes high resistance and that prevents your battery from receiving a full charge. But you’re still wondering why the jumper cables didn’t start the car since you’re providing a direct connection to the battery terminals, right?

When you clamp the positive jumper cable to the positive terminal and wiggle it to get good contact, you have a direct connection to the starter motor. When you connect the negative jumper cable to a metal portion of the engine, you’re relying on the negative battery cable, terminal and battery post to complete the circuit. That’s where you get into trouble if the battery posts/terminals are corroded. In other words, you still have extremely high resistance in the circuit and that reduces current flow.

Short or dead cell in the battery prevents starting, even with a jump start

Let’s assume your battery posts and terminals are clean so we eliminate the corrosion issue. You connect the jumper cables and you still can’t get the starter to crank. All you hear is a click or nothing at all. If the car battery in the dead vehicle failed due to an internal short or bad cell, it creates such a huge drain on the jumping vehicle, that it can’t start your engine. In other words, all the jumping power is diverted to the short circuit inside the battery.

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

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

jumper cables

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 start 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

How much power can your jumper cables carry?

Vehicle computers need a minimum of 9.6-volts to operate and command the fuel injectors and ignition system. Starters really need a minimum of 10-volts to operate properly.

So the jumper cables you use have a direct affect on your ability to jump start the engine. Remember, you need AT LEAST 9.6-volts to start your engine. Plus, the listing below assumes that the battery terminal connections are CLEAN

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.

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.

©, 2019 Rick Muscoplatl only carry 500 amps

Posted on by Rick Muscoplat

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