Jump Start A Car

[iPrabhath]

Hello friends,

I recently saw that when we jump start a car, the negative terminals of the charged battery and dead battery are not directly connected(shown in attached image). Is there any specific reason behind this? What is wrong with connecting negative terminals directly?

[Crosswind]

AFAIK there's nothing wrong with connecting the negative to negative or to earth, IF the negative is properly earthed. If not, there is a chance of sparking. So to avoid that risk, it's better to connect to the body of the car.

[terrabytetango]

Normally, the final connection when hooking up two cars will cause a spark. Since lead acid batteries produce hydrogen as a by product when discharging, there might be a chance of the gas igniting, so as a precaution it is stated to make that connection to earth instead of the (-)ve terminal of the battery. I personally have never followed that rule and have hooked up red-red and black-black without any issue, but don't blame me if something explodes when you try it.

[Schiffer]

Normally, the final connection when hooking up two cars will cause a spark. Since lead acid batteries produce hydrogen as a by product when discharging, there might be a chance of the gas igniting, so as a precaution it is stated to make that connection to earth instead of the (-)ve terminal of the battery. I personally have never followed that rule and have hooked up red-red and black-black without any issue, but don't blame me if something explodes when you try it.

i too have jump started before like that (red to red, black to black) and after the spark, nothing actually happens.

but tbh i didnt knew about this earth thing before.

guess from now on, im going to earth just to be on the safe side.

[harshansenadhir]

I'd like to see someone following this guide on a valkswagen beetle or anyother positive grounded vehicle.

[MasterDon]

Connect the negative wire(on the dead car) to a bare metal part like the engine block itself.Anything else is not recommended,and always remember to attach the negatives last and to remove them first.

[Sylvi]

Members,

Any one can jump start a car if the polarity of the jump starting vehicle is correctly connected.That means (+) Positive wire of the Jump start battery has to be connected to positive terminal of the Vehicle battery..( --) Negative wire of the jump start battery has to connect to Negative terminal of the Vehicle battery.

Mr.Harshan Senadhira's post says about Positive ground Vehicle. Polarity has to be examined before any one try to jump start a vehicle.

I came across a used Vehicle battery which was totally dead and it was in the port of Colombo few years back two mechanics Jump started the vehicle then there was a sound from the alternator and car started but alternator was not generating. Moment they removed the jump start cables engine stopped. Battery was a very big 120Amps 12 Volt battery which Mechanics had brought with jump start cables. They could not install that battery on the car due to the size was different. The two mechanics were not electricians.

I examined this vehicle the fault was the mechanics started the vehicle with changed polarity due to this the alternator diodes got burnt off. later I got that owner to get a another battery of the existing size and fix the same and started the car had to replace the diode plates on the alternator. All diodes were cracked from the plates.

If polarity is changed some times electronic circuits can damage, if they are not build with polarity change safety is installed by the manufacture on the electronic circuits, most electronic circuits today will work only on correct polarity.

About the spark when the jump start cables are fixed that is a normal thing due to existing battery do not have correct voltage because it is discharged and jump start battery has the correct voltage.

Sylvi Wijesinghe.

[Reckless]

Is it normal that when the starter motor cranks the battery voltage drops to about 9V then jumps to 14V when the car starts and alternator kicks in?

Edited November 3, 2011 by Reckless

[iPrabhath]

Members,

Any one can jump start a car if the polarity of the jump starting vehicle is correctly connected.That means (+) Positive wire of the Jump start battery has to be connected to positive terminal of the Vehicle battery..( --) Negative wire of the jump start battery has to connect to Negative terminal of the Vehicle battery.

Mr.Harshan Senadhira's post says about Positive ground Vehicle. Polarity has to be examined before any one try to jump start a vehicle.

I came across a used Vehicle battery which was totally dead and it was in the port of Colombo few years back two mechanics Jump started the vehicle then there was a sound from the alternator and car started but alternator was not generating. Moment they removed the jump start cables engine stopped. Battery was a very big 120Amps 12 Volt battery which Mechanics had brought with jump start cables. They could not install that battery on the car due to the size was different. The two mechanics were not electricians.

I examined this vehicle the fault was the mechanics started the vehicle with changed polarity due to this the alternator diodes got burnt off. later I got that owner to get a another battery of the existing size and fix the same and started the car had to replace the diode plates on the alternator. All diodes were cracked from the plates.

If polarity is changed some times electronic circuits can damage, if they are not build with polarity change safety is installed by the manufacture on the electronic circuits, most electronic circuits today will work only on correct polarity.

About the spark when the jump start cables are fixed that is a normal thing due to existing battery do not have correct voltage because it is discharged and jump start battery has the correct voltage.

Sylvi Wijesinghe.

Will a car start with power supplied at reverse polarity?

For this to happen , there must be a rectifier in between battery and the starter motor.

Or else

At reverse polarity starter motor must turn in the reverse direction(clockwise or anticlockwise).Will engine start in the reverse direction?

[Sylvi]

iPrabhath,

During my Electrical carrier of few decades I never came across a rectifier between the starer motor cable and the battery.

Almost all starter motors are build with Field coil motors, self starter motors will never turn on opposite direction. there is a method of wiring to the Self starter motors to turn on clockwise and anti-clockwise. I have come across self starter motors on automobiles which turns opposite direction then motor is fitted to the engine from behind the engine gear box side. Normally the self starter motor is fitted from the engine side

Please note if a Self starter motor turns opposite direction the Bendix drive wheel which get engaged to the fly wheel to rotate the engine will not engage although the solenoid pushes the drive wheel it will not take the load. Because it is designed to take the load of the engine only to one direction as soon as the engine starts the drive wheel get push back with free wheeling.If not the self starter motor will run at very high speed and damage the total self stator motor. I have come across plenty of motors when the drive wheel get jammed and get locked the self srarter motor runs with the engine above said fault will be the final result.

Only motors with permanent Magnets will rotate clock vice and anti clock vice when you change the Polarity of the battery power leads. Or it has to be a different design with different winding on the motor

If you want any more information on changing rotation on motors please do post.

Sylvi Wijesinghe.

[Sylvi]

Is it normal that when the starter motor cranks the battery voltage drops to about 9V then jumps to 14V when the car starts and alternator kicks in?

'Reckless',

Yes you are correct in future please check Voltage at the battery terminals as well as the engine. That means when you operate the starter motor at battery it will drop few volts due to high discharge. One lead on the battery and check other voltmeter lead on the engine do operate the starter motor. the voltage drop has to be equal as on the battery and the lead on the engine.

If there is a difference in voltage it is necessary to have a earth from the battery heavy cable which can take a load of 200 amps.

( Welding cable with two terminals soldered or crimped) to the Engine. all cars come with engine earthed to the body but due to old age some cars have this problem.

Early days 4CV Renault cars had this problem in that car 6v Battery was in front, there was a long heavy cable runs to the behind engine to the solenoid of the self starter motor, then the earth cable was earthed to the body when the car was new it was OK,

After running few years there was a voltage drop so the modification what I did was to fix a bracket behind of the car and have the battery behind

With two short cables jointing the starter motor solenoid and earthing the body. As well as the engine with the cable which car had from front to rear which supplied power to the starter motor.

Sylvi Wijesinghe.

[Reckless]

'Reckless',

Yes you are correct in future please check Voltage at the battery terminals as well as the engine. That means when you operate the starter motor at battery it will drop few volts due to high discharge. One lead on the battery and check other voltmeter lead on the engine do operate the starter motor. the voltage drop has to be equal as on the battery and the lead on the engine.

If there is a difference in voltage it is necessary to have a earth from the battery heavy cable which can take a load of 200 amps.

( Welding cable with two terminals soldered or crimped) to the Engine. all cars come with engine earthed to the body but due to old age some cars have this problem.

Thanks for your answers Sylvi, think I may have a battery problem. My car starts only at the 2nd attempt in the morning. Starter motor cranks slowly at the 1st attempt and all the dash lights go dim. but in the 2nd attempt it start instantly and can start without any problem for the rest of the day. I checked the alternator with the voltmeter and the simple head light test and it seems ok.But today hoping to go to a battery joint to be sure.