# Technical/Basic question about jump starting a car



## amodoko (May 29, 2011)

Kind of a basic question about jump starting a car. I know the process but want to understand something about safety. I know the process goes that you hook the positive of the good battery to the positive of the bad battery, then the negative of the good battery to the engine block on the car that is dead. I know you hook the negative on the engine block of the dead battery instead of the negative post of the bad battery because the jumper cables are live and you don't want a spark near a battery that may be venting flammable gases which can cause an explosion. 

But why do you hook the negative of the good battery directly to the negative jumper cables instead of grounding that on the engine block too? Is that because it doesn't have the opportunity to spark because the circuit isn't completed until the last negative clamp is attached to the engine block of the dead car and thus, it can't spark unless you mess with it while the circuit is complete? I would assume it will still vent flammable gases so wouldn't it be safer to just hook the negative jumper cable to the engine block of the good car also? I would think that the circuit would still be complete and able to charge the dead battery if you had the negative cables hooked up to each car's engine block while the positives were attached to each battery's positive posts since the negative post is grounded... just like the engine block, right? 

Sorry, I'm up late and can't sleep and my mind was just wondering this. Hopefully it makes sense


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## gregzoll (Dec 25, 2006)

You could hook to a ground point on the car being used to jump the other. Only with older batteries, that you have the lids, and not working in a ventilated space, yes you could get a spark.


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## SuperJETT (Oct 27, 2012)

When connecting to the good battery's ground, there is no path for current at that point so no chance for a spark, plus you're eliminating any bad connections. On the dead car's side, you'd rather prevent a spark than eliminate a bad connection because a bad connection just means you won't start the car while a spark could be really bad. My grandmother's battery blew up when I was a kid, thankfully there wasn't anyone around the front of the car.


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## amodoko (May 29, 2011)

Thank you both for answering my questions, much appreciated


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## gregzoll (Dec 25, 2006)

SuperJETT, usually exploding batteries are caused by shorted plates inside, over voltage, or vented gasses from it, which all it needs is a spark to go kapow.


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## iamrfixit (Jan 30, 2011)

Take all the necessary connection precautions most definitely, but be aware a battery explosion can still occur even when you do everything right. 

I have had two different batteries explode while jump starting. Neither of those occurred while I was connecting the jumper cables. Both times the cables had been connected for a short time to allow the battery to charge a bit. Right after I began to crank the engine over; boom, the battery blew. The first time it was really a wonder nobody got hurt, several people were standing right around the front of the car. 

You can not believe how violently they blow up and how incredibly loud it is. It will leave your ears ringing for some time afterward and parts of the battery flew 40-50 feet from the car. The second time was on a skid steer; the battery was well contained inside the engine compartment and not accessible with jumper cables. The jumper cables were connected to remote terminals specifically for jump starts or charging. All that was left on both occasions was the bare plates sitting on the bottom of the case. The entire top and sides of the battery case and all the acid was completely blown away. The vaporized acid fills the air with a terrible sulfur smell, so thick you can taste it and causes a burning, tingling sensation on your skin. Not something I care to repeat!


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## amodoko (May 29, 2011)

Whoa, iammrfixit, that sounds scary. I definitely do not want that happening ever. I'm assuming when the first battery blew up on the car that you were cranking, it had blown up simply because of some sparking when the engine was cranked due to voltage drain/loss in the circuitry of the car's electrical system within the engine compartment that ignited the flammable gases? Or maybe there were shorted plates in the battery. I woudn't think it would be due to over voltage though (thanks gregzoll).

But regardless, that sounds a bit scary. My parents don't drive their cars much now that they are much older, and their new batteries tend to die a lot due to them not driving much, so they just have Triple AAA come to jump their cars a bunch since they have a membership that gets them unlimited roadside assistance. I bought them battery tenders, but after hearing this stuff, I think I would rather them just call Triple AAA and have those guys jump their cars because you can still have a battery explode on battery tenders if you do not set them up properly and in the correct order.


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## iamrfixit (Jan 30, 2011)

I would guess it was due to internal problems with the battery in both cases. Heavily discharged batteries emit a lot of extra gasses when they are rapidly charging. When connected to a running vehicle your dead battery is receiving a very fast charge, possibly as much as 100+ amp depending on the vehicle you are jumping from. Neither of these batteries acted quite right from the start. Normally a dead battery requires little more than clamping on the cables and turning the key. Unless it is extremely cold, the vehicle is severely flooded, or has other problems preventing it from starting normally it should take little more than a quick jump. None of these conditions were present yet the engines did not seem to crank right over with a jump. I suspected internal shorting, you never know when that might happen.

My wife drove our van to work one day and a few minutes later tried to run an errand, the van would not start. I got there about ten minutes later and it would not even light the dome light. Not normal for sure, there had not been time for the battery to discharge even if something were simply left on. I connected the cables and it arced a lot, seemed to really pull my truck rpm down. I quickly disconnected and checked the volts. I found it had almost zero volts then noticed the battery was very warm. When I tried to remove it, the case was so hot you could not touch it without gloves. The battery had seemed completely normal just minutes before when she drove to work. 

In the case of your parents I would definitely use the battery tenders. That will far increase the life of the battery over letting it discharge and jump starting. I use a Deltran tender on my motorcycle and it has increased the life of the $150 battery from lasting barely 2 years to 6 years and still going. Not just me, several friends with bikes have had similar experience. My charger came with a pigtail that can be installed so the charger is simply plugged in, no need to even access the battery or open the hood. These chargers are a very low amp float charge, only charging when the battery requires it. The charger can be left connected indefinitely and the battery will always be fully charged without overcharging. I guarantee the battery will last far longer this way.


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## amodoko (May 29, 2011)

That's some solid information, much appreciated bud


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## cleveman (Dec 17, 2011)

Don't take any chances.

Remember the ABC's of safety: Always Be Careful.

That being said, I would just push-start the car.


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## amodoko (May 29, 2011)

hahaha, manual transmissions, yes... but most modern automatics should not be push started due to the higher rolling speed requirements and the potential to damage the car... I believe modern automatics just aren't meant to be push started.


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## polarzak (Dec 1, 2008)

amodoko said:


> I believe modern automatics just aren't meant to be push started.


It can be done without any damage. There can be damage to the body of the vehicles while pushing ed (or towed) at a fairly high speed to to overcome the fluid coupling (transmission) between the engine and the wheels. The procedure is not for the faint of heart.


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