# Why does jump starting a car not burn the battery?

## Main Question or Discussion Point

So I have a good car battery at around 12 V and a bad car battery discharged to 6 V. I hook up the batteries (sitting in the cars) with jumper cables. Now, before I turn the ignition switch on the dead battery car, there is effectively a circuit between the the good battery and the bad battery. The wire resistance and battery internal resistances are negligible, won't that mean a very large current will follow from one battery to the other creating a dangerous situation?

Thanks,

## Answers and Replies

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berkeman
Mentor
Overall a dangerous situation, yes. A lead acid car battery that measures 6V is very dead, and may not respond to being recharged. Can you just recycle it and buy a new car battery?

Also, turning on the ignition switch in the dead battery car is not the issue. It's connecting the good battery to the bad battery with jumper cables. When you connect the negative jumper cable (which you do last, right?), that's when the high charging current starts to flow...

anorlunda
Mentor
Yes you can get a lot of current and it could be dangerous. However, the dead battery is more likely to be at 11 volts or better.

Also, when you get 100 amps or so flowing, the internal resistance and the jumper cable resistance aren't so negligable. You could easily get 100 amps for a short time, but I don't think you could get 500 amps.

Thanks for the quick replies.

berkeman: What I was trying to say is that when the dead car is being started, the starter resistance will be even smaller than the internal resistance of the dead battery so the current should flow from good battery to the (bad) engine instead of into the dead battery

anorlunda: the internal resistance of a car battery should be around 0.001 ohms. My standard 12 feet 8 gauge wire should have resistance 0.008 ohm. Together their resistance is only 0.01 ohm. The voltage difference is 6 volts which would lead to 600 amps! But I could be wrong. I heard somewhere that internal resistance increases as a battery is discharged.

I guess before people jump start another person's car, they should really test the voltage terminal of the "dead" battery for exactly this reason.

By the way, after another day I measured the dead battery and it went from 6 volts to 0.08 volts.

256bits
Gold Member
anorlunda: the internal resistance of a car battery should be around 0.001 ohms. My standard 12 feet 8 gauge wire should have resistance 0.008 ohm. Together their resistance is only 0.01 ohm. The voltage difference is 6 volts which would lead to 600 amps! But I could be wrong. I heard somewhere that internal resistance increases as a battery is discharged.
That's not how it works.
You still need a certain amount of voltage to reverse the chemical reaction within the battery to charge it up again.
So if you hook up a "dead" battery to a fully charged one, very little current should flow.
Unless of course there is a short within the dead battery, in which case you would use that resistance to get a high current.

meBigGuy
Gold Member
Basically what happens with a very dead car battery is that the battery seems to charge very quickly, but has no capacity. The battery only draws current to store energy as charge.

The low series resistance you mentioned is a discharge phenomena for a healthy battery.

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CWatters
Homework Helper
Gold Member
In other words the 6V battery won't stay 6V. As soon as you connect the 12V battery the voltage will rise very rapidly.

jim hardy
Gold Member
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The wire resistance and battery internal resistances are negligible, won't that mean a very large current will follow from one battery to the other creating a dangerous situation?
Depends on the state of discharge.

Think about what's happening around the battery plates
Current has to flow through the electrolyte by ion migration
In a battery that's really badly run down the electrolyte is depleted of ions and the plates are coated with sulfate, both of which increase its resistance.
As you see, at 1 volt per cell electrolyte molality is quite low. Few ions left to conduct.

That's why your charger when connected to an absolutely dead battery initially shows low current .
A battery that's not so badly run down will accept more current.
That's where you can get into trouble with sparks and hydrogen.
I had a battery explode in my face once - an internal intercell connector was cracked raising internal resistance. Thinking it was just badly run down I charged it overnight . That made hydrogen.... When daughter hit the starter, "Ka-Pow" the whole top of the battery flew by my head , acid spattering my shirt pants and face. Ruined a good pair of dockers and a dress shirt. Good thing i was wearing glasses....
With the top gone i could see the cheap internal construction of that battery. High current to the starter had melted the last sliver of that cracked intercell connector and ignited the hydrogen.
Lesson - dont scrimp on a car battery. .

Here's an interesting link...
volts vs molality plot came from there

old jim

dlgoff
Gold Member
Depends on the state of discharge.
A very good article. I've always liked http://batteryuniversity.com/learn/. They have a neat New Lead Acid Systems article where Carbon is added to the negative plate.

Scientists have known for years that sulfate accumulation prevents the classic lead acid from delivering sustained performance; partial charge and aging are the main culprits. The Advanced Lead-carbon (ALC) solves this by adding carbon to the negative plate (cathode). This turns the battery into a quasi-asymmetric supercapacitor, improving charge and discharge performance.

anorlunda
Mentor
anorlunda: the internal resistance of a car battery should be around 0.001 ohms. My standard 12 feet 8 gauge wire should have resistance 0.008 ohm. Together their resistance is only 0.01 ohm. The voltage difference is 6 volts which would lead to 600 amps! But I could be wrong. I heard somewhere that internal resistance increases as a battery is discharged.
The dead battery could be at 6V, but I said it is more likely to be around 11V. That is especially true if you judge it as dead just because your car won't start.

(12V - 11V) / 0.01 ohms = 100 amps which is what I said you could get for a short time.

Besides, you seem to have neglected. CWatters.

In other words the 6V battery won't stay 6V. As soon as you connect the 12V battery the voltage will rise very rapidly.

also keep in mind that as you connect the cables most of the current runs from the good battery into the cables then into the dead battery terminals, but it doesnt go into the dead battery most of that current just follows the leads attached to the battery and into the starter motor, but it only goes into that motor once you turn the key and the solenoid moves and connects the incomming terminals, so when you hook up the cables themselves to the deadbattery only as much current flows as the dead battery consumes,

I can't give a fancy formula but having helped out many times to other poeple with these cables I can say there isnt an awful amount of current flowing into dead battery's themselves.only a few times i have even got a decent crack and a small spark once i connect the second cable connector and complete the cirucit.

now I assume there are cases when the dead battery is not simply dead but is close to short circited or something along those lines then it might drawn a dangerous amount of current and destroy the good battery that is being attached to it.but then again 256bits already mentioned that in post #6

I have a quick question.
I had a battery on mu car that was stoped almost 2 years.

So i started the car manually and i was able to get it cherged just enough to make it start.

3months ago i borrowed a good battery for a friend of mine. That old battery is now stoped and i measured 8v on it.

Ill give tje good one back. Do you guys think that is a good idea to put it those "battery suplements" and ride my car back home (250kms far) so it would recharge again?

Or should i just give up on that battery?

meBigGuy
Gold Member
Dead lead acid batteries suffer from sulfation. It's possible that a desulphation additive and a slow recharge will rejuvinate the battery, but I don't know how completely (I've never done it). My tendency would be to get a new battery since unreliable batteries are not worth the time.

CWatters
Homework Helper
Gold Member
I use lead acid batteries to charge model aircraft. Thefastes way to kill a lead acid battery is to fully discharge it and leave it like that. I ruined several that way. They are never quite the same. I now always recharge them asap after use and keep them on a float charger.

dlgoff
Gold Member
Do you guys think that is a good idea to put it those "battery suplements" and ride my car back home (250kms far) so it would recharge again?
Are you sure they are getting a full charge to start with? Maybe your charging device isn't providing a solid enough voltage?
... i measured 8v on it.
Maybe a rectifier in the charging circuit has been damaged?

jim hardy
Gold Member
2019 Award
Dearly Missed
3months ago i borrowed a good battery for a friend of mine. That old battery is now stoped and i measured 8v on it.
a weak alternator can give confusing symptoms. It'll make a few amps which will charge your battery when you are not using accessories like headlights and heater/airconditioner that draw considerable current. So the symptom is your battery will be run down after driving at night or with heater/AC fan running, but a daytime trip without them will charge it back up.

The test is to run the car with a voltmeter attached(many cars have one in the instruments) and read battery voltage.
It should read between 14 and 15 volts and hold there as you turn on headlights, heater/AC fan, rear window defroster, seat warmers, any other high power loads......

Two causes of weak alternator output are
1. worn brushes inside the alternator (cheap and easy fix if you can find new brushes)
2. loose fan belt - more common than you'd think in days before advent of modern serpentine belts with tensioner.

Run that voltage test . If you find weak alternator output , fix that and see if your battery comes back to life.
Lacking a voltmeter - go to a dark road at night and stop the car edit - but leave engine idling. Observe how bright are your high beam headlights. Turn on heater/AC , windshield wipers, any other loads. If lights dim perceptibly have a battery shop check your charging system. If fan belt starts screeching - it's loose.

EDIT Late entry - if you have HID(xenon) or LED headlights the dark road test won't work - find a voltmeter or battery shop. Most Walmarts can do the check.

old jim

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6 Volts on a 12 volt car battery is not only a flat battery, it is a failed batter we used to use a rule of thumb that 10.1 (Open circuit) volts is the minimum a car batter should even be, any lower means it has failed and should be replaced.

It also could be quite dangerous to dump a lot of power into such a batter.

A 12v battery at 6v will likely take very little charge amperage from the fully charged battery.

.

meBigGuy
Gold Member
Just to reiterate what I think I said previously. When a lead acid battery degrades from being at low charge for a long period of time, it suffers from sulfation. The effect is that it takes very little charge to make it appear charged, and it discharges equally rapidly. In other words, its capacity drops --- a lot.

See http://batteryuniversity.com/learn/article/sulfation_and_how_to_prevent_it and 100 other pages on "lead acid sulfation".

Low voltage does not mean low resistance to charge currents.

Now, if the low voltage occurs for other reasons (physical damage or shorted cells) then bad things can occur when charging or jumping.