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Car battery

Q

Qacer

Platinum Member
I'm curious. If an engine is running and you take out the battery, would the car still remain running?

I'm curious because someone told me that their car died on the way home because the battery went completely dead. I thought the battery was just used to jump start the alternator, and technically, once the alternator is running, then you have a little generator in your car.

Any insights? Thanks!

 
My Maxima's battery died a few weeks ago and ran fine before that. My old Eclipse used to eat batteries (about every 2 years or so) and it would die at idle when the battery was dead.
 
Some will, some won't. My old "80 Ford truck could run off the alternator all day long with no battery. I've seen some that won't (can't remember what though). I haven't tried it on my 99 Jeep yet. Couldn't tell you what the deciding factor is though.
 
Yes, the car should keep running without a battery.

However, it's not a good idea to disconnect or connect the battery while the car is running due to the possibilty of power spikes.

"I'm curious because someone told me that their car died on the way home because the battery went completely dead."

They likely had a bad alternator, meaning the car was running on the battery alone, and the car quit when the battery ran down.

 
as soon as you jump it the alternator begins feeding juice back into the battery, at a much higher rate than what it takes to start your car.

as far as i know, your car runs wholly on the alternator after it starts, but like the other guy said, i wouldn't disconnect it while it's running.
 
Originally posted by: DaedalCipher
as soon as you jump it the alternator begins feeding juice back into the battery, at a much higher rate than what it takes to start your car.
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Not even close. A typical starter for a 4 or 6 cylinder engine will pull between 100 and 200 amps without a load on it and well over 250 amps when cranking the engine (large V8 engines can draw over 350 amps when cranking). The alternator will charge the battery with a couple amps at most. The alternator charges the battery at a much slower rate than the battery discharges during starting.

Originally posted by: DaedalCipher
as far as i know, your car runs wholly on the alternator after it starts, but like the other guy said, i wouldn't disconnect it while it's running.
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Once the engine has caught, yes, the alternator should be able to produce enough current to sustain the engine.

ZV
 
Assuming that your alternator is fully functional then the answer is definitely ...
 
Some new will, most won't. I've seen cars with perfect alt's die after disconencting the battery. Most older ones will as all they need to run is spark. The reason most newer ones won't as they are using more juice than the alt can put out coompared to what is stored in teh battery.
 
I would guess that the ones that don't run without batteries are due to the alternator using the battery to smooth out the power coming out of it, and no battery means crappy regulation. A battery with a shorted cell will also drag the alternator down.

Now, as ZV mentioned, the average car takes a couple hundred amps worth of power to kick over, this is why batteries are often rated in "cold cranking amps". Or the jolt it can give out for a short time to run the starter. Your typical alternator however is around 70-80 amps max. Trucks often have ones in the 100-160A range. But not all of that power goes to the battery, I would guess between the ignition, ECM, fuel pump, and other normal things in a car, 20A is taken before you turn the AC or your headlights on. Doing that probably takes another 30, which leaves 20A to charge up your battery after you start your car.

Oh yeah, and the reason why diesels have two batteries, is because 650CCA isn't going to kick over that kind of compression and warm the manifold heaters. Once they're running, you could happily disconnect one battery.
 
"The reason most newer ones won't as they are using more juice than the alt can put out coompared to what is stored in teh battery."

That would cause a charging system light on the dash to illuminate, imo.
 
Originally posted by: LTC8K6
"The reason most newer ones won't as they are using more juice than the alt can put out coompared to what is stored in teh battery."

That would cause a charging system light on the dash to illuminate, imo.
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Or a number of other lights to go out later down the road... 🙂
 
Originally posted by: LTC8K6
"The reason most newer ones won't as they are using more juice than the alt can put out coompared to what is stored in teh battery."

That would cause a charging system light on the dash to illuminate, imo.
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The light illuminates when the alt isn't charging above 12 volts, or at least 99.9% of the cars are that way. A battery can be below 12 volts and you will never see the alt light or if you unhook the battery and just run off the alter (older car) you will never see the light.

I don't know of anyone that had the alt light come on and it was telling them the battery was dead.



 
IIRC, the light also illuminates if current is flowing in the wrong direction. That is, from the battery. If the alternator were not supplying enough current to run the car, current would begin to flow from the battery, illuminating the charging system light, and telling you your alternator is not up to snuff.
 
Originally posted by: Demon-Xanth
Now, as ZV mentioned, the average car takes a couple hundred amps worth of power to kick over, this is why batteries are often rated in "cold cranking amps". Or the jolt it can give out for a short time to run the starter. Your typical alternator however is around 70-80 amps max. Trucks often have ones in the 100-160A range. But not all of that power goes to the battery, I would guess between the ignition, ECM, fuel pump, and other normal things in a car, 20A is taken before you turn the AC or your headlights on. Doing that probably takes another 30, which leaves 20A to charge up your battery after you start your car.

Oh yeah, and the reason why diesels have two batteries, is because 650CCA isn't going to kick over that kind of compression and warm the manifold heaters. Once they're running, you could happily disconnect one battery.
Click to expand...

No automotive designer in his right mind would set up a car's electrical system to charge an automotive battery at 20 amps though. You'd have electrolyte boiling off if the driver took a long road trip. A typical in-home "fast-charger" for an automotive battery will charge at 10 amps and IMO even that's pushing it. Most automotive electrical systems are designed to trickle-charge the battery enough to "top it off" by the time the car has been running for 20 minutes or so. I'd imagine that they're operating in the 2 amp range.
 
Originally posted by: Zenmervolt
No automotive designer in his right mind would set up a car's electrical system to charge an automotive battery at 20 amps though. You'd have electrolyte boiling off if the driver took a long road trip. A typical in-home "fast-charger" for an automotive battery will charge at 10 amps and IMO even that's pushing it. Most automotive electrical systems are designed to trickle-charge the battery enough to "top it off" by the time the car has been running for 20 minutes or so. I'd imagine that they're operating in the 2 amp range.
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My cheap charger has a 2A and a 6A setting.
 
The car's electrical system charges the battery similar to the way a battery charger does.

A 10A charger only charges at that rate when the battery is deeply discharged. The rate tapers off as the battery regains it's charge.

If the battery is deeply discharged, the alternator charges it at it's highest rate at first, then gradually tapers off the curent as the battery regains it's charge.

The current delivered to the battery by the alternator via the charging circuit is proportional to the state of discharge of the battery, limited to the system's maximum current rate, of course.

The alternator only provides the amount of current necessary to run the vehicle and charge the battery and no more. It responds to the demands proportionally. The battery is just another load on the alternator like the headlights. Large load when the battery is discharged, and small load when the battery is charged.




 
All dead batteries are not the same, AFAIK the battery is needed to complete the electrical circuit so even if the battery will no longer hold a charge as long as current still passes through it the alternator will keep the car running once its started.

On the other hand if the battery has a dead cell (internally shorted out) current from the alternator can't pass through so the car will die.
 
"On the other hand if the battery has a dead cell (internally shorted out) current from the alternator can't pass through so the car will die. "

IIRC, this will simply cause the remaining 5 cells to be overcharged by the alternator. The car should not die.

The cell would have to be open to stop current flow, and that would be the same as the battery being absent, and this should not keep the car from running.
 
Originally posted by: Zenmervolt
Originally posted by: Demon-Xanth
Now, as ZV mentioned, the average car takes a couple hundred amps worth of power to kick over, this is why batteries are often rated in "cold cranking amps". Or the jolt it can give out for a short time to run the starter. Your typical alternator however is around 70-80 amps max. Trucks often have ones in the 100-160A range. But not all of that power goes to the battery, I would guess between the ignition, ECM, fuel pump, and other normal things in a car, 20A is taken before you turn the AC or your headlights on. Doing that probably takes another 30, which leaves 20A to charge up your battery after you start your car.

Oh yeah, and the reason why diesels have two batteries, is because 650CCA isn't going to kick over that kind of compression and warm the manifold heaters. Once they're running, you could happily disconnect one battery.
Click to expand...

No automotive designer in his right mind would set up a car's electrical system to charge an automotive battery at 20 amps though. You'd have electrolyte boiling off if the driver took a long road trip. A typical in-home "fast-charger" for an automotive battery will charge at 10 amps and IMO even that's pushing it. Most automotive electrical systems are designed to trickle-charge the battery enough to "top it off" by the time the car has been running for 20 minutes or so. I'd imagine that they're operating in the 2 amp range.
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True, I was just stating how much overhead the car has to charge up the battery. 200A for 5 seconds worth of cranking can be replenished in 10 minutes at 2A charge.
 
Originally posted by: Demon-Xanth
200A for 5 seconds worth of cranking can be replenished in 10 minutes at 2A charge.
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not always, ot depends on how much is in the bank and the size of the battery. You could recharge it in a minute with my 200 amp alt, but it will only put a "top" charge on it and will be dead quicker than before. I'm talking real life experiance, not theory or numbers that should equate.
 
Originally posted by: Demon-Xanth
Originally posted by: Zenmervolt
Originally posted by: Demon-Xanth
Now, as ZV mentioned, the average car takes a couple hundred amps worth of power to kick over, this is why batteries are often rated in "cold cranking amps". Or the jolt it can give out for a short time to run the starter. Your typical alternator however is around 70-80 amps max. Trucks often have ones in the 100-160A range. But not all of that power goes to the battery, I would guess between the ignition, ECM, fuel pump, and other normal things in a car, 20A is taken before you turn the AC or your headlights on. Doing that probably takes another 30, which leaves 20A to charge up your battery after you start your car.

Oh yeah, and the reason why diesels have two batteries, is because 650CCA isn't going to kick over that kind of compression and warm the manifold heaters. Once they're running, you could happily disconnect one battery.
Click to expand...

No automotive designer in his right mind would set up a car's electrical system to charge an automotive battery at 20 amps though. You'd have electrolyte boiling off if the driver took a long road trip. A typical in-home "fast-charger" for an automotive battery will charge at 10 amps and IMO even that's pushing it. Most automotive electrical systems are designed to trickle-charge the battery enough to "top it off" by the time the car has been running for 20 minutes or so. I'd imagine that they're operating in the 2 amp range.
Click to expand...

True, I was just stating how much overhead the car has to charge up the battery. 200A for 5 seconds worth of cranking can be replenished in 10 minutes at 2A charge.
Click to expand...

Yup, that's where the beauty of it all comes in. Even at a slow rate (better to hold a charge, as mooseracing pointed out) it doesn't take long to replenish the power taken to start the car because even though it's high draw it's a very short duration.

LTC8K6 is correct that a charging system will vary the rate of charge dependent on the battery's state, but I still am not really sure that a car's charging system is truly designed to apply anything other than a "maintenance" charge.

ZV
 
I read somewhere that the charge rate is very high right after you start the car, but only for a short period. 20-30A after start, 30-50A for a deeply discharged battery, but only for a minute or two. Then down to a 10A or so rate until the battery is topped up, then less than 1A for maintenance.

If anyone has an older car with an ammeter, you will see that needle swing way over for a deeply discharged battery, and then gradually move back to the center as the charge rate drops.

Those were a much better indicator to you of what was going on with the battery, imo.

Of course those were generators, but when you saw that needle come back, you knew you were okay. 😀
 
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