Roll starting with a disconected battery?

[illusion88]

A week ago a replaced my battery. It had been dying alot. I was parking on hills for fear of not being able to start the car. I tried to charge the battery on a charger but it didnt work. So one walmart trip and $65 later I had a new battery.

All week it had been starting fine, no problems. Then last night, when I got back in the car after getting a few things at a 711 it didnt start! Oh no! Lucky, I was on a hill and was able to get it going. On the drive home I was thinking my alternator was the real culprit. The only reason the battery lasted this long was because it was brand new and I was only driving about once every two days.

The next morning I poped the hood and checked it out. Turns out, the negative battery cable was a bit loose. I tightned it down and all is well.

My question..... How was I able to roll start the car if the battery was disconnected? When I went to start the car at the 711, the starter engaged but the engine didnt turn over. When I tried it the next morning it was completly dead. Could it have been that the movement of the car shook the cable back on (it really wasn't that loose, but I guess it was enough)? Or am I confused how a roll start actually works....

Car: (her name is Clair BTW)
01 Civic
Manual transmission (duh)
87k miles
White
Old tires
Brakes that need replaced
Can roll start like a champ!

 

[Billb2]

Cars will run with the battery disconnected (or removerd, for that mater), they just can't be started with the starter then. The electrical system doesn't know whether it's getting power from the battery or from the alternator.

 

[LTC8K6]

I'm curious as to what the OP thinks the alternator does. The alternator provides all of the power to the vehicle while the car is running. You can remove the battery once the engine is running. (Don't do that, though. It's not really good for the electrical system.)

When you roll started the car, the alternator was turning and providing power, just like it always does. So of course you can roll start the car without a battery.

If you turn the engine, you are turning the alternator, and you have power. Power to the ignition means the engine can start.

Barring any odd problems of course, such as a shorted battery or something else that is robbing the system of power.

Some trucks use a compressed air powered starter. They don't really need a battery at all as long as there is air in the tank. They sound just like the air impact wrenches used to put on tires when they start. Con-Way freight trucks come to my shop all the time and their trucks have compressed air starters.

 

[Zenmervolt]

Originally posted by: Billb2
Cars will run with the battery disconnected (or removerd, for that mater), they just can't be started with the starter then. The electrical system doesn't know whether it's getting power from the battery or from the alternator.

Many newer cars cannot be started without a battery installed. The ECU needs to be receiving power before the engine starts to turn over and the fuel pump needs to pressurize the fuel lines. Since neither of those two requires very much power, even a "dead" battery that cannot spin the engine will almost always still provide enough current for those to happen and thereby allow push-starting.

You are right that once a car is running it is theoretically possible to remove the battery and the car will continue to run (since the battery is wired in parallel with the car's electrical system and not in series). However, given that many electrical systems use the battery as a buffer to prevent current spikes it is generally not recommended to run a modern engine without a battery present.

To the OP's question:

The battery was not disconnected. A slight looseness in the cable is not enough to cause a fully open circuit, just enough to introduce additional resistance in the circuit. If the battery had been disconnected, the starter wouldn't even click, much less engage.

ZV

 

[lurk3r]

I may be wrong, but iirc an alternator cannot work without a battery. All ZV's points about computers also apply, but even on an older carburated car, without some voltage the alternator cannot function.

 

[Demon-Xanth]

An older non-EFI car can be roll started (assuming it's a manual) because the alternator will give enough juice to get a spark from the coil. If it's either EFI or automatic, get yourself a battery because that thing isn't going anywhere.

I think the last transmission with a pump on the output shaft was the 727 (but don't quote me on that), any transmission since can't drive the engine if it's not already turning.

 

[Zenmervolt]

Originally posted by: lurk3r
I may be wrong, but iirc an alternator cannot work without a battery.

You are wrong. Neither an alternator nor a generator needs a battery connected to work.

ZV

 

[Blunc]

an alternator or generator works because the electronic principle of a wire/coil moving thru a magnetic field generates voltage.
unless an alternator or generator is built with real magnets in it, they will not work without a power source connected.

alternators and generators have "electromagnets" built in, if there is no power to the electromagnet there will be no magnetic field created.

 

[cprince]

Originally posted by: Blunc
an alternator or generator works because the electronic principle of a wire/coil moving thru a magnetic field generates voltage.
unless an alternator or generator is built with real magnets in it, they will not work without a power source connected.

alternators and generators have "electromagnets" built in, if there is no power to the electromagnet there will be no magnetic field created.

I have been wondering about this for a while. From what I found out, backup generators have enough residue magnetism to get the field going. But if the generator was on a load when the engine is off, the residue magnetic field will be dissipated and you need to re-excite it. But I think for an automotive alternator, the iron in the core might be permanently magnetized--albeit weakly.

 

[LTC8K6]

Originally posted by: lurk3r
I may be wrong, but iirc an alternator cannot work without a battery.

So why does your car run without the battery in it?

 

[Zenmervolt]

Originally posted by: Blunc
an alternator or generator works because the electronic principle of a wire/coil moving thru a magnetic field generates voltage.
unless an alternator or generator is built with real magnets in it, they will not work without a power source connected.

alternators and generators have "electromagnets" built in, if there is no power to the electromagnet there will be no magnetic field created.

Unless the alternator has been sitting unused for a very long time there is generally sufficient residual magnetization for it to self-excite. In theory, you're correct, but in actual application that's such a rare occurrence that it can be disregarded in the vast majority of cases.

Some alternators, like the alternator in my Harley, use permanent magnets to avoid this issue entirely.

ZV