ALL of the energy consumed by electrical accessories in car ultimately comes from the alternator.
(Of course, the energy to run the alternator comes from the mechanical rotation of the engine, which in turn gets its energy from fuel. Thus, the energy to power all electrical accessories on a car ultimately originates from the fuel.)
When it comes to automobiles, it is best to think of the 12V battery as being a gigantic, 1,000,000,000,000,000,000,000 Farad capacitor (which it isn’t, but that’s another story). When you turn the key to start the car, energy stored in this “jillion Farad capacitor” is used to turn the engine over and start the car. Once the car has started, the engine (via the alternator) recharges this “jillion Farad capacitor” so it can start the car the next time. In addition to recharging the “capacitor,” the alternator is supplying energy to all the electrical accessories (under normal circumstances).
There has been some discussion on whether-or-not you need a battery AFTER the car has started.
Theoretically, you do NOT need a battery after the car has started. On older cars you could disconnect the battery after start-up and it would usually run fine. In fact, this was how some people used to test alternators: they would start the car, then disconnect one of the battery cables. If the car continues to run (and all electrical accessories continue to work) the alternator was deemed good. But I wouldn’t do that with newer cars. Despite the fact that the alternator alone is supplying the energy needs of all the electrical accessories (again, under normal circumstances), modern electrical systems still want the battery to be “in the circuit.” I’m not exactly sure why this is true; perhaps new regulators also measure charging current in addition to battery voltage, and “freak out” if they don’t detect any current.
Michael Craft, Electrical Engineer
University of Dayton
Dayton, OH
You most certainly CAN push-start modern cars with manual transmissions, and it’s not dangerous…unless you have no idea how to do it. All you need to do is to get the car rolling at about a normal walking pace, whether by pushing it or letting it roll down a gentle slope, put the car in second gear, TURN THE IGNITION ON, dump the clutch, and when the engine starts to fire, feather the throttle and put the clutch back in immediately.
It’s also worth noting that race cars don’t even have batteries - anything that doesn’t contribute to going fast, stopping quickly, or cornering reliably is left out. That’s why, when race drivers kill their engines in the pits, EVERYBODY comes over the wall to bump start it.
Hooking up a car to another to start it is not always a good idea, the electronic engine management systems in modern cars do not like it and you may end up killing the ‘brain’ or seriously shortening its life.
Depending on how well the alternator has been designed you can run a car without the battery but some alternators can supply damaging voltages as their speed goes up and for some reason the voltage regulators can’t cope without the battery, probably because they use it as a voltage referance.
This can fry the rectifier and any other electronics too.
No, they are just dummy parts put there to justify the high price tags on cars these days.
Seriously now, the reason you cannot take the battery out of the system is that it regulates the voltage. The rectifying diodes are high amp - low reverse voltage and, if you take the battery out, the reverse voltage will damage the diodes and short them. This happens often in boats where you have several banks of batteries and a swith that allows you to disconnect all. Even though it clearly says “do not set to ‘none’ while motor is running” people just do not pay attention… and repair shops make a good living.
>> An alternator will generate a lot more current than a battery ever will.
Necros, where do you get this info? A car alternator may be rated at 60 amps which it would give under ideal conditions, not under normal conditions. Most of the time it is giving a fraction of that. A car battery, while cranking the motor can easily be putting out 4 or 5 times that number of amps.
It happens indeed–one time my cousin was leaning over his bass boat (unloading gear after a fishing trip) when the battery blew up. My dad (he was there; I was at school) said it sounded like a +p .357 magnum. Luckily it blew out the sides, and the only damage to my cousin was a loss of hearing for a few hours. This cousin, BTW, became a firefighter because they work 2 days and are off 5–that way he gets to go fishing A LOT.
In school I drove a toyota five speed. 3 of the 6 years I had it, the starter was fried. I could push start that baby by rolling back off of a curb stone or tree root I had cleverly parked upon. In a pinch, I could push it on the flat fast enough to jump in and bump the clutch to start it.
Later I got a VW with fuel injection, and an electric fuel pump. The starter was fine, but if you ever let the battery DIE, there was no push starting it, because there was no juice to move fuel to the cylinders. So to address the OP, alternators keep the battery charged, so you can spark the monster to life again.
I used to drive an Escort GT 5-spd. The battery was terminally dead, as I always forgot to turn off my headlights after arriving to morning classes.
The engine was small enough that a five or ten foot roll was enough to start it in second, and if I was on a flat surface, I could run beside it, pushing it fast enough in neutral to jump in, turn it to start, slam on the clutch, put it in gear, and pop the clutch. Scrreeeech and away I go.
After a few weeks of this, and a bad starter, to boot, I just gave up on key-starting it. Roll or push start was what I always relied on.
It also had fuel injection and an electric fuel pump. When the battery was so dead that THOSE didn’t kick in, I had a trick. I’d get it rolling downhill fast, then put it in first and pop the clutch. I’d let the engine chug along as a compression brake long enough that the battery would charge enough for the fuel pump to kick on, then I’d put it in second, let off the clutch again, tap the gas a bit, then away I go.
That fuckin’ engine was like an ox. Too bad I ran it dry of coolant and warped the head.
It bloody damn well better do something. I shelled out $250 (with labor) for a new one a couple of weeks ago.
Seriously, you can take my favorite med school learning approach–to understand what something does, look at what happens when it fails.
–The battery light, which normally comes on briefly when the car is started and then goes off, does not go off. (You’re running on the battery.)
–After a while, the CD player kicks out and the clock starts to flicker. (The battery gives out.)
–When you turn the car off, it will not start back up again. You do get a rapid clicking noise rather than the singular grunt or half-hearted turning over of a dead battery.
–You call AAA. They tow you to your friend the mechanic. The mechanic makes this month’s boat payment.
Fortunately, this happened in my driveway, which by my calculations has slightly less than a 50% chance of happening.
On not being able to push start modern cars with manual transmissions.
I can answer that with another question. If the car will not start/run unless the clutch is depressed, then how does it run at all? Logicly it would die the minite you put it into gear and feathered out the clutch to go.
The assertion that it can not be done because the starter solinoid will not engage without the clutch depressed is nonsense at best.
The solinoid simply engages a pinion gear to the flywheel that rotates the engine. This in turn produces rotation that starts the car. The clutch mearly disengages the transmission, preventing the car from moving during this process.
The clutch saftey is a dummy device, installed to prevent over stressed housewifes with 5 screaming kids from engaging the starter while the vehicle is in gear. This in turn prevents the vehicle from lurching into the brand new Lexus in the adjacent stall. This helps the manufacturer avoid lawsuits from said Lexus owner.
(note to the humor impaired, feel free to substitute “white male executive” for housewife in above paragraph.)
During a push start, the clutch IS engaged, producing the rotation that is required to get the whole works running. The batery is dead, get it? The whole starter system is essentially non-functional.
Note that it might well be impossible to push start a car with a dead battery if the battery is REALLY dead. For while the battery acts as neither a capacitor nor a “primary” voltage regulator on modern vehicles, it is still an essential part of curcuit. Not to mention the source of power for the mysterious black box.
AFAIK, the clutch safety is connected to the ignition wiring. That is, turning the ignition switch with the clutch out does not complete the circuit; you need to depress the clutch and turn the key to complete the circuit. Since you’re not using the “starter” position on the ignition switch when you push-start a car, the clutch safety is out of the loop. You should be able to push-start any manual-transmission car.
(FWIW, my '79 Porsche 911SC had fuel injection, and I could push-start it. The several Chevy Sprints I’ve driven had clutch safety switches and they could be push-started. My '94 Yamaha has a clutch safety switch, and it can be push-started. Piston-powered airplanes, due to their magnetos, can be hand-started. I blew it by getting an automatic transmission in my '99 Jeep Cherokee, so if I lose my battery in the bush I’m screwed.)
