Car battery -- charging questions

[donpacino]

When somebody's alternator brushes wear down and lose spring tension the symptom is the alternator is rendered incapable of full output.
It shows up on a long trip on a hot night when running nearly everything; alternator can't keep up so battery runs down to point it won't start after a refueling stop and they need a jump..
Before serpentine came along a loose alternator belt would give same symptom..

Thats a good point. I tend to forget about degregation of system components.

 

[jack action]

I spent 6 years in the US Navy, as an electrical operator, and to my recollection, I NEVER disconnected the generators, from the grid.
This just sounds counterintuitive to me.

Someone please jump in here, and save me!

It is not physically disconnected by someone. I like this air compressor analogy from madelectrical.com:

The voltage regulator limits the maximum voltage in the electrical system. In the compressed air system the pressure regulator limits the maximum pressure. The voltage regulator will also cause the alternator to produce more output, when voltage (pressure) at the electrical system is low. And in the compressed air system, the pressure switch will turn on the compressor when system pressure gets low.

Lights, ignition, and accessories use power from the electrical system. Every time we switch an accessory ON, more power is drawn from the system. Voltage (electrical pressure) drops as power is drawn from the system, and then the voltage regulator causes the alternator to make more current. And in the compressed air system an impact wrench, blowgun, paint gun, or the fitting for filling a tire, can all use power (compressed air) from the system. When we use compressed air from the system, PSI (air pressure) drops, and the regulator turns the compressor ON. In the electrical system, the voltage regulator “turns the alternator ON,” or “turns OFF the alternator” as needed to maintain voltage at the proper level. And in the air compressor system the pressure regulator stops and starts the compressor as needed to maintain the proper level of pressure.

 

[OmCheeto]

It is not physically disconnected ...

That's all I needed to hear.

Thanks!

 

[jim hardy]

Someone please jump in here, and save me!

automobiles - ... they evolved,

so as soon as you say "Here's how it works" somebody will take those words to heart only to find he hard way his is different.

In the days of DC generators (until early 60's) indeed a relay in the voltage regulator disconnected the generator from the battery whenever it was making insufficient voltage to overcome the battery.
Some early alternators did the same thing, i had a ~'65 Ford Falcon with such a relay in its regulator. .
But my '62 Chrysler had no such relay, it relied on the rectifier inside the alternator to prevent reverse current from battery .

Everything I've encountered since ~1970, GM Ford and a friend's Mercedes, relied on the rectifiers , there is no disconnect. The big wire at rear of alternator is "hot" at all times.
That invites trouble when getting a jump-start as follows:
When somebody hooks the jumper cables backward, that forward biases the rectifiers inside the alternator and often wrecks them. They fail shorted so no longer block reverse current. . Now the battery will discharge through the alternator probably overheating its windings and ruining it. If you're lucky the fusible link in alternator wire will open before the wiring harness melts.

Introduction for shade tree mechanics here:
http://www.ratwell.com/technical/ChargingSystem.html
He shows the Bosch system i found on my sister's Mercedes Benz
a friend's Ford van was identical just had different letters on the terminals.

Observe the "warning lamp" is the source of field current when first starting up.
Once the alternator makes a few volts it provides power to the D+ terminal for its field, "pulling itself up by its bootstraps" if you will.
But the bulb by itself doesn't pass enough current to initiate the "bootstrap".
My friend's Ford had a ~5 ohm resistor right across the rear of the lamp socket to deliver a couple amps of bootstrap current. When that resistor failed, the symptom was of course the light stays on because the alternator can't bootstrap itself into operation. My friend found if he over-revved his engine the light would go out - it bootstraps from the trickle of lamp current if you spin it uncomfortably fast .
We installed a temporary pushbutton across the lampholder terminals for "bootstrap" until we could locate a resistor. .

Sorry to be so un-academic in a physics forum
but physics majors have car troubles just like the rest of us... I hope this helps somebody figure theirs out .

old jim

 

[SteamKing]

I spent 6 years in the US Navy, as an electrical operator, and to my recollection, I NEVER disconnected the generators, from the grid.
This just sounds counterintuitive to me.

Someone please jump in here, and save me!

I bet in the Navy, you used alternators instead of generators, even though you called them generators.

In the car battery charging circuit, the voltage regulator does not trip the alternator completely off-line when the battery is charged, it reduces the amount of current generated by the alternator by reducing the amount of current sent to the alternator's field coils. If the field coils are not energized, the alternator can't make any additional current to send to the rest of the electrical system.

Like any self-contained power plant, a ship's electrical generating plant must contain a means whereby one alternator can be properly shut down for maintenance or repair without requiring the entire plant to be shut down. It might be terrifically inconvenient, but it can be done. I sailed on a merchant ship which lost one of our two turbo-alternators while at sea. We had to use the back-up diesel generator (Fairbanks-Morse opposed piston engine) and put it on-line in place of the damaged steam turbine alternator. Since the damage couldn't be repaired outside of a dockyard, we used the F-M generator the rest of our trip.

 

[OmCheeto]

I bet in the Navy, you used alternators instead of generators, even though you called them generators.

I don't remember anyone mentioning "alternators" while I was assigned to the surface fleet, and the submarine I was on had at least two things they called "motor-generators", MG's for short.
The DC end of the MG's ran through so many brushes, it was ridiculous!
Much worse than my '61 VW beetle.

On both sides of the surface, we call the things that made AC "turbo-generators", TG's for short. "SSTG" stood for "Ships Service Turbo Generator".
Both vessels were built around 1959, and I was assigned to them between 1979 and 1983.
Not sure what the kids nowadays are playing with. Could quite well be "alternators".

In the car battery charging circuit, the voltage regulator does not trip the alternator completely off-line when the battery is charged, it reduces the amount of current generated by the alternator by reducing the amount of current sent to the alternator's field coils. If the field coils are not energized, the alternator can't make any additional current to send to the rest of the electrical system.

That sounds better.

Like any self-contained power plant, a ship's electrical generating plant must contain a means whereby one alternator can be properly shut down for maintenance or repair without requiring the entire plant to be shut down. It might be terrifically inconvenient, but it can be done. I sailed on a merchant ship which lost one of our two turbo-alternators while at sea. We had to use the back-up diesel generator (Fairbanks-Morse opposed piston engine) and put it on-line in place of the damaged steam turbine alternator. Since the damage couldn't be repaired outside of a dockyard, we used the F-M generator the rest of our trip.

Ah ha! You said generator!

 

[jack action]

Some early alternators did the same thing, i had a ~'65 Ford Falcon with such a relay in its regulator. .
But my '62 Chrysler had no such relay, it relied on the rectifier inside the alternator to prevent reverse current from battery .

The relay is an intrinsic component of a [mechanical] regulator. That component was replaced by an electronic version around 1970. With the smaller size, they were able to incorporate the regulator within the alternator. Here's a picture of such a regulator with the rectifier beside it (which is not the same thing):

Your '62 Chrysler had an external mechanical regulator too for sure. Unless you had it after 1970 and someone converted it.

 

[jim hardy]

Your '62 Chrysler had an external mechanical regulator too for sure.

Yes. Only one coil in it to control voltage. No current coil because an alternator is inherently current limiting . No disconnect relay because of the rectifiers inside alternator. I marveled at the simplicity.
That year they used a grounded field alternator the regulator supplied + to the field from ignition switch.
By 1965 they'd changed to an ungrounded field alternator, the regulator providing ground instead of + .

Of course they're all electronic now as in your picture so you can no longer see by eye and feel with a fingertip how they operate because nothing moves..
The 50's and 60's was a great time to learn basics because , well, everything you encountered was pretty basic.

old jim

 

[collinsmark]

I don't remember anyone mentioning "alternators" while I was assigned to the surface fleet, and the submarine I was on had at least two things they called "motor-generators", MG's for short.

I can't speak for the specific MGs on the submarine, but I might be able to add some insight to the word "alternator."

The word alternator is an archaic term that has somehow survived the test of time, but usually only when applied to the automotive field. Some terms that have not survived are condenser which is now replaced by capacitor, and rheostat which is replaced by either potentiometer or variable resistor. For some reason, alternator remains the most common term for an automobile's AC generator.

There is some history behind this. Some time ago (even a bit before my day), solid state rectifiers -- what we now call diodes -- that were practical for high current, automotive applications, didn't exist. Vacuum tubes could do it, but those aren't necessarily practical for a car engine. So the car's battery was recharged via an actual DC dynamo generator*. Dynamo generators have lots of problems such as being difficult to regulate and requiring more maintenance due to commutator wear. So in the 1960's, when silicon diodes became affordable, the automotive industry switched to using AC generators, a.k.a. alternators, converting the AC signal to DC using the solid state silicon diodes. Getting rid of the problem prone, unreliable DC generators was a big deal. For awhile, having an alternator was an indication (to the common person) that the car was designed with high-tech, state-of-the-art quality. I'm guessing that's why the name stuck.

But in other fields this apparatus is just called a generator. And if you want to get specific then it's an AC generator (and if you want to get really specific it's a synchronous machine or synchronous AC generator). For example, the steam turbine at your local power plant is physically connected to and drives a really, really big version of your car's alternator. But they don't call it an alternator. They just call it a generator.

*(Dynamo generators are still used today on small engines -- like the one that powers your lawnmower. The principle involves a spinning magnet, typically attached to the flywheel [in small engines], that spins around, occasionally passing by a coil of wire. In combination with a capacitor (a.k.a condenser) and a commutator a DC voltage source can be produced**.)

**(Edit: I should clarify that if your lawnmower is manually started, has no fancy-schmancy frills, and doesn't have an onboard battery, then there is no need for the additional components for DC conversion. In this case, the electrical system is called a magneto system.)

 

[Averagesupernova]

In the days of DC generators (until early 60's) indeed a relay in the voltage regulator disconnected the generator from the battery whenever it was making insufficient voltage to overcome the battery.

Known as the cutout relay. The cutout relay is not part of regulation. It was typically housed in the same package as the relay that regulated voltage but it served a different purpose. Actual generators with segmented commutators would typically use an external package that housed these two relays and sometimes a relay that regulated current. This relay had a coil that passed ALL of the charging current and would open the field circuit when the charging current exceeded a preset level.
-
Edit: Jim I know that you know this but there was a post that implied (in my opinion) this relay was part of regulation and of course it is not.

 

[jim hardy]

Edit: Jim I know that you know this but there was a post that implied (in my opinion) this relay was part of regulation and of course it is not.

Thanks ! Instruction pamphlet here for those old Delco regulators
in case anyone is into ~1930 thru 1963-ish cars...

http://ruiter.ca/mc/info/PDFs/1R-118.pdf

Cutout relay is just that, it does the job of a big diode, just prevents the battery from discharging backward through the generator.
Big diodes weren't common until the 1960's.
Regulator coils are just that, their contacts "tap dance" to control generator voltage and current.by interrupting field current tens of times per second. You can feel them working with a fingertip. You can push on them and have your helper( or lovely assistant? ) watch generator respond on the vehicle's ammeter.
Think of them as electromechanical operational amplifiers -- Not so reliable as solid state but a lot easier to troubleshoot ..

The Chrysler alternator regulator i described had only a voltage regulating relay so was much smaller.

old jim

 

[OmCheeto]

I can't speak for the specific MGs on the submarine,

Nor, no matter how much I actually know about them, can I.

Submarine Power & Propulsion
Maneuvering Room Consoles

Supervised by the engineering officer of the watch, one petty officer mans each of these three consoles to monitor and control the submarine's entire nuclear power plant. The console to your left controls the steam turbines. The center console is the nuclear reactor control panel, while the right-hand console controls the electrical system.

Displaying consoles like these in public, something never before done, has required modifications to protect sensitive classified information about the design and operation of nuclear-powered submarines. Where necessary, scales on instrument faces have been modified, instrument labels altered, or instruments repositioned, and some classified nuclear instrumentation has been removed.

All I can say is that everything worked quite well. And it appears that they've kept up the good work since I got out.

Forbes
...
The Nuclear Navy has logged over 5,400 reactor years of accident-free operations and traveled over 130 million miles on nuclear energy, enough to circle the Earth 3,200 times.
...

but I might be able to add some insight to the word "alternator."
...

Thanks! Naming new things that are similar, but different, seems to be a constant problem.

The word alternator is an archaic term that has somehow survived the test of time, but usually only when applied to the automotive field.
...
**(Edit: I should clarify that if your lawnmower is manually started, has no fancy-schmancy frills, and doesn't have an onboard battery, then there is no need for the additional components for DC conversion. In this case, the electrical system is called a magneto system.)

I have an AC electric lawnmower.
But I do have a not so fancy-schmancy outboard motor, that has an electric starter, magneto spark device, but no alternator.
Hence, why if you've seen pictures of any of my boats, there is usually a solar power generator(??) somewhere in the picture.

Actually, even if the motor did have an alternator, I'm pretty sure the leaks in that stupid boat would have drained the battery.

hmmmm... Now I've forgotten what the question was...

Hello,

I am a Telecommunications student who does not have a lot clue about cars.Does a car charges up its battery when it moves? What power supply is used for lighting in car, the battery? So if car charges the battery when it moves, why sometimes external/additional charging of battery is vital ?

I see this was answered in the 2nd post.

 

[nsaspook]

The regulator output voltage will not depend on the battery. If the output voltage is too high, it is because the regulator is not working properly.

Even the electronic voltage regulation is slow in response to a 'Load Dump' event. Don't ever, ever, ever disconnect the battery while the alternator is running.
https://en.wikipedia.org/wiki/Load_dump

http://www.littelfuse.com/~/media/e...n_automotive_environment_application_note.pdf

 

[jack action]

Don't ever, ever, ever disconnect the battery while the alternator is running.

Well, if the battery is fully charged, I can disconnect it without any consequences. Also, @jim hardy is safe with his '68 Ford truck with no electronics. So maybe one or two ever are enough ...

 

[OmCheeto]

Even the electronic voltage regulation is slow in response to a 'Load Dump' event. Don't ever, ever, ever disconnect the battery while the alternator is running.
https://en.wikipedia.org/wiki/Load_dump

http://www.littelfuse.com/~/media/e...n_automotive_environment_application_note.pdf

Interesting. I'm not sure I've ever heard of that.

And from the second article, another good reason to make sure your battery connections are tight.

A load dump may result from a battery disconnect resulting from cable corrosion, poor connection or an intentional battery disconnect while the car is still running.​

 

[Averagesupernova]

Well, if the battery is fully charged, I can disconnect it without any consequences. Also, @jim hardy is safe with his '68 Ford truck with no electronics. So maybe one or two ever are enough ...

Go ahead and try it. Let us know how it turns out. I have a number of years experience in the automotive electronics industry and it is never ever a good idea.

 

[jim hardy]

Hence, why if you've seen pictures of any of my boats,

Wood boat, white mercury, cable steering... Makes my nostagia ache.
...
Just today i acquired a 1960 Evinrude 5½ Fisherman...

 

[Averagesupernova]

Wood boat, white mercury, cable steering... Makes my nostagia ache.
...
Just today i acquired a 1960 Evinrude 5½ Fisherman...

Yeah I hadn't noticed the cable steering til you mentioned it. I have a Johnson 40 with cable steering that has been setting for years. I have a early 60's StarCraft cabin cruiser with a 100 Merc on it. I need to take the time to actually use it though.

 

[jim hardy]

Load dump... worst when the alternator is heavily charging a run down battery. There's a lot of energy stored in the alternator's magnetic field that must go someplace.
That's why better automotive electronics is designed for ~ 200 volt surge.
That's also why one should keep his battery cables in good repair, ie tight and clean. I won't use those replacement cable ends with the wimpy clamps.

attention to the little things keeps the big ones away.

 

[jack action]

Go ahead and try it. Let us know how it turns out. I have a number of years experience in the automotive electronics industry and it is never ever a good idea.

Oh! It's not a good idea! Note that I kept at least one ever . But I know that doing it under certain circumstances may not hurt anything, especially if there is no electronics to hurt. I usually keep the three consecutive ever for stuff like welding a leaky gas tank!

I'm the kind of guy who always cuts the power before working on any electrical system and uses appropriate instruments to make measurements. Some backyard mechanics and DIY I know - who like checking if there's current by making sparks - still wonder how come I never had "the chance" of being electrocuted. Well that's my trick!

 

[Planobilly]

A short history of how we got battery power in cars to begin with.

As best that I understand, the storage battery was invented in 1796. About 115 years later a few storage batteries were placed in cars. That was around 1911. Before that dry cell batteries were used to power a horn which I assume was the first electrical powered device in cars. I assume lights came next and so it goes until today where one may find a 300 watt stereo in a car. There are also electronic automatic breaking system being developed that I suspect would be useful to those who have the 300 watt stereo turned up to the max...lol

All this need for safe more or less low voltage and more or less high ampere electrical power relates to advancements in cars and the many uses of internal combustion engine and the electrical equipment around it. As engines needing electrical power came into use in boats and aircraft the designs of the batteries and charging systems changed and became more complex.

When I was a kid 6 volt batteries were common in cars. Back in the 50's there were systems on boats and aircraft that I was around that were 24 volt and 32 volt. Those larger boats had large diesel engines with starters requiring more electrical power to start and generators that put out the required amps.

In general terms the batteries primary purpose was to start the engine and in some cases to maintain power to the ignition system at least in cars. In today's world the smart phone charger in a car could be considered by some to be the the primary purpose of the electrical system...lol

The first cars I drove would stop running pretty soon if the generator stopped providing current to the battery. This type of problem was overcome in aircraft that I flew by the use of a magnetos. There were two separate magneto systems and two sets of spark plugs in each cylinder to address the possible failure of the battery and or charging systems. Piston driven aircraft to this day still use that magneto system due to reliability issues with batteries and charging systems.

As has been stated in some of the post in this thread these batteries and charging systems change in design on a regular basis. I assume these changes will continue into the foreseeable future. The Federal Highway Administration states that american drivers drive around 2.5 trillion miles per year. I don't want to do the math but that adds up to a lot of amps generated and a lot of potential buyers for gadgets to make use of those amps.

Cheers,

Billy

 

[jim hardy]

The first cars I drove would stop running pretty soon if the generator stopped providing current to the battery.

When the generator quits you're relying on the battery of course.
In the era you describe,
battery and coil ignition draws only an amp or so,
fuel pump is mechanical,
so if you turn off all the electrics the battery will carry you all day maybe a week.
But at night your headlights will run it down quickly.

My first encounter with low alternator output was ~1968.
I was in Alabama headed back to school in Missouri when noticed the alternator couldn't keep up with the headlights Ammeter showed slight discharge with headlights on, slight charge with them off... Voltage readings confirmed it.
Knowing i had eight hours of night driving to go i decided to reduce electrical load - removed right low beam headlight and right taillight, verified with voltmeter i was now above 12.6 volts so at least trickle charging.
Made it back to school, took alternator apart and carried it into Professor's office . He was delighted to help. , pointed out worn brushes and introduced me to concept of "Armature Reaction" .
We covered synchronous machines in more depth in his course a few weeks later, but the physical visualization was a great help to me .. I'm thankful for the lesson that alternator gave me.
Tinkering is good. Some of us learn our basics that way.

Nowadays cars are utterly dependent on alternator to go beyond a very few miles.
If yours quits, shut off everything especially airconditioner, roll down windows and limp home.
old jim

 

[jack action]

A short history of how we got battery power in cars to begin with.

You missed the most important reason (only reason?) that caused every car having a battery:

The battery has only one purpose: Powering the engine starter.

From http://www.motorera.com/history/hist06.htm:

Until 1908, motorists warned pedestrians to "move it" by shouting or by pressing a pedal to clang a bell. Neither method was as raucous as the electric horn, which got its name, the Klaxon, from the Greek word klaxo, meaning "to shriek."

Between 1908 and 1911, the few autos that had Klaxons used dry cells to operate them. [...]

By 1911, storage batteries had attained a degree of reliability exceeding that of dry cells; they lasted at least a month.

[...]

The few manufacturers who adapted the storage battery to work the Klaxon then looked around to see what else they could do with the excess current the storage battery provided -- and found electric lights.

[...]

Other manufacturers revived the dynamo, which had been around for some time. [..] The battery then didn't have to be taken out of the car every month for recharging.

A problem still presented by those first dynamo-equipped cars was battery overcharging. However, this trouble was resolved with the development of a variable speed regulator by DELCO. It was first used in the 1912 Cadillac, which displayed another feature that set the auto industry on its head: the self-starter.

Once they adopted the self-starter, auto manufacturers had to adopt the battery/generator system to work the starter. However, the system put out a much more current than the starter, lights and horn needed, and carmakers realized they could harness this current experience. and use it for igniting the fuel mixture. The magneto then became obsolete.

[...]

As public confidence in the reliable battery/generator/self-starter system soared, it soon replaced the magneto in all GM cars. GM enjoyed a sales boom, and the remainder of the auto industry soon adopted the system. Of the 462 models shown at the 1911 New York Auto Show, only 19 had battery/generator systems, and they all had backup magnetos. Of 119 makes displayed at the 1924 New York Show, 110 had storage battery/generator systems and self-starters.

 

[Planobilly]

"If yours quits, shut off everything especially airconditioner, roll down windows and limp home."

Well Jim...If you have no air conditioning this time of year here in Florida you need to call a tow truck with AC to ride into get you home...lol

Billy

 

[Averagesupernova]

Knowing i had eight hours of night driving to go i decided to reduce electrical load - removed right low beam headlight and right taillight, verified with voltmeter i was now above 12.6 volts so at least trickle charging.
old jim

I love this. How many college kids do you think would be able to do this today?

 

[OmCheeto]

When the generator quits you're relying on the battery of course.
...
Tinkering is good. Some of us learn our basics that way.

Nowadays cars are utterly dependent on alternator to go beyond a very few miles.

1.12 miles, to be precise, if you owned a 1994 Chrysler LeBaron convertible, with a V6. (I love Google Earth!)

1. Left parking spot at work. Drove 0.16 miles, heard some unfamiliar noises, along with some flashy dash board lights. Pulled over, turned off the car, and saw something rolling down the hill. Picked it up, and could not identify it. Popped the hood, and noted that the serpentine belt was a bit loose, as in, lying in a heap. Decided that the thingy that had fallen off was the some kind of spring loaded tension device. Determined that there was no way to maintain tension on the serpentine belt, even though I had a roll of duct tape in the car.
2. Started car, and drove 0.55 miles to the nearest convenience store, as I needed supplies, as this was going to be a LONG day. Turned off engine, got some bottled water, and the engine would not start. Being that I was somewhat of an expert in battery temperament, I let it sit for 15 minutes. The car then started right up.
3. Drove the car 0.41 miles back to my parking spot. Turned off the car. Just for kicks, I tried starting it. Barely a click.
4. Came back the next day, with all manner of alternative energy supplies. Successfully made the 11 mile trip home safely, and fixed that darned problem.

If yours quits, shut off everything especially air conditioner, roll down windows and limp home.

old jim

Old cars(and boats) make for some fun adventure stories.

ps. Good thing that whole story started on a Friday afternoon, and the sun was out all day Saturday.

 

[jim hardy]

...

Successfully made the 11 mile trip home safely, and fixed that darned problem.

11 miles ? Didn't overheat ? Probably that was the 3.0Liter engine? Its water pump is driven by timing belt not serpentine .

Old cars(and boats) make for some fun adventure stories.

I'll second that. It's what we learn by those (mis)adventures that so enriches our life.

 

[OmCheeto]

...

11 miles ? Didn't overheat ? Probably that was the 3.0Liter engine? Its water pump is driven by timing belt not serpentine .

Yes. I was most fortunate.
But don't get me started on "let's put the thing that goes out the most, in the most inaccessible spot. That'll get Om and Old Jim to buy new cars..."
I chucked my 2nd to last car, just because it's water pump was inside the stinkin' engine.
Just checked my new(ish) '09 vehicle. (Yes, I eventually gave up, on my '80s & '90s POS*s...)
The water pump is external.

I'll second that. It's what we learn by those (mis)adventures that so enriches our life.

As they say, what doesn't kill you...

* POS:

 

[Guineafowl]

Sorry to (slightly) hijack this thread, but does anyone have a comment about jump starting modern cars? I've had advice that ECUs and other units that talk over the CANBUS can be fatally damaged by jump starting. I was advised to 'be careful', but without an explanation of exactly how. Apparently the acceptor car's alternator can produce a voltage surge once started that feeds into the donor car's electronics and buggers them.

I would have thought that A) The acceptor car's alternator would be regulated not to do this, and B) The batteries linked in parallel would buffer any surges effectively.

 

[jim hardy]

I would have thought that A) The acceptor car's alternator would be regulated not to do this, and B) The batteries linked in parallel would buffer any surges effectively.

There are two dangers that i know of

one was mentioned earlier in the thread,
when the strong donor battery is connected to the weak acceptor battery only a small current should flow - enough to raise the weak battery to 12.6 volts.
The trouble is, if the cables get hooked backward the strong battery does its best to reverse the polarity of the acceptor electrical system
which applies -12 where +12 belongs
In datasheets for automotive IC's you'll see "reverse polarity protection" mentioned. Manufacturers learned about reverse jumpstarts the hard way.
And about under-the-hood temperatures.

The other is , since garages get a lot of calls in dead of winter when the starter is asked to crank an engine with oil that's thick as molasses,
it used to be common to hook two batteries in series and jump-start with 24 volts which makes the starter really go.
Hence the 40 volt input voltage tolerance on that automotive IC above.
Hint - don't have any lights on when receiving a 24V jumpstart.

But I think the practice is very rare anymore.

 

[donpacino]

Sorry to (slightly) hijack this thread, but does anyone have a comment about jump starting modern cars? I've had advice that ECUs and other units that talk over the CANBUS can be fatally damaged by jump starting. I was advised to 'be careful', but without an explanation of exactly how. Apparently the acceptor car's alternator can produce a voltage surge once started that feeds into the donor car's electronics and buggers them.

If they designed the electronics correctly, the canbus should be galvanicly isolated (ideally optically) from the power bus. This means that no voltage surge should damage other devices on the communication buses. This doesn't necessarily protect the device seeing the voltage surge Like jim said it is common for automotive parts to be rated for 40V or more. They will also have surge ratings, and should be designed to handle jump starting the battery.

 

[OmCheeto]

...
Hint - don't have any lights on when receiving a 24V jumpstart.
...

Since the OP's question has been answered, and we now are apparently going off on car electrical tangents, I thought I should share one more story, as it involves "headlights" as a voltmeter, and crappy battery connections.

About 25 years ago, my friend called me, and asked me for some help.
At the time, he lived about 30 miles away, but at the moment, was only 1/2 mile from my house.
So I drove over, and he said that there was something wrong with his car, which at the time was a 1965 Dodge Dart. (He now drives a freakin' Ferrari 308. I hate him.)
Anyways, he said his girlfriend had just left, and he had replaced the starter in his car 3 TIMES, but the car still wouldn't start.
Which somewhat reminded me of 10 years earlier, when I replaced my carburetor 3 times in one day, only to have the clerk give me the stink eye when I asked for a fourth, as all of his carburetors were defective. But that's another story.
So anyways, I had my friend turn on his headlights.
They were quite bright, so I knew the battery was charged.
Then with the lights on, I asked him to start the car.
The lights went out.
I told him to stop, turn off his lights, and give me his lug wrench.
He was somewhat perplexed, as changing his tire didn't seem to be the right option for fixing a "starter" problem.
Anyways, I rapped the top of both of his battery connectors, and said "try now".
The car started.

For the next 5 years, he sent all of his girlfriends to my house, as I was apparently some type of automotive electrical genius.

 

[Tom.G]

RE Post #64

but does anyone have a comment about jump starting modern cars?

RE Post #48

Even the electronic voltage regulation is slow in response to a 'Load Dump' event. Don't ever, ever, ever disconnect the battery while the alternator is running.

I haven't investigated these approaches in detail but they seem to fit the requirements.
1) Once the disabled car is running, shut off the engine of the rescue car before disconnecting the jumper cables.
That way you don't risk a 'Load Dump' event from the rescue car alternator.

2) An even safer approach would be shut off both engines before disconnecting the jumper cables. Obviously this would have to be after the disabled battery had gained enough charge to operate the starter.

P.S. A few months ago a neighbor didn't do something 'right' when jumping a girlfriends car. His repair bill was just over US$3000.

 

[arydberg]

I believe Ford has a 130 amp alternator in some of their vehicles.

Sometimes these are for Police cars.

 

[Guineafowl]

Sometimes these are for Police cars.

Our Land Rover has a 150 A one as standard. Probably to cope with the extra load of the PTC element in the heater matrix, and the beefier battery in a diesel car.