On 15-6-2009 19:26, Orchid XP v8 wrote:
>>>> BTW a beating heart is absolutely not gruesome, I would rather say 
>>>> beautiful. I'd be happy to show you if you happen to be in Amsterdam 
>>>> some time and an experiment is running. ;)
>>>
>>> I once saw an operation on TV. They cut open a guy's chest, ripped 
>>> his ripcage apart, and inside was his heart. I swear, it honestly 
>>> looked like it was trying to CLIMB THE HELL OUT OF HIS BODY AND RUN 
>>> AWAY!! O_O
>>>
>>> *shivers*
>>
>> Was that a dramatazation or an actual procedure?
> 
> It was meant to be the real thing. Apparently there was something or 
> other wrong with this guy's heart, so they were going to rip it out and 
> stick a new one in. (I wasn't really paying attention.) The new heart 
> looked rather limp and pale. But the still-beating one looked evil! (And 
> several sizes too big for the chest cavity, BTW...)

I think I would not use 'rip' and 'stick' to describe the procedure. 
That the heart to be explanted is big and ugly is to be expected. The 
ones I see are all failing hearts and these are huge, and if the patient 
is older they are yellow with fat. Yes, I do sometimes see them. Guess 
what we are doing with them...

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>> A pretty accurate model of a battery is a perfect voltage source in 
>> series with an "internal" resistor.  This then nicely explains how the 
>> more current you try to draw the lower the apparent voltage will be at 
>> the terminals of the battery.
>
> As far as I'm aware, resistors affect only current, not potential.

That statement doesn't even make sense.  The voltage across the terminals of 
a resistor is equal to the current going through it times the resistance, 
that's all you need to know and then you can figure out any resistor circuit 
with a bit of algebra.

By the way, notice how the lights dim on your car (and maybe your radio also 
shuts off?) when you start the engine?  That's because the starter motor is 
drawing a huge current and causing the battery voltage to drop to way below 
12V.

Your lights are connected directly to the battery supply, so they dim, and 
the radio might only be specced to work down to 8V or something, in which 
case it shuts itself off until the voltage comes back up again.  Some radios 
can survive very low voltages for short periods of time, so they usually 
stay on while the engine is being started, but not on all cars.

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>> As far as I'm aware, resistors affect only current, not potential.
> 
> That statement doesn't even make sense.

As the resistence of a circuit increases, the current flowing through it 
decreases. But that doesn't affect the actual potential across the circuit.

Or rather, that's what the books I've read claim, anyway...

> By the way, notice how the lights dim on your car (and maybe your radio 
> also shuts off?) when you start the engine?

Yeah. That's because there's a limit to how much current the battery can 
supply.

Indeed, I'm told the only reason cars use lead acid batteries (which are 
very heavy yet don't actually hold that much charge) is so that they can 
briefly deliver the astronomical current required to turn the starter 
motor. From the sources I've read, if it weren't for that, you'd only 
need a battery a fraction of the size to run the spark plugs, headlights 
and other electrical bits.

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Invisible wrote:
>
> As the resistence of a circuit increases, the current flowing through it
> decreases. But that doesn't affect the actual potential across the circuit.

That's as long as the resistance on the feeding wires is small enough to
be uncompareable to the resistance of the actual device. Actually, even
then the change of the potential is just so small that you can't measure it.

> Or rather, that's what the books I've read claim, anyway...

I bet they are "for teh n00bz" -level books :).

> Yeah. That's because there's a limit to how much current the battery can
> supply.

And that's because of the internal resistance. If there were no such
thing, what would limit the current?

> Indeed, I'm told the only reason cars use lead acid batteries (which are
> very heavy yet don't actually hold that much charge) is so that they can
> briefly deliver the astronomical current required to turn the starter
> motor. 

Actually cars use lead acid batteries because they still are just the
best way to deliver that current reliably. NiCD -batteries can't stand
uneven charging, NiMH -batteries can't deliver enough current IIRC,
Litium batteries (both LiPo and Li-ion) can't stand water-freezing
temperatures. It all actually makes sense :).

> From the sources I've read, if it weren't for that, you'd only
> need a battery a fraction of the size to run the spark plugs, headlights
> and other electrical bits.

Actually you wouldn't need a battery to run those things, you'd just
need a battery/capacitor to regulate the current coming from the alternator.

-Aero

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>> Or rather, that's what the books I've read claim, anyway...
> 
> I bet they are "for teh n00bz" -level books :).

Well, it does say "ages 5 and above"...

>> Yeah. That's because there's a limit to how much current the battery can
>> supply.
> 
> And that's because of the internal resistance. If there were no such
> thing, what would limit the current?

I'm sure there's a logic there somewhere - it just doesn't make any 
sense to *me*. ;-)

>> Indeed, I'm told the only reason cars use lead acid batteries (which are
>> very heavy yet don't actually hold that much charge) is so that they can
>> briefly deliver the astronomical current required to turn the starter
>> motor. 
> 
> Actually cars use lead acid batteries because they still are just the
> best way to deliver that current reliably. NiCD -batteries can't stand
> uneven charging, NiMH -batteries can't deliver enough current IIRC,
> Litium batteries (both LiPo and Li-ion) can't stand water-freezing
> temperatures. It all actually makes sense :).

...as I said, you need the wattage to turn the starter motor. ;-) If you 
only needed electricity for the sparks and lights and stuff, you 
wouldn't need nearly as much power. (Freezing would still be an issue 
tho...)

>> From the sources I've read, if it weren't for that, you'd only
>> need a battery a fraction of the size to run the spark plugs, headlights
>> and other electrical bits.
> 
> Actually you wouldn't need a battery to run those things, you'd just
> need a battery/capacitor to regulate the current coming from the alternator.

Not if you expect the headlights to work while the engine isn't running. ;-)

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andrel <a_l### [at] hotmailcom> wrote:

> Thanks for assuming that there are only kids in this newsgroup. ;)
>

Only?


> I disagree, a lie for children is when you explain something highly
> simplified and/or deliberately wrong because the actual facts are to
> difficult. Suggesting that the heart is driven by the nervous system is
> simply wrong and what actually happens is just as easy to understand.
> Involving the brain will in a bright child immediately lead to questions
> what happens when you sleep or are unconscious. The child might even
> fear to go to sleep. It does not explain coma or how a paralysed person
> can live. So, please, if you have children, tell them a simplification
> of the truth, but never that your brain dictates when the heart has to
> beat.

OK I stand (or sit) corrected. You’ve taken the heart out of my interest :)

Stephen

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Invisible wrote:
> 
> Well, it does say "ages 5 and above"...

As assumed :).

>>> Yeah. That's because there's a limit to how much current the battery can
>>> supply.
>>
>> And that's because of the internal resistance. If there were no such
>> thing, what would limit the current?
> 
> I'm sure there's a logic there somewhere - it just doesn't make any
> sense to *me*. ;-)

That logic IS the internal resistance. You'd just need to put some mind
to it (and maybe read a book for ages 15 and above), and you'd certainly
figure it out.

>> Actually cars use lead acid batteries because they still are just the
>> best way to deliver that current reliably. NiCD -batteries can't stand
>> uneven charging, NiMH -batteries can't deliver enough current IIRC,
>> Litium batteries (both LiPo and Li-ion) can't stand water-freezing
>> temperatures. It all actually makes sense :).
> 
> ...as I said, you need the wattage to turn the starter motor. ;-) 

Yes. You can have the wattage from numerous different battery types. The
problem is that you can't have the wattage from any other battery type
reliably enough, or in a package that actually fits in the car.

> If you
> only needed electricity for the sparks and lights and stuff, you
> wouldn't need nearly as much power. (Freezing would still be an issue
> tho...)

Yep, indeed it would. Lead acid handles freezing temperatures incredibly
well and in small package.

BTW, when there's *really* cold outside, the battery won't give as much
current out, thus not starting the car. To beat this you can use the
internal resistance to heat up the battery a bit (eg. by putting your
park lights on) and then start the car. Viva la resistance!

> Not if you expect the headlights to work while the engine isn't running.
> ;-)

True. The headlights are traditionalle 55W halogens and tail light
(which are lit when headlights are lit) 5W halogens (and there's usually
4 of them). Parks are also usually 5W halogens, that makes
55*2+5*6=140W, which makes ~12A of current, which makes under 6 hours to
drain the normal ~60Ah battery empty or less to drain it to the point
the current won't be enough for starting the engine. Therefore it's a
good idea not to use the headlights too much while the engine ain't
running ;-).

-Aero

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> That logic IS the internal resistance. You'd just need to put some mind
> to it (and maybe read a book for ages 15 and above), and you'd certainly
> figure it out.

LOL! Now I have the mental image of electronics textbooks having age 
certificates.

Stuff like "Certificate 18; Contains non-conventional current flow 
direction and nonlinear diode behaviour. May be shocking to certain 
readers." ;-)

Obligatory XKCD quote: http://www.xkcd.com/567/

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> NiMH -batteries can't deliver enough current IIRC,

NiMH should be fine for current, they are used in electric vehicles after 
all.

I suspect the main drawback of NiMH is that it self-discharges pretty 
quickly compared to lead acid.  Your car might not start after leaving it 
for a few months, which would be unacceptable to most car makers.

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scott wrote:
>> NiMH -batteries can't deliver enough current IIRC,
> 
> NiMH should be fine for current, they are used in electric vehicles
> after all.

That's the one I wasn't sure about, therefore IIRC :).

> I suspect the main drawback of NiMH is that it self-discharges pretty
> quickly compared to lead acid.  Your car might not start after leaving
> it for a few months, which would be unacceptable to most car makers.

Might be. Anyway, it has such great disadvantage compared to lead acid
on this matter, it's not used :). We can be sure that car manufacturers
and battery manufacturers have calculated it.

-Aero

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Eero Ahonen <aer### [at] removethiszbxtnetinvalid> wrote:
> scott wrote:
> >> NiMH -batteries can't deliver enough current IIRC,
> >
> > NiMH should be fine for current, they are used in electric vehicles
> > after all.
>
> That's the one I wasn't sure about, therefore IIRC :).
>
> > I suspect the main drawback of NiMH is that it self-discharges pretty
> > quickly compared to lead acid.  Your car might not start after leaving
> > it for a few months, which would be unacceptable to most car makers.
>
> Might be. Anyway, it has such great disadvantage compared to lead acid
> on this matter, it's not used :). We can be sure that car manufacturers
> and battery manufacturers have calculated it.
>
> -Aero

The main advantage of lead acid batteries is they can be charged with constant
voltage where other batteries like NiCd, NiMh, LiIon or LiPo need specialized
charge electronics to limit the charge current and monitor the cell voltage.
Not so for lead acid. The only thing you need in the car is a voltage regulator
which regulates the output voltage of the alternator to roundabout 14V and the
battery is happy.
Also: Lead Acid Batteries like it if they are fully charged against for example
NiCd thad lose capacity very quickly if you have them laying around fully
charged.

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Eero Ahonen wrote:

> 
> That logic IS the internal resistance. You'd just need to put some mind
> to it (and maybe read a book for ages 15 and above), and you'd certainly
> figure it out.
> 

I was going to say something like the reaction rate inside the battery 
caps the amount of current the battery can supply, but I guess that 
would be under internal resistance.

> 
> Yes. You can have the wattage from numerous different battery types. The
> problem is that you can't have the wattage from any other battery type
> reliably enough, or in a package that actually fits in the car.
> 

Lead-acid works well in this application because it can tolerate only 
brief cycles. If you drain a lead acid battery completely, it won't 
recover well. But the advantage of that chemistry is the fast reaction 
rate, which allows the battery to supply a huge amount of amperage in a 
very short time. Other chemistries can tolerate deep cycling (near full 
discharge), but cannot tolerate the very small discharge/recharge cycles 
that are required in a car, e.g. NiMH and NiCd both tend to develop a 
"memory" (NiMH significantly less so, but it does happen) Other 
chemistries don't like being cycled frequently, and will wear down rapidly.

>> If you
>> only needed electricity for the sparks and lights and stuff, you
>> wouldn't need nearly as much power. (Freezing would still be an issue
>> tho...)
> 
> Yep, indeed it would. Lead acid handles freezing temperatures incredibly
> well and in small package.
> 
>

The alternator usually provides all of the current needed to run the 
accessories when the engine is on. This can be proven if you've ever had 
to jump an entirely dead battery and drive to the local auto shop to buy 
a replacement...

-- 
~Mike

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scott wrote:
> which case it shuts itself off until the voltage comes back up again.  

I always thought they were just wired up to work in the "on" position but 
not the "start" position.

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

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> The alternator usually provides all of the current needed to run the 
> accessories when the engine is on. This can be proven if you've ever had 
> to jump an entirely dead battery and drive to the local auto shop to buy a 
> replacement...

A lot modern cars already disconnect (electronically) a partially-charged 
battery while cruising to reduce fuel consumption.  Only when you lift off 
the throttle or brake will it connect it up to be charged, thus avoiding 
wasting fuel to charge the battery.

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Darren New <dne### [at] sanrrcom> wrote:
> scott wrote:
> > which case it shuts itself off until the voltage comes back up again.
>
> I always thought they were just wired up to work in the "on" position but
> not the "start" position.

Depends how the car is wired.
Same with the headlights. Some cars shut them off while starting, others don't.

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>> which case it shuts itself off until the voltage comes back up again.
>
> I always thought they were just wired up to work in the "on" position but 
> not the "start" position.

Before cars turned into computers, the radio system typically was just 
connected directly to the battery through a fuse, and has an "ignition" 
signal line to indicate whether it should be on or not (this signal is on in 
both the "on" and "start" position, obviously).

Car manufacturers give all sorts of detailed specs for radio systems, 
detailing exact input voltage patterns and what the system must or must not 
do.  For older radio systems they only had to work down to 10 V or 
something, and when the voltage dropped below that they had to behave 
sensibly (ie not blurt out massive clicking noises or static).  This was 
purely because it is cheap and simple to make a radio work with a supply 
between 10-16V compared to 0-16V (yes, newer radios do have to theoretically 
work with a 0 volt supply for a fraction of a second).

Recently though, especially with these systems where the engine is switched 
off every time you stop, the radio system is required to keep working during 
engine start events.  There are quite detailed specs about what the radio 
must "survive" in terms of supply voltage fluctuation, because the supply 
voltage is anything but smooth in a car.

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scott wrote:

> Recently though, especially with these systems where the engine is 
> switched off every time you stop, the radio system is required to keep 
> working during engine start events.  There are quite detailed specs 
> about what the radio must "survive" in terms of supply voltage 
> fluctuation, because the supply voltage is anything but smooth in a car.

Did you see the one on Fifth Gear where they decided to test the theory 
that a car can survive a lightning strike by... driving a car into a 
giant laboratory and hitting it with lightning, with the presenter still 
sat inside?

IIRC, afterwards the car worked great. But the radio was behaving very 
strangely...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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