> 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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