High voltage to power electric cars.

[sinebar@bellsouth.net]

Please bear in mind that I am not an electrical engineer so my
knowlege is somewhat limited. Now my question.
I presume that storing high voltage at low current is more efficient
than storing low voltage at high current? I also presume that
capacitors can be charged much faster than a lead acid battery? So I
was thinking why not charge up a bank of high voltage capacitors with
high voltage low current and then step it down to low voltage high
current using a transformer which then could maintain a charge on a
conventional lead acid battery that is operating the cars electric
motor. My thinking is that the caps could be charged fast and the
high voltage which is useless to run an electric motor, could be
converted to high current which is usefull for running a motor or
maintaining a battery charge. Now, could such a system have the
advantage of extending the milage of an electric car significantly
with out having to wait hours to charge?

 

[John Larkin]

On Sat, 10 May 2008 12:04:07 -0700 (PDT), "sinebar@bellsouth.net"
<sinebar@bellsouth.net> wrote:

Please bear in mind that I am not an electrical engineer so my
knowlege is somewhat limited. Now my question.
I presume that storing high voltage at low current is more efficient
than storing low voltage at high current?

Not really. There's a very wide voltage range over which electronic
chargers and motor controllers can work well.

I also presume that
capacitors can be charged much faster than a lead acid battery?

Only because capacitors store such a tiny amount of energy compared to
batteries.

What matters is joules (or horespower-hours) per pound and per cubic
foot. Changing the voltage rating of a capacitor or of a battery
doesn't materially alter that ratio [1], and batteries are hugely
better than caps, by a factor ballpark 1000:1.

John

[1] any bunch of batteries, or of capacitors, can be connected in
series (high voltage, low current) or in parallel (opposites) but the
energy storage capacity doesn't change. There's no free lunch.

 

[Michael Black]

On Sat, 10 May 2008, sinebar@bellsouth.net wrote:

Please bear in mind that I am not an electrical engineer so my
knowlege is somewhat limited. Now my question.
I presume that storing high voltage at low current is more efficient
than storing low voltage at high current? I also presume that
capacitors can be charged much faster than a lead acid battery? So I
was thinking why not charge up a bank of high voltage capacitors with
high voltage low current and then step it down to low voltage high
current using a transformer which then could maintain a charge on a
conventional lead acid battery that is operating the cars electric
motor. My thinking is that the caps could be charged fast and the
high voltage which is useless to run an electric motor, could be
converted to high current which is usefull for running a motor or
maintaining a battery charge. Now, could such a system have the
advantage of extending the milage of an electric car significantly
with out having to wait hours to charge?

JUst because there is good reason to have high voltage/low current

long distance transmission lines does not mean that batteries should
be the same thing. As recently discussed, transmission lines use
low current to avoid losses, and thus in order to transmit enough
power they use high voltage.

What's "efficiency"? You can take one single point, and it can
be very efficient, but looking at the overall system, it can be
inefficient. This isn't just about power storage, but about the
actual motor used for propulsion, and the charging system may
also come into effect.

Now, maybe there are good high voltage motors, and good reason
to use them. I don't know. But unless they exist and for
good reason, then you have to deal with converting a higher
voltage to the needed voltage, and that's bound to introduce
ineffeciency. Likewise, one has to charge them, and unless
one can deal with the charging at the higher voltage, one
has to do conversion, again introducing inefficiencies.

Of course, those other points become far more relevant because
there is no suggestion that higher voltage/lower current batteries
are more efficient.

Note that one traditional thing that runs off batteries are the
pre-atomic submarines. They'd run on the surface at night off
diesel motors, which were better than electric, and then during
the day when it was risky, run under the surface off electric
motors. The batteries were charged during the evening when
the diesel motors were running. You couldn't use diesel motors
underwater because there was no way to vent them.

But, one thing they did for "efficiency" was in how they combined
the batteries. Put them in series to get higher voltage, put
them in parallel for greater current. Read up on that sort
of thing, and it might help you build the better electric
car.

For that matter, there was a time when electric cars were "common",
right in the early days of cars. Look into what was used then.

As for capacitors, they take less time to charge compared to
batteries because they barely have any storage capacity. Charge it
up in a hurry, and you basically used up the charge in a hurry too.
It takes longer to charge a battery because that battery when
charged will supply the needed voltage and current for quite some
time.

Michael

 

[BobG]

Look at the Maxwell Boostcap web site. They make a cap the size of a D
cell that is 350 Farads, but only 2.7V. They put a bunch of em in a
box that will handle 125V and 100s of amps in and out for many
seconds. This is just the boost you need when your electric car starts
out. When cruiseing, 60V would be ok, but when accelerating, a big
relay could clack the cap bank in series with the battery and youd
have a big boost voltage to get up to speed. Also good for catching
all the juice the motor is putting out when slowing down. Better than
heating up the brakes.

 

[sinebar@bellsouth.net]

On May 10, 6:02 pm, "Tom Biasi" <tombi...@optonline.net> wrote:

sine...@bellsouth.net> wrote in message

news:50dba210-dad9-466e-a0aa-db9fcc05f2ed@m3g2000hsc.googlegroups.com...

Please bear in mind that I am not an electrical engineer so my
knowlege is somewhat limited.  Now my question.
I presume that storing high voltage at low current is more efficient
than storing low voltage at high current? I also presume that
capacitors can be charged much faster than a lead acid battery? So I
was thinking why not charge up a bank of high voltage capacitors with
high voltage low current and then step it down to low voltage high
current using a transformer which then could maintain a charge on a
conventional lead acid battery that is operating the  cars electric
motor.  My thinking is that the caps could be charged fast and the
high voltage which is useless to run an electric motor, could be
converted to high current which is usefull for running a motor or
maintaining  a battery charge.  Now, could such a system have the
advantage of extending the milage of an electric car significantly
with out having to wait hours to charge?

And, where are you going to get that DC transformer.

I believe DC can be converted to AC for driving a transformer. I
think it can be done with an inverter.