Question: capacitors as special use batteries?

[msadkins04@yahoo.com]

I recall from my hobbyist days that high-voltage capacitors can keep
a
charge for a significant amount of time, especially if they are cut-
off from the circuit subsequent to charging.

I'd like to know whether it might be feasible, from a technical and
cost-effectiveness standpoint, to use a large array of specially
designed capacitors to hold excess electricity from solar plants for,
say, periods of a week or more. This would be helpful to bridge gaps
caused by overcast weather, and for other purposes.

I know that there are potentially problems with dielectric breakdown
and leakage. On the plus side, such "batteries" could be charged and
discharged very quickly, as needed, and without the complications
involved in conventional batteries using chemical electrolytes.

 

[Michael Black]

On Wed, 9 Jul 2008, msadkins04@yahoo.com wrote:

I recall from my hobbyist days that high-voltage capacitors can keep
a
charge for a significant amount of time, especially if they are cut-
off from the circuit subsequent to charging.

The fact that something can keep a charge for a significant amount of

time is irrelevant. The fact that those capacitors are isolated from
the circuit so there is virtually no load is the giveaway. If you
keep a load on the capacitor, it will discharge. A really high impedance
load will not affect the voltage on the capacitor for a long time, but
the moment you try to draw any real current and the voltage on the
capacitor goes away immediately.

Capacitors don't have the capacity to do what you want. They are
good at storing voltage, they are lousy at storing power, and you need
power (ie voltage times current) if you want to do anything with this.

You will very easily learn that you would need massive capacitors (in
terms of capacitance and thus in terms of physical size) before you
could even come close to doing anything useful this way.

Spend your efforts on lowering current draw, which then requires less
power. I have one of those "atomic clocks" that is at least four years
old and still running on the two AA batteries that I put in when I bought
it. That's low current, considering there is radio circuitry that has
to come alive each night to get the update from the time station. No need
to fuss with large capacitors, or rechargeable batteries, it just does
its job well with very little current draw.

In a lot of cases, you can't do that, since the device is required
to supply power of some sort (like that stereo amplifier over there).
Radios are another place where low current isn't necessarily the best
route; I just got one of those crank radios and the crank function is
great, the radio is horrible, likely the result of low current being
seen as the primary design factor, rather than good reception.

Michael

 

[Alan Nishioka]

On Jul 9, 10:50 am, "msadkin...@yahoo.com" <msadkin...@yahoo.com>
wrote:

I'd like to know whether it might be feasible, from a technical and
cost-effectiveness standpoint, to use a large array of specially
designed capacitors to hold excess electricity from solar plants for,
say, periods of a week or more. This would be helpful to bridge gaps
caused by overcast weather, and for other purposes.

Modern capacitors are starting to have enough capacitance to be used
in this sort of application.

Here is a research starting point:
http://en.wikipedia.org/wiki/Supercapacitor

Alan Nishioka
alan@nishioka.com

 

[Eeyore]

"msadkins04@yahoo.com" wrote:

I recall from my hobbyist days that high-voltage capacitors can keep
a charge for a significant amount of time, especially if they are cut-
off from the circuit subsequent to charging.

I'd like to know whether it might be feasible, from a technical and
cost-effectiveness standpoint, to use a large array of specially
designed capacitors to hold excess electricity from solar plants for,
say, periods of a week or more. This would be helpful to bridge gaps
caused by overcast weather, and for other purposes.

I know that there are potentially problems with dielectric breakdown
and leakage. On the plus side, such "batteries" could be charged and
discharged very quickly, as needed, and without the complications
involved in conventional batteries using chemical electrolytes.

Basically no. The energy stoage capacity per volume / weight is absurdly
low when compared with any other method.

So-called 'super caps' have niche applications but will never be used
like that.

Graham

 

[John]

On Wed, 9 Jul 2008 10:50:31 -0700 (PDT), "msadkins04@yahoo.com"
<msadkins04@yahoo.com> wrote:

I recall from my hobbyist days that high-voltage capacitors can keep
a
charge for a significant amount of time, especially if they are cut-
off from the circuit subsequent to charging.

I'd like to know whether it might be feasible, from a technical and
cost-effectiveness standpoint, to use a large array of specially
designed capacitors to hold excess electricity from solar plants for,
say, periods of a week or more. This would be helpful to bridge gaps
caused by overcast weather, and for other purposes.

I know that there are potentially problems with dielectric breakdown
and leakage. On the plus side, such "batteries" could be charged and
discharged very quickly, as needed, and without the complications
involved in conventional batteries using chemical electrolytes.

The capacities aren't yet at that level, but one company is now
marketing a capacitor powered cordless drill with a recharge time of
less than 2 minutes.

..

 

[John Larkin]

On Thu, 10 Jul 2008 17:28:00 -0400, John <look@sig.net> wrote:

On Wed, 9 Jul 2008 10:50:31 -0700 (PDT), "msadkins04@yahoo.com"
msadkins04@yahoo.com> wrote:

I recall from my hobbyist days that high-voltage capacitors can keep
a
charge for a significant amount of time, especially if they are cut-
off from the circuit subsequent to charging.

I'd like to know whether it might be feasible, from a technical and
cost-effectiveness standpoint, to use a large array of specially
designed capacitors to hold excess electricity from solar plants for,
say, periods of a week or more. This would be helpful to bridge gaps
caused by overcast weather, and for other purposes.

I know that there are potentially problems with dielectric breakdown
and leakage. On the plus side, such "batteries" could be charged and
discharged very quickly, as needed, and without the complications
involved in conventional batteries using chemical electrolytes.

The capacities aren't yet at that level, but one company is now
marketing a capacitor powered cordless drill with a recharge time of
less than 2 minutes.

.

And I bet the drilling time is a lot less than that.

John

 

[BobG]

Tecate sells the Maxwell Boostcaps that are the size of a D-cell,
2.7V, 350 Farads. Its takes several minutes to charge one up on the
lab supply supplying 2.5V 3A. Several 1000 watt-sec.

 

[John Larkin]

On Fri, 11 Jul 2008 08:16:36 -0700 (PDT), BobG <bobgardner@aol.com>
wrote:

Tecate sells the Maxwell Boostcaps that are the size of a D-cell,
2.7V, 350 Farads. Its takes several minutes to charge one up on the
lab supply supplying 2.5V 3A. Several 1000 watt-sec.

You're not going to get more power out, on average, than you put in.
Drills use a lot of power, so if a 2-minute charge yields 2 minutes of
drilling, it's going to be a *big* charger.

Batteries aren't as efficient as caps (joules inut) but batteries
tend to run constant-voltage during use. Caps lose voltage linearly,
so will need downstream electronic switching regulators to keep up the
load as they discharge.

John

 

[BobG]

On Jul 11, 6:03 pm, John Larkin

You're not going to get more power out, on average, than you put in.
Drills use a lot of power, so if a 2-minute charge yields 2 minutes of
drilling, it's going to be a *big* charger.

Batteries aren't as efficient as caps (joules inut) but batteries
tend to run constant-voltage during use. Caps lose voltage linearly,
so will need downstream electronic switching regulators to keep up the
load as they discharge.
====================================I bet you could build form-fit-and-function battery packs for a drill

using boostcaps and a wide input range switcher like you said. You
drill for a couple minutes then change the pack out. The charger could
hump 50 or 100 amps into the pack being charged and charge the thing
up 10x faster than it discharged under use. Better than the charge for
several hours arrangement they have now.

 

[Jasen Betts]

On 2008-07-12, starfire <starfire151@cableone.net> wrote:

What we really need is a 200F, 100VDC-rated cap the size of an 0805
surface-mount device

Someting like that would be dynamite!

Infact, better than dynamite.
Dynamite only manages an energy density of 4300J/g at about 0.002g each,
mass for mass your (dream) caps would pack 1000 times the punch when fully
charged.



Bye.
Jasen

 

[Ross Herbert]

On Wed, 9 Jul 2008 10:50:31 -0700 (PDT), "msadkins04@yahoo.com"
<msadkins04@yahoo.com> wrote:

:I recall from my hobbyist days that high-voltage capacitors can keep
:a
:charge for a significant amount of time, especially if they are cut-
ff from the circuit subsequent to charging.
:
:I'd like to know whether it might be feasible, from a technical and
:cost-effectiveness standpoint, to use a large array of specially
:designed capacitors to hold excess electricity from solar plants for,
:say, periods of a week or more. This would be helpful to bridge gaps
:caused by overcast weather, and for other purposes.
:
:I know that there are potentially problems with dielectric breakdown
:and leakage. On the plus side, such "batteries" could be charged and
:discharged very quickly, as needed, and without the complications
:involved in conventional batteries using chemical electrolytes.


At this stage supercapacitors are only useful and cost effective for supplying
short term, high current delivery.

In 2000,CSIRO and other Australian auto companies developed a hybrid electric
demonstration vehicle called aXcessaustralia LEV
http://www.csiro.au/solutions/aXessaustralia.html

CSIRO developed a lead-acid battery with combined supercapacitors to produce a
battery pack which was about 20% the weight of battery packs normally used in
electric vehicles. The Supercaps were charged by regenerarive braking and
supplied the short term boost current for rapid acceleration. When running on
electric only, the vehicle was capable of around 20 minutes use in urban
environment.

 

[John Larkin]

On Fri, 11 Jul 2008 23:01:00 -0600, "starfire"
<starfire151@cableone.net> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:3tlf74tlds3t6q5hg86rn5a6ffkq4ekl5n@4ax.com...
On Fri, 11 Jul 2008 08:16:36 -0700 (PDT), BobG <bobgardner@aol.com
wrote:

Tecate sells the Maxwell Boostcaps that are the size of a D-cell,
2.7V, 350 Farads. Its takes several minutes to charge one up on the
lab supply supplying 2.5V 3A. Several 1000 watt-sec.

You're not going to get more power out, on average, than you put in.
Drills use a lot of power, so if a 2-minute charge yields 2 minutes of
drilling, it's going to be a *big* charger.

Batteries aren't as efficient as caps (joules inut) but batteries
tend to run constant-voltage during use. Caps lose voltage linearly,
so will need downstream electronic switching regulators to keep up the
load as they discharge.

John



You're absolutely right, John. A couple of minor points, though...

Caps lose voltage exponentially, not linearly. That's one of the downsides
of using them as battery replacements.

Linearly with constant-current load. Drooping exponential with
constant-resistance load. If you hang a switching regulator on the
output and drive a constant-power load, current will increase as
voltage drops, which is a growing exponential upside-down, because the
load is a negative resistance.

How many watts does a drill need? A lot, I think, if it's doing any
serious drilling.

We've been using supercaps as intermittent storage devices and they have
their good sides and bad sides. No matter what the size of the cap, if the
load is fairly low, the dissipation of energy through the load happens very
quickly. Since E = 1/2 C * V^2 (energy = 1/2 * capacitance * voltage
squared), if a 4F cap is charged to 5VDC, it has an initial energy value of
50J.

A AA alkaline battery can deliver a couple of kilojoules. So spending
multiple dollars for 50 joules isn't very appealing

John