Re: Question: capacitors as special use batteries?


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:3tlf74tlds3t6q5hg86rn5a6ffkq4ekl5n@xxxxxxxxxx
On Fri, 11 Jul 2008 08:16:36 -0700 (PDT), BobG <bobgardner@xxxxxxx>
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 in:out) 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.

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. When connected to a load R, in one RC time constant of time, the
energy has dropped to 6.8J or roughly 86% of the total initial energy is
dissipated in the first RC time constant.

Another downside of using supercaps... their intrinsic voltage rating is
usually very low, typically around 2.5VDC to 2.7VDC. To get any sizeable
voltage rating (for instance 100VDC) you need a LOT of caps in series, which
significantly drops the capacitance value. Even the caps rated at 5.0VDC or
5.5VDC are typically made up of a set of the lower voltage-rated caps.

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

Dave


.

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