Re: Using Ultra Capacitors to Replace Batteries

On Tue, 17 Jul 2007 22:14:29 -0400, the renowned krw <krw@xxxxxxxxxx>
wrote:

In article <arlq93hblejlkuae64nj8fgpufo4qljvgv@xxxxxxx>,
speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx says...
On Tue, 17 Jul 2007 14:50:39 -0400, the renowned krw <krw@xxxxxxxxxx>
wrote:

In article <fs2q93pee6dutpk95ggdbq6f6osdnclqnq@xxxxxxx>,
speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx says...
On Tue, 17 Jul 2007 14:22:44 -0400, krw <krw@xxxxxxxxxx> wrote:

In article <Xns997064D4F2AC7damon161attbicom@xxxxxxxxxxxxxx>,
damon1SIX1@xxxxxxxxxxxxxx says...
Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in
news:469C0E6A.BB11045D@xxxxxxxxxxx:

D from BC wrote:

Can ultracaps outperform batteries?

Not normally, although the ultra caps used in regenerative braking
systems in electric vehicles have the advantage of being able to
accept charge faster than a battery will.That's the only reason
they're there.

Another important difference between batteries and capacitors
is the flatness of discharge voltage; a battery is nearly flat
over most of its range. I don't think a capacitor, no matter
how large, will be remotely as flat.

Yeah, kinda. Since C = Q/V, the voltage across the capacitor is
proportional to the charge on the capacitor, I.e. the discharge curve
is linear.

Depends on the load- if it's drawing constant power like a regulated
SMPS with constant load (or a switching motor controller with constant
output power) it curves downward as the voltage drops and the current
therefore increases.

Of course! You added a squared term in there. Battery discharge
curves (at least all I've encountered) show charge vs. voltage.
Since that was the comparison...

The cheap primary batteries such as carbon-zinc and alkaline (for some
reason) usually are specified with constant resistance load, so the
current drops as they discharge (and the voltage drops a lot as they
are used up).

Actually, that makes some sense for primary cells (think
flashlights).

Yes, that's probably the origin of it, predating radios and such like
(or maybe just the A battery on early portable radios).

More expensive ones like NiMH are typically spec'd with constant
current load as you suggest (but they are fairly constant voltage
throughout most of the useful part of the discharge). I have not seen
one spec'd with constant power load, although that's probably more
typical of today's loads (maybe with some pulsed aspect to the
loading).

Are DC motors constant power? I know induction motors are, more or
less.

Hmm.. if you just hook them across the battery (as in a non-electronic
toy) they're more like constant current with a constant load-- they'll
slow down (less back EMF) to deliver similar torque. So if junior is
running his toy truck up an incline, the constant current test should
be fairly close to the real operating conditions.

I don't think you get the negative resistance characteristic showing
up that strongly except with electronic switching regulators.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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