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Discussion Starter #1
Re: [EVDL] Who sells controllers for ultra-caps ?

In doing my "homework" over the past few years... I have come to believe
that Super-Caps, Stiffening Caps, or Even the Front end Invert-Caps in the
controllers can SIGNIFICANTLY alter the batteries life.

We all know that HEAVY loads above the "normal" C rating of the battery
significantly degrades a battery.. . What I believe is that there are
HIDDEN current spikes that are caused by the controllers that are not
adequately filtered by the controllers front end caps. The Spikes, although
extremely narrow, are many times the average current we see on our
instrumentation, but they are responsible for heating and erosion of the
Batteries. This hidden load is purely reactive/capacitive and serves no
purpose... THIS is where I believe adding additional very low ESR Caps to
the front will help. They don't need to be big, because their effect is in
the sub-1/10th of a second range, more in the switching frequency
harmonics...

While this additional reserve up front could help the Battery, it COULD hurt
some lesser controllers, as the Spike that we are not seeing was limited by
the battery, is no longer and thus the semiconductors now could see that
inductive SPIKE on the front of each switch...

Ideally, you could have a TUNED RLC filter between the battery and the
controller that would active filter spikes from the battery, but provide
clean current to the controller...currently we just have a RC with the L
after the semiconductors...obviously there is a BIG C in the controller, but
again it is being switched and at the current we can see that causes some
interesting spikes that can't be all filtered with just capacitors... Some
people have experimented with BIG inductors on their B+ lines with antidotal
success stories...

Just my non-scientific $.02

Bob Sisson
1993 Geo Metro Convertible Project
Gaithersburg MD

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of tomw
Sent: Saturday, November 06, 2010 9:26 AM
To: [email protected]
Subject: Re: [EVDL] Who sells controllers for ulta-caps ?


My impression was that most applications use the supercapacitors in parallel
with the battery pack to limit current spikes to/from (if have regen) the
pack. If you pull 400A at 115V for 20 seconds during acceleration, that is
about 920 kJ or about 0.26 kWh. The energy discharged from the caps is
0.5CV*V, so for a 5V change in voltage (120-115) that would be about 31.3 kJ
or about 0.009 kWh. If my numbers are correct, I don't see how this helps
reduce battery current spikes all that much. It seems more cost effective
to size your pack at larger Ah capacity to limit the current spikes to less
than 3C (for LiFePO4 cells), so they have a small effect on cycle life.
--
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http://electric-vehicle-discussion-list.413529.n4.nabble.com/Who-sells-contr
ollers-for-ulta-caps-tp3029625p3029965.html
Sent from the Electric Vehicle Discussion List mailing list archive at
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Discussion Starter #2
Re: [EVDL] Who sells controllers for ultra-caps ?

Bob Sisson wrote:
> I have come to believe that Super-Caps, Stiffening Caps, or even the
> Front end Invert-Caps in the controllers can SIGNIFICANTLY alter the
> batteries life.

There is some truth to what you're saying, but it seems to have been
greatly exaggerated by the magnifying glass of marketing spin. For
example, the absurdly exaggerated claims for auto audio "stiffening
caps", or comments on the benefits of controller input capacitors when
going from *no* capacitors (GE EV-1) to a modest bank of cheap
capacitors (Curtis) or good ones (Zilla).

> We all know that HEAVY loads above the "normal" C rating of the
> battery significantly degrades a battery.

Well, going from *no* capacitors (GE EV1 controller) to modest
capacitors (Curtis controllers) has been shown to add 20-30% to range.
The increase in battery life is detectable, but probably less. There has
been no noticeable further improvement going from modest to great
capacitors (Zilla). The better capacitors have more to do with reducing
EMI and extending controller life than they do with helping the batteries.

> What I believe is that there are HIDDEN current spikes that are
> caused by the controllers that are not adequately filtered

Nothing is hidden from a man with a good oscilloscope! :) Controller
builders know about these spikes. If they're smart, they deal with them;
otherwise they'll soon be adding dead parts to their controller
development bucket.

> THIS is where I believe adding additional very low ESR Caps to the
> front will help. They don't need to be big, because their effect is
> in the sub-1/10th of a second range, more in the switching frequency
> harmonics.

The spikes you refer to are there, and of *far* higher frequencies --
in the microsecond range. A good controller will have film or other
types of capacitors to deal with them. If the designer *doesn't* deal
with these spikes, they will murder the transistors and diodes long
before they hurt the batteries!

> Ideally, you could have a TUNED RLC filter between the battery and
> the controller that would active filter spikes from the battery, but
> provide clean current to the controller.

This is called soft-switching or quasiresonant switching. It's been used
for decades in high-end switching power supplies, and in mature
industrial controllers, but is barely begun to be applied to EV
controllers. I think designers are so busy fighting with the big issues
that they haven't gotten around to dealing with the subtleties of soft
switching yet.

Note that supercap and ultracaps are not low-ESR capacitors in the sense
we are talking about here. They are still basically electrolytics, and
have similar ESRs to batteries. The best batteries still have lower ESRs
than the best supercap or ultracaps. The only margin for improvement is
when the best supercaps/ultracaps are used with the *worst* batteries.
Maybe this makes sense; maybe not -- it all hinges on the economics.
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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