I'm looking for a rechargable battery or ultracapacitor that has the
best power/weight ratio as possible given that it will be drained in
about two minutes (probably this means you need very low internal
resistance).  Ultracapacitors seem to be a little on the heavy side
based on the data I've seen.  I'm looking for a total of 20,000 watts
or more, so fairly large batteries/caps should be OK.  I'm assuming I
will have to combine a bunch to get the 20,000 watts.  Please produce
a chart of power/weight ratios for some of the lightest alternatives. 
Number of charge/recharge cycles would also be a good thing to include
in the chart.  If two minutes is too fast a drain time (violates
maximum currents) please use the maximum current the device can
handle.

---

Request for Question Clarification by chromedome-ga on 31 Jul 2002 06:33 PDT

Hi, Larry95!

You've given us some rather intriguing questions to work with, haven't
you?  I've consulted a few of my local suppliers on this one, and
they're reluctant to make a firm recommendation without a) the voltage
output you expect to require; and b) a sense of the application
involved, to the extent that you're willing to divulge it.

Given that, I anticipate being able to answer your question within the
next 24-48 hours.

-Chromedome

---

Clarification of Question by larry95-ga on 31 Jul 2002 23:27 PDT

Hi chromedome-ga,

Thanks for working on this.  I'm not that concerned about voltage, I'd
like to drive some sort of brushless electric motor, and those can be
quite flexible in voltage requirements (few volts to few hundred I
would guess).  That's the basic application.

Thanks!

---

Request for Question Clarification by ozguru-ga on 14 Aug 2002 01:49 PDT

Dear larry95,

I believe that the following paper answers your question about
specific energy (table 4 - lithium). It also provides data for the
number of charge discharge cycles (table 7 – nickel-x) and specific
power (table 5 – lead acid). It also contains many other comparison
tables that may be of interest, particularly involving relative costs.

Anant D. Vyas, Henry K. Ng, Danilo J. Santini, and John L. Anderson,
Batteries for Electric Drive Vehicles: Evaluation of Future
Characteristics and Costs through a Delphi Study
http://www.transportation.anl.gov/ttrdc/pdfs/TA/103.pdf
_____

This paper provides interesting comments regarding ultra capacitors:
Toyota did not use them because of their lower energy density.
However, although their energy density is 1/10 that of pulse
batteries, their cycle life may be 10 times that of the pulse battery.

Walstrom, Jons: Energy Storage Technology for Electric and Hybrid
Vehicles
http://www.kfb.se/ehvproge/m-99-23.pdf
_____

Other links

Ultracapacitors
http://www.maxwell.com/pdf/uc/datasheets/BMOD0115A09.pdf
Maher, Bob, Ultracapacitors and the Hybrid Electric Vehicle
http://www.ec-central.org/magazine/art_4_mar_apr_01.pdf
_____

Rechargeable battery comparisons
http://www.buchmann.ca/Article4-page2.asp
_____

Search strategy:
Highest energy density
highest current batteries lead acid lithium ultracapacitor
_____

I would be happy to respond to further clarification queries. Table 5,
specific power gives the best indication of the relative current
delivering capacities of the batteries considered as practical choices
for the  relatively high current application of electric vehicle power
(hundreds of amps rather than mA). I did not specifically address your
two minute draining concept as this would seem to be very dependent
upon the amphour rating of the specific battery. (For a price, it is
possible to purchase high current versions of many battery
technologies: lead acid, lithium and nickel cadmium). Ie Price and
depth of discharge cycle may then become as important as the density
criteria.

Please let me know if you consider this a satisfactory answer to your
question, so that I can repost as an answer or provide further
clarification.

Regards,

---

Request for Question Clarification by chromedome-ga on 15 Aug 2002 14:43 PDT

Hello again, larry95:

This has proven a little more "interesting" to research than I'd
originally anticipated.  In part, this is because my local battery
gurus turned out to be less obliging when it didn't mean a sale for
them!

A couple of points which will help me fine-tune things for you:  

1) In your discussion/dismissal of ultracapacitors, you mentioned that
they are "a little heavy" for your application.  In order to help
whittle down the number of options, what is the maximum size/weight
that you would be comfortable with?  (or range of maximum
sizes/weights, ie can be "X" large as long as it weighs less than "Y",
otherwise not).  This has a bearing on battery selection, as well.

2) You've asked to see the number of charge/recharge cycles, where
available, which would indicate that ongoing use is projected.  Now,
in the longer term, as you know, battery chemistries differ in how
they react to deep discharges.  How important is this in your
reasoning?  Do we need to take loss of capacity into account, or does
that fall under the heading of "valuable but not crucial" information?

Try contacting Electrovaya at www.electrovaya.com or call
905-855-4610.  They manufacture superpolymer lithium-ion batteries.
Joel

Given the 2 minute duration, the best bet is some of the high rate
NiCad batteries.  The best readily available cell is the Sanyo 2400
SCRC.  It can be discharged with pretty good effiicency in about 2.5
minutes, or about a 60 amp current.  Voltage is about 1 volt per cell
so about 60 W per cell, so about 350 cells.  They are about $5 to 7
per cell, and about 66 grams per cell, so thats about 10 peak W per
dollar.  With appropriate chargers, expect hundreds of charge cycles
in this kind of use, although performance will deteriorate.  If your
output current requirement means running cells in parallel, you have
to be careful about the pack design, and use isolation diodes.

NiMH cells still do not have the peak power of NiCd cells.  They do
best at a 5 minute discharge or longer.

Li-Ion cells are getting a lot better.  They are about 3 times the
energy density of NiCd, but they want at least a 15 minute discharge
time, so power density is still half of the best NiCd's.  It will be
interesting to see what they can do in a few more years.  I dont know
how the lifetime is at these high discharge currents.  Cost is much
higher than the NiCd's, probably about one Watt per $.

Hello, Larry95:

I'd been working on other questions while awaiting clarification on
this one, and overlooked the impending expiry date.

If you are still interested in an answer, I have located a number of
batteries with good power/weight ratios and low internal resistance;
and also many ultracapacitors with excellent power/weight.

I'm just trying to find the equations for calculating the maximum
discharge rate/minimum discharge time, now.

Please advise?

-Chromedome

Thanks, for some reason, I can't change the exp. date, so I'll post it
again at the same price.  Would love to see the info!

-Larry