Newsgroups: comp.robotics
Path: brunix!sgiblab!news.cs.indiana.edu!umn.edu!gold!roger034
From: roger034@gold.tc.umn.edu (Brynn Rogers)
Subject: Re: Battery Current Sourcing
Message-ID: <CsAsyn.6y5@news.cis.umn.edu>
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Organization: University of Minnesota
References: <rowleyCsACpI.9AC@netcom.com>
Date: Sat, 2 Jul 1994 05:35:04 GMT
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In article <rowleyCsACpI.9AC@netcom.com>,
Thomas Rowley <rowley@netcom.com> wrote:
>Does anyone know the non-destructive current sourcing capabilities of
>Nicads? or gel cells for that matter. While batteries have a nominal
>amp-hour rating, this value does not indicate the maximum rate that the
>battery can be discharged. Also the RC hobby catalogs describe "SCR"
>Nicads which are optimized for rapid charge/discharge. Some 1 A-H SCR
>Nicads are described as bing able to provide 20+ amps for short periods!
>
>In particular, I need to provide 3-4 Amps for periods of 15-20 minutes
>before recharging. What is the smallest rechargeable Nicad that I could
>use?


By the numbers, 4 Amps x 20 Minutes(.33hours) = 1.33 Ah or 1330 Mah.

At the minimum, you need a Cs (sub-C, not quite as big as a C cell) 
but it is probably easiest for you to get C cells.

There are many types of NiCad cells and some are much better at high
current draw (and charging) than others.  It is the way the battery is
made that has the biggest effect on this.   For the best high current
draw/charge battery you need something SAFT calls Sinter-Sinter construction,
and other people have this too by maybe other names (sinter-plate or
sinter-foam come to mind.)

We use a 10 C cell pack (2400 Mah rated, 2800 Mah typical) SAFT SE type cells
which can easily supply 5 Amps for a half hour or so.  It can also be charged
at a 1C (or even 2C) charge rate, as long as you have a thermistor on the
pack and terminate charging when it is full.

   When we were picking out what cells to use (a learning experience) we 
tried some cheaper panasonic P-220C cells, a 2200 Mah C cell that is
press-plate construction (I think) and it could not supply us the current
we needed.  We make thermal printing machines and this 2 inch machine
requires about 4 amps typical to print.  In the Panasonic spec for these
batterys the fastest charge rate you can give them is 730 Ma, or  C/3.
(Panasonic likes to call that 'Quick').

   The one advantage the Panasonic cells (or any plain NiCads) have over
the Saft (or any high charge/discharge rate batterys) is that they have a 
very low self discharge rate.  You could charge up these panasonics and leave
them on the shelf for 3 months and they would still be 80% full.  The SAFT
cells would be totally dead at that time.  If you never wanted to discharge
at a greater than C/4 rate (or charge faster than C/3) the panasonic cells
would be better for you.  They could not cut the mustard for our portable
application.

  
   Most batterys you see at radio shack or where ever are the cheap kind
like the panasonics  (may actually be panasonic, they are the biggest mfg.).
Only at hobby stores where the RC car/boat/plane/??? people shop will you
find the good fast charge/discharge cells.

  From what I hear, Sanyo makes the best cells (and most expensive), 
followed by SAFT, then everybody else.

  FYI     SAFT America Inc.
          Portable Battery Division
          711 Industrial Blvd.
          Valdosta, Georgia 31601
          912-247-2331


P.S.  The better panasonic cells (which we never tested, 
      our purchasing department didn't want to cough up that much dough
      [I think they are more expensive than the nice SAFT cells we use])
      are  P-280CR  2800 Mah   2800 Ma  max charge rate (it says here
      2800 Ma x 1.5 Hours,  there are losses when charging [heat!])
  
      If I am not mistaken, both these types of Panasonic cell are available
      from Digikey.


Brynn Rogers     roger034@gold.tc.umn.edu



--
Brynn Rogers     roger034@gold.tc.umn.edu

