Newsgroups: comp.robotics
Path: brunix!uunet!munnari.oz.au!bunyip.cc.uq.oz.au!s1.elec.uq.oz.au!ward
From: ward@s1.elec.uq.oz.au (Thomas Ward)
Subject: Re: General Battery Characteristics?
Message-ID: <CA8oxJ.145@bunyip.cc.uq.oz.au>
Sender: news@bunyip.cc.uq.oz.au (USENET News System)
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Organization: Dept. of Elec. and Comp. Engineering, Uni of Queensland
References:  <2249t2INNqo7@uwm.edu>
Date: Fri, 16 Jul 1993 04:10:31 GMT
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In article <2249t2INNqo7@uwm.edu>, rick@ee.uwm.edu (Rick Miller) writes:
|> I'd like to find out the "typical" characteristics of ordinary batteries.
|> You know, the ones you buy anywhere... AA, C, D, and 9-Volt.
|> 
|> Has anyone got (or know where to get) typical info on these?  I'd like to
|> know stuff like:
|> 
|> 	What's the amp-hour rating?  (How is this rating tested?)
|> 	What's the internal resistance?  (Does it change with use?)
|> 	Are there curves to show battery characteristics? (What are they?)
|> 
|> RICK MILLER            <rick@ee.uwm.edu> Voice: +1 414 221 3403 FAX: -4744
|> 16203 WOODS            Send me a postcard, and I'll return another to you!
|> 53150-8615 USA         Sendu al mi bildkarton, kaj mi redonos alian al vi!

The amp-hour rating is the number of amps you can draw from a battery
for one hour.  (Actually, it's usually expressed over more than 1 hour,
e.g. 10 hours)
This means that a 1Ahr battery could supply you with 1 amp for 1 hour or
100mA for 10 hours - pick any current and time that multiply together to
give 1 Ahr.

Don't expect to get 10 amps for 6 mins though, the maximum load does depend
on the internal resistance of the battery. (You couldn't draw this much from an
alkaline AA cell for eg.)

Theoretically, you could measure the internal resistance of the battery by shorting it
out and seeing how much current flows (Stick an ammeter across it). Then divide this into
the batterie's voltage (Ohm's law -> V=I.R).  You'd want to make sure that your 
ammeter could handle the current however, and you certainly wouldn't want to do this to
a car battery (You'd blow everything in site).  The safest way is to put an external
resistance in series with the battery (in series with the ammeter) to ensure that you
don't suck too much current out (i.e. for 1.5V battery you might use a 10 ohm resistor)
Then measure the current - if the internal resistance was 0 ohms, you would get 150mA
flowing.  If it was 10 ohms, then you would get I=V/R = 1.5/(10+10) = 75mA flowing.
In other words, divide the cell voltage by the current, and then subtract the externally
applied resistance to find the internal resistance.  You could get to the same solution
by measuring the voltage drop across the load instead of looking at the ccurrent.

The most common curve for batteries is voltage vs time for a set load.  This is
easy to do just by putting a resistor across the battery and measuring the voltage
supplied into the load at regular intervals.  You might want a couple of curves
(maybe one for 10mA, 100mA, 1A etc.)
Be careful if you go for high currents - A 1 ohm resitor across a 1.5V battery
capable of supplying a large current would cause 1.5A to flow.  Thsis would 
dissipate 1.5Vx1.5A=2.25W in the resistor - go for at least a 5W reseitor.

To find out the number of AHr in a battery, just draw this curve, and see how long
you can supply whatever current you are drawing before significant voltage drop,
and multiply them together.

As a rough guide, Leas-acid and NiCads have fairly low internal resistance
Alkalines have more and standard dry cells have the most.  The smaller the cell
the greater it's internal resistance.
You'll get a couple of AHr out of a alkaline AA, and about 500 or 600mAHr out
of an AA NiCad.  You'll only get a couple of hundred of mAHr out of a 9-Volt
though.


Hope this is of some help,

Tom Ward (ward@s1.elec.uq.oz.au)
