> >> Making the test rig, etc, is easy enough, but I'd like
> >> to then make this info available online, but I don't know
> >> if that'd open me up to legal action on the part of a
> >> battery manufacturer who's displeased with my
> >> findings.
> >
> >Could happen. I've seen a number of comparative battery tests
> >punblished over the years. Being careful to publish only
> >accurate data and a few disclaimers may be wise. "Just becaise
> >Ev-O-Vac batteries performed superbly when I tested them doesn't mean ... "
> 
> I suspect that part of "covering your backside" would consist of including
> any identifiable batch numbering and such info in any data you publish. This
> gives both you and the manufacturer an "out" in that you can claim that you
> have not tested any other batches in a manner that allows the result to be
> extended to any other batch, and it allows the manufacturer to identify any
> process problem they may perceive has caused a problem if they think your
> results are not what they expect.

Good idea.  I'm not going to turn testing and posting the results of battery
quality into a lifelong hobby, I'd just like to have the info for my purposes
and I'm sure many of you would, too.

> For Russells testing with converters, it may be sufficient to use a
> switching regulator as the negative impedance load. A small one such as the
> LT1676 or LT1776 from Linear Technology would be a good starting point for
> up to about 500mA load.

That's not a bad thought, either- no better way to simulate the decreasing
efficiency of a switching supply than to use a switching supply!

> Another style of test you may want to implement is to test the internal
> impedance. Do this by having a constant low current load (say 10mA) and
> periodically turn on a higher current load (say 100mA, depending on battery
> size). Measure the voltage on both low load and high load during the
> lifetime of the battery. A switching converter such as those above would be
> good for this as they have a shutdown pin, making the load switching easy.

My very simple-minded consideration of this whole idea implies to me that
measuring the constant-current discharge would likely be both the easiest
and the best place to start.  Measure constant current discharge, and every
30 seconds (or 1 minute, or 20 minutes, or whatever the appropriate period
is) remove the load and measure the cell voltage with a very high input
impedence ADC.  From there, it's trivial to calculate the internal resistance.

Furthermore, by testing a type of cell at different discharge rates and
graphing out the data, one can easily find the total energy in the cell
(in Joules, not this mAH business), as well as getting a feel for how
discharge rate affects energy available to the load.

I'm not as blasphemous as Russell; I was going to use a PIC18F2320
to monitor the cell voltage and use PWM to drive a current source,
with some kind of feedback to keep the current constant.  I was thinking
of storing the data in flash for later upload to a PC, but a constant
connection to a PC is possible, too (although my PC client program
skills are wanting).  I figure integrating cell voltage over time times
current would give me a reasonable approximation of Joules deliverable
to load at a particular current draw.  Of course, load currents can
be highly variable, but the system I'm thinking of could easily be
made to slowly ramp up the current draw, or provide any other
discharge curve.

Mike H.

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