This is my first post. I have been lurking around this group for a
few years...learned a lot! I have a question with only a little metal
content. I need to test 123 camera batteries (3 volts). My plan is
to hook up the battery in series with a resistor and then check the
voltage while under load. Does this sound like a reasonable plan?
What I need to know is: how much resistance would be good for
simulating the load of a digital camera? I would mount it on a small
block of wood and solder (metal content) the resistor between two
clips that would hold the battery. What voltage would be reasonable
for a "good" battery? How many watts should the resistor be rated
V=IR, Voltage = Current X Resitance
W=IV, Wattage = Current X Voltage
If you have an ohmmeter you can determine aproximately what the current
consumption of your camera is by measuring resistance at the battery
leads. Invert the positive and negative test cables of your meter and
take a second measurement, also take two measurements with the diode/
semiconductor option on your ohmmeter. Sometimes the battery feeds a
protection diode and taking straight ohm measurements gives you a bogus
reading. Once you know the aproximate resistance of your camera, use a
resistor about that size and of a wattage 2X the dissipation of the camera.
This won't work... The load presented by electronic equipment simply cannot
be measured with an ohm meter. The reason is that the equipment contains
many transistor junctions that don't turn on unless sufficient voltage is
applied. And the quiescent current is a tiny fraction of the dynamic
(cycle) current. What you need to do is to hook a current meter between the
battery and the camera with jumper wires. The easy way to do this is to get
an extra pair or two of hands involved to hold the kludge together while you
take the measurement. An old analog meter will work better than a digital
because it will respond more quickly, and you will need a quick response to
get an idea of the peak current when the camera is operated. Then,
calculate the value of a resistor that will draw somewhat (maybe about 1.5x)
more current than the camera needs.
But, a better approach would be to get the engineering data on the battery
in question (should be available on the manufacturer's web site...). It is
a safe assumption that the engineers knew what they were doing and chose a
battery adequate to run the camera... What you want is a voltage vs.
current or discharge curve so that you can determine the condition of the
battery (never mind the camera...).
There is, of course, one big bug in the whole idea of battery testing. In
many types of batteries, the voltage drops fairly quickly during the initial
usage, then drops very slowly over most of the battery's useful live, then
drops precipitously at the end. So, it may be very difficult to determine
the difference between a battery with 90 percent of its life left and one
with only 20 percent.
In short, a battery that is "mostly" used up may well be able to put out
enough current at sufficient voltage to run the camera, but will fail in
very short order.
That's right, most digital cameras probably have some sort of DC to DC
converter as a power supply. I foolishly was thinking of an analog
camera where the battery was only driving the light meter.
What kind of chemicals are used in your "123" batteries. The CR 123
batteries are commonly Lithium. The voltage across the terminals of a
Lithium CR 123 is fairly constant till the final few percent of the battery
life. So, it is very difficult (impractical) to judge the life remaining
life in a Lithium cell when only its terminal voltage is known.
If you have rechargable 123 cells, you probably wouldnt be asking this
The "resistor load" will be usefull for determining which batteries are
A 50 ohm across the two CR 123 cells in series wil draw about 120 mills.
I'd guess that current is close to what your camera needs for some
functions. A 1 watt resistor wil serve your needs if you choose to use a
50 ohm resistor.
On 5 Mar 2007 09:12:18 -0800, firstname.lastname@example.org wrote:
I'd test with 3 ohms. Some digicams draw more than an amp, some
less, but 3 ohms would work fine for testing. You'd want a 3 or 5
watt resistor. Some discharge curves for 123 cells can be seen here:
I sure need to laern to read better. I read the original post as
Batteries in series. He had written that the battery is in series with a
resistor, not two batteries in series. But, I guess he really wants to
load the battery with a resistor across it and read the voltage across the
resistor. I should have read more carefully.
But, is it really practical to evaluate a Lithium CR123 cell by measuring
the voltage across it while it is loaded to draw an amp?
You might try simply putting the cells in question into the camera and
see what the built in battery meter says. It already knows what the
load situation is.
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