How To Build A Battery Tester

  If you've ever wonder which of your battery packs is your strongest,
I"ve got an inexpensive discharge meter for you.  With this discharge
meter (resistor bank), you can measure and record a pack's capacity.
Here is what you'll need:

Charger
Digital Voltmeter
Stopwatch
20A Resistive Load (Read On)



  The first 3 items are easy to obtain.  The 20A load can be built by
placing 27, 1-ohm, 10W resistors in a series/parallel combination.  These
resistors can be found at Radio Shack (part no. 271-131).  In the figure
above, you can see how I assembled the resistors in nine groups of three
series resistors together.  After soldering the nine groups of 3
resistors,  I attached them with braided battery wire.  I completed the
discharge meter with connectors for my battery.  I also added test points,
which provide a place to attach the voltmeter.  I used a set of Radio
Shack speaker terminals for the test points.  If your voltmeter leads are
equipped with alligator clips, the test terminals won't be necessary.

  You have to plot the battery discharge curves on a chart.  It's easy to
draw the chart on graph paper or a computer.  For the graph, I used a
range from 4.5 volts to 8.5 volts.  If you use a 7-cell pack, you might
want to extend the range.  The time axis runs from 0 to 5 minutes in
15-second steps.  I also used a data table, which is convenient for
recording the voltage readings obtained while discharging a battery pack.

  This Is What Your Finished Resistor Bank Should Look Like:




 CAUTION!  The resistor bank gets very hot.  The resistor bank is
handling 120 watts.


THE TEST

  Now that you've assembled the discharge meter, let's look at the
procedure of recording the data.


  Charge The Battery Pack
  Attach the voltmeter at the test points
  Start the stop watch and quickly plug in the battery pack.
  As quickly as possible, look at the voltmeter and record the zero-time
voltage reading.  The meter reading will be changing fast, so write down
the highest number that you see.
  With your stopwatch, record the voltage reading of the battery pack
every 15 seconds.  You'll have plenty of time to record the data.
  Continue taking voltage readings until the battery voltage falls just
below 6 volts.  I prefer to stop the recording after the 4:15 mark.
Don't let the batteries drop below 5.5 volts.
  Plot the data.
 

  Your Data Readings Should Look Like This:



ELAPSED TIME 
VOLTAGE READING
:00 
7.46
:15 
7.15
:30 
6.98
:45 
6.79
1:00 
6.78
1:15 
6.78
1:30 
6.76
1:45 
6.74
2:00 
6.72
2:15 
6.71
2:30 
6.68
2:45 
6.65
3:00 
6.62
3:15 
6.59
3:30 
6.56
3:45 
6.54
4:00 
6.50
4:15 
6.46


  With this resistor bank, you'll be able to test your batteries for the
best pack.  It could also be used to evaluate different brands and types.

 CAUTION!  The resistor bank gets very hot.  The resistor bank is
handling 120 watts.
 

  These plans were originally published in the March 1992 issue of RCCA.