FIGURE 5. Block diagram of the test system.
accuracy is of academic value only.
Look at the accompanying plots (in Excel) of a
number of tests that I ran on eight Ryobi 18 volt
battery packs. You can see that quite a few of the
packs had very low mAh capacity. However, if you
compare by colors (same battery pack), you can see
that some of the tests also had acceptable
results. The reason is that a full charge/
sometimes restore a NiCad battery’s capacity to
near normal. If it doesn’t, then that pack is
probably defective. You can also tell by the plots
whether a pack has any failed cells by a step
voltage drop early in the discharge cycle.
Figure 3 shows the actual data used to
create the plots in Excel. If you look at it closely,
you can see that the Charge/Discharge (test)
cycles resulted in some packs more than
doubling in mAh capacity (see tests B5B and B5C).
If you have any battery packs that are NiMH, you
will see a big difference in the discharge curves. The
NiMH will have a higher and more constant voltage
during discharge, but have a big step drop at the end
when it reaches its capacity end. The three dark
colored discharge curves (Figure 4) are NiMH
FIGURE 6. Test system electronics’ housing and
FIGURE 7. The electronics consisting of the A/D modules,
control circuit, and power supply.
Check out the block diagram of the original
test setup in Figure 5. To make the connection to
the battery easier, I modified each battery charger
by adding terminals that connected to the
charger’s battery terminals. Make sure that you
understand that with some battery chargers there
can be lethal voltages at these points if plugged
into 120 VAC with no battery pack installed!!!
NEVER PLUG IN THE BATTERY CHARGER WHILE
TESTING OR PREPARING TO TEST A BATTERY
I have used DGH and Advantech serial A/D
modules but any A/D that you can easily interface
(serial A/D, serial DMM, Ethernet, USB) to your PC
will work. The software provided is for example
only and you will have to modify it (within VC#) to
work with whatever A/D system you are using.
A/D modules with a PC interface are readily
available for a reasonable cost. With some
persistence and patience, you can find a useable
A/D module at even less cost (auction, closeouts,
or using a microcomputer chip with A/D).
In terms of actual hardware, there are no real
critical components. However, for the voltage
divider you should match values as closely as
40 SERVO 07.2010