hit Start. The latest version uses relays to connect the
battery pack to the load that is activated by the
program when the Start Battery Test button is hit.
The program then makes various calculations and
saves each reading to the Output Data file. The final
calculation is the mAh rating which is the last line in
// we compute the following;
// - total voltage drop from first batt
// to last batt value (at cell=1v)
// - MAH by average current/elapsedtime
double dElapsedTime = Convert.ToDouble
double MAH = (dAvgCurrentToCellOneV *
((dElapsedTime) / 3600)) 1000;
// avg current (in amps) time (in hours)
// = amp hours 1000 = MAH (millamp hours)
The program measures the time it takes for the
battery pack to reach a cell voltage of 1.0 volts. It
also measures the average current for this same time
period. It then computes the mAh rating by
multiplying the Average Current TimeElapsed (in
The program saves the calculated data into a CSV
data file that is easily imported into Excel. It would
not be difficult to create an actual Excel data file, but
that’s for a future enhancement. It is also not
necessary to use Excel since the last line in the data
file contains the summary data, including the
calculated mAh capacity of the battery tested.
I hope this gives you some ideas on how to test
and salvage all those rechargeable batteries that you
have lying around. I spent less than $100 on
everything and have saved probably more that triple
that amount by restoring many of the batteries that
were not performing. SV
DGH and Advantech A/D modules are available
I bought all of my A/D modules used
for a fraction of the new price.
I also have used an inexpensive DMM w/RS-232.
I check eBay ( www.ebay.com), Craigslist
( http://detroit.craigslist.org/), and also use
Google Shopping ( www.google.com/products).
You just have to be patient
since they don't show up very often.
42 SERVO 07.2010
How to do this
project for peanuts!
Ihave saved almost $300 using my battery testing setup. A typical cordless tool battery costs $30-$40
each, with some costing $50-$60 each. The methods
and equipment I chose will do the job of battery
testing quite well, but you can accomplish the same for
possibly under $30.
Let’s start with the load resistors. You can use
high power surplus resistors if the values result in
approximately 12 to 24 ohms (for 12V to 24V batteries)
and are capable of handling 20-30 watts. It’s even
cheaper to use automotive bulbs. Two 1156 bulbs in
series is 12. 2 ohms (hot) and they are under a $1 each.
If you want to save more money and not use sockets,
just solder the wires to each bulb, but make sure they
do not touch anything as they get very hot. I
recommend using a relay to turn the load resistor on
and off. If you don’t have an output to control the
relay, then just wire a switch in series with the resistors
or bulbs. Then when you are ready to begin, flip the
switch on and start your testing.
The other major component is the A/D system.
If you have a DMM with an RS-232 output, then that is
all you will need. Connect the RS-232 output to your
PC. Then, modify the software to work with the DMM.
Another option is an A/D demo kit that has an RS-
232 output. I have seen some of these for as low as
$20. Many manufacturers offer these demo/evaluation
kits or boards to showcase a particular A/D chip. For
battery testing, even an eight-bit A/D would suffice.
Another option is a cheap PIC kit with A/D. Many
of these have an RS-232 interface that can be used to
program the chip, and then can be used as an interface
to a PC.
You could even dispense with using a PC and
build a completely stand-alone battery testing system.
Just use an inexpensive micro with A/D and an LCD
display. I have seen some Stamp modules with A/D for
$20 and an LCD for under $10. If you shop around, you
could spend as little as $30 for the entire battery test
system as either a stand-alone or one with a PC
It will take some programming changes but all the
principles are in the software program available from
the SERVO website ( www.servomagazine.com).
Basically, all you have to do is measure the starting
voltage and the ending voltage (when each cell = 1V)
and the time elapsed. Then, based on your load
resistance, compute the average current using the
starting current and ending current. Multiply the current
times the elapsed time and you have the battery’s mAh
capacity. Even using an eight-bit A/D and measuring
the voltage every 30 seconds will still come up with a
usable mAh calculation.