possible. You can compensate for any errors in
the voltage divider by using a calibration factor in
the test program. The load resistor value should
be within range of the estimated mAh capacity
and be able to handle and dissipate the power.
You can either just connect the battery to the
load resistor with a jumper lead or use a digital
I/O line (from the A/D module) to drive a relay to
handle the connection. The relay should be
spec’d to handle the actual load current plus a
safety margin (100%), and the relay driver can be
just about any Darlington transistor with enough
gain to drive the relay and a current rating to
handle the relay.
If you add current sensing, the current sense
resistor must be low enough to minimize the
effect on the load, yet large enough for the A/D
to measure. One advantage of the Advantech
module is that each channel’s gain is
programmable. You can measure the voltage
with one channel and the current with another
channel.
If you do not use a software-controlled relay
to make/break the connection to the load
resistor, you will have to monitor the battery
voltage and disconnect the load when the
battery voltage drops to one volt per cell.
Discharging much beyond that point is not
recommended as it can degrade a battery’s
performance.
I am also considering building a completely
stand-alone microcomputer-based battery
tester/charger. You could take the basic design
described here but use a microcomputer (chip or
board system) with A/D and digital outputs, just
interfaced to an LCD display. It would be best to
use a system with a high level language, as the
math involved would be much easier to program
that way. The tester could be integrated into a
standard battery charger, but with relays to
separate the charge and test functions. For our
PC-based system, I charge the batteries using a
separate charger unit. That way, I can test one
battery while recharging another.
The Software
The software was written in Microsoft Visual
C# 2008 Express Edition which is available for
free from Microsoft.
When the Start Battery Test button is hit, the
software initializes the serial port, creates the
Output Data file, turns on the load relay,
initializes a one second timer, and begins taking
measurements. In the early setup, I would
just connect the load with jumper clips, then
FIGURE 8. Access points to the battery terminals.
FIGURE 9. Block diagram of
the enhanced system.
FIGURE 10. Test system software control panel.
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