operate the equipment they are trying to power; kind of
like the older NiCads but for different reasons. Lithium-ion
technology is here to stay, but it’s still a work in progress.
Battery Capacity — C
Battery capacity, labeled as the letter “C,” is the
battery’s current-energy rating as in 2,450 mAh (
milliamp-hours). I used an Energizer AA size, model NH15BP- 2 for
testing. That means this particular battery can (theoretically)
produce 2,450 mA for one hour, or 4,900 mA for 30
minutes, or 9,800 mA for 15 minutes (I doubt it). In reality,
batteries do not respond in such a linear fashion. Battery
manufacturers typically specify a discharge load of 0.1C or
one-tenth of current capacity when rating their batteries;
then they simply multiply this number by 10 for the
published rating. Therefore, a battery that can deliver
245 mA for 10 hours will be rated at 10 x 245 or
2,450 mAh as in our test battery — specsmanship at
its best or worst depending on your point of view.
The value of 0.1C has been chosen by manufacturers as it illustrates battery capacity in the best light
because of the lower discharge value. A faster discharge rate will most certainly result in less energy
being available from the battery, and therefore the
measured capacity will be lower. It’s like specifying
fuel mileage for cars; you can’t get the same range
or mpg if you drive 70 mph versus seven mph. So,
who can you believe — their specs or your tests?
full sunlight and allow the battery to begin to charge.
Figure 5 shows the beginning of a normal charge cycle
with the nominal voltage and current flowing from the
solar panel into the battery. Notice that the battery voltage
is above the 1.2 volt rating during charging; this is normal.
Also notice that the current from our small solar panel is
about 126 mA, which is a trickle charge level. Since our
test battery is rated at 2,450 mAh, it will take about 24
hours “in the bright sun” to completely charge it at this
small current level. Therefore, the charging cycle may have
to be extended over several days to acquire a full charge.
The charging time is totally dependent on your solar panel’s
voltage and current capability. If you use a larger panel with
greater current output, the charge time will be less. All
things considered, it won’t hurt if you never get it to a fully
charged state. Again, NiMH batteries don’t suffer the
memory effect of partial charging so don’t be concerned.
Figure 1 – Battery Charge Test Bed for the Parallax BS2 Processor.
Setting Up the Test Bed
You can determine the battery capacity under
load by carefully discharging the battery while
measuring the power at specific time intervals.
However, you can’t do it unless it’s charged to
some degree. This, then, brings us to the first phase
of our experiment: charging the battery — slowly
and carefully so as not to damage it. The hardware
setups are illustrated in Figure 1 and Figure 2 for
the Parallax BS2 and PICAXE 28X2, respectively.
You will need to wire the circuit and download the
firmware to the micro.
Note: If your battery is already fully charged or
partially so, it is best to discharge it to approximately
0.25 volts (but not 0 volts — never fully discharge it)
using a 10 ohm resistor before going to the next
step. Use the test bed setup in Figure 3 or Figure 4
to monitor the voltage levels. It basically swaps the
battery for the solar panel as the voltage source and
uses the 10 ohm load to discharge the battery. You
can monitor the voltage level on the computer.
Return to the test bed setup in Figure 1 or Figure 2
when the battery is discharged to about 0.25 volts.
Figure 2 – Battery Charge Test Bed for the PICAXE 28X2 Processor.
Figure 3 – Battery Discharge Test Bed for the Parallax BS2 Processor.
Part 1: Manual Charging
Set up the test bed in Figure 1 or Figure 2 in
Figure 4 – Battery Discharge Test Bed for the PICAXE 28X2 Processor.
October 2009 47