Figure 5 –
Start of Battery
cold (see Figure 5). So much for battery
designers making it simple for us common
folk to design battery chargers for them;
but that’s the fun part, right?
Another method of detecting the end
of charge is by using a temperature probe
on the battery and looking for Delta– T or
the change (rise) in the cell temperature.
However, this is for high current, fast charge
systems. Since we are trickle charging our
batteries with the small solar panel, there is
no need to do this; however, I wanted to
make you aware of it should you ever want
to use a larger, more powerful solar panel
for charging. Just remember: As with solar
panels, heat is the enemy of batteries, so
keep things cool.
Figure 6 shows what to look for as the battery
reaches its fully charged state (if you have the time and
patience to do so). Notice the slight peak in the voltage
(solid plot line) before it dips back again; this means that
the battery has reached its fully charged state. You will
generally find this type of noticeable peak when the
battery is being charged at 1C or more. If you are using a
solar panel that delivers this kind of amperage, this is what
to expect. Otherwise, you will experience plots like those
with the dashed and dotted lines at lower currents. With
lower currents, the voltage peak is much less pronounced
and is even non-existent at very low trickle rates like our
small solar panel produces.
Delta-V or Delta-T
We’ve already seen an example of Delta– V or the
“change in voltage” in Figure 6 that indicates the end of
the charge cycle. After the cell is fully charged and as it
begins to overcharge, the voltage polarity of the
electrodes inside the battery will begin to reverse, and this
will cause the battery voltage to peak then decrease
slightly. However, a disturbing characteristic about NiMH
batteries is that “false” Delta– V events can occur before
the real one does. New NiMH batteries can exhibit these
voltage peaks early in their cycle, especially when they are
Figure 6 — General Charge Curve for Three Current Inputs.
One at a Time or All Together
Another “gotcha” about NiMH batteries (unlike
Lithium-ion or lead acid) is that you just can’t detect when
the battery is fully charged by monitoring the voltage
alone. Nickel metal hydride batteries like nickel cadmium
don’t have a “float charge” voltage; they are current-based
animals and, as such, each cell needs to be charged
individually — not in series or parallel like lead acid or
Lithium-ion — especially when using a high current charge.
Take this into account if you build a multi-cell charger,
because some cells will take on more current than others
even when they are completely charged! This means battery damage will occur and about the only thing to do is
balance the current input in some way, like using resistors
in series with “each” battery in order to limit the current.
Again, we won’t worry about this for our simple one cell
charger, but just be aware of this for future reference.
Part 2: Measuring
With a fully or partially charged battery, now comes
the second phase of our experiment: measuring battery
capacity. For this test, we want to measure power into a
resistor load over a specific amount of time, a.k.a., energy.
The battery’s mAh rating is not particularly useful in the
real world as pointed out earlier. Instead of mAh, we want
wH or watt-hours, since a watt (voltage x current) takes
into account any variation in battery voltage, as well as
current, and gives a truer measure of instantaneous power,
as well as power over time (energy).
In order to measure the battery’s stored energy, we
will apply the same 10 ohm resistor load and periodically
measure the power at set time intervals. This will be easy
with the graphic software since all we need to do is hook
things up and watch what happens. A feature of the
graphic software allows you to capture an entire screen
image for later viewing and analysis. This saves trying to
copy down the voltage, current, power, and resistance