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Lithium Ion Battery |
Lithium Ion Current Capacity Vs Voltage |
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AA
Battery Holder |
White LED Voltage Vs Current |
Solar Powered Spotlight |
Based on the battery curves shown, it looks like a good voltage to
terminate a discharge test would be at about 3 volts. Also, as shown by the typical
white LED forward voltage Vs current curve, it looks like when the LED reaches 3 volts,
the forward LED current would be well below 10ma. That means the light emitted by
the LED would be about half as much as the 20ma figure. That seems like a good target.
So, the two curves converge nicely.
I wanted a way to test these and other batteries using an automated method.
I want to be able to start a test, then walk away and come back later to see the results.
The circuit I came up with is shown below. A 9v battery powers the whole system.
A dual op Amp is used to control the current drawn from the battery under test and to also
monitor the battery voltage. A low power +5v regulator is used as a voltage
reference and a power supply for the logic devices. A CD4013 dual flip/flop is used
as a set-reset latch. A pushbutton switch is used to start the test, which latches
the flip/flop. When the battery voltage drops below 3.0v, the test is halted and the
flip/flop latch is reset. CD4060 is configured as 0.00277Hz clock. The CD4060
has an oscillator and a 14 stage counter. With the component values shown, the
frequency is adjusted to 45.5Hz. By the time this master clock works its way through
the 14 stage binary counter, the square wave signal which emerges has a period of one
pulse every 360 seconds. The output of the counter is connected to a pulse generator
which shapes the leading edge of the low frequency clock signal into a single 100ms pulse.
That pulse is connected to an n-channel MOSFET as a switch. The transistor is
connected to a modified pedometer, which counts the pulses and displays the total number
of pulses. With everything running right, the modified pedometer displays hours and
tenths of hours for the battery discharge test. If the lithium battery has anything
like a 2 Amp-hour capacity, the total discharge time with a 20ma load should be about 100
hours. Such a battery would make an excellent power source for a solar powered spot
light.
Link to Poor Man’s Pedometer |
