The Dead 9V Battery
I’m sure that by now, most of you have come across a
circuit called a “joule thief.” This is a simple self-oscillating
voltage booster which most people build to use with either
an AA or AAA battery. Any “dead” 1.5 volt battery will
do, where the voltage has dropped to about 1.0-1.1 volts
or lower. The fact is that you can use a good battery as
well, and probably every one of the small flashlights on the
market that use only one AAA or one AA battery is using a
joule thief circuit.
This will be a great way to use the dead AA and AAA
batteries I end up with from battery-operated things around
the house. So, what can one do with all those dead 9V
batteries? These are generally down to about six volts
and seem to be useless since they are rather low current
batteries even when new.
The circuit I want to discuss here is one that would
be a great project for a newbie to learn about
soldering on. All that is needed is an ultra-bright
white LED, a 220-330 ohm 1/8-1/4 watt resistor,
a “dead” nine volt battery, battery holder, and
something to put it all in. You can add an on-off
switch or even an on-off-on switch.
A word of warning here for those who
use rechargeable batteries: Do NOT use a
rechargeable battery in one of these flashlights.
It will be dead when you take it out because the
simple version of these circuits will use the battery
down to below a half volt. Being one of the more
efficient projects I’ve built, this circuit will still
be putting out usable light down to about a quarter volt.
This is way below where a rechargeable battery would be
ruined. I went with an on-off-on switch because an on-off
version wouldn’t have the option of driving the 3V ultra-bright white LED directly. You could also build two circuits:
one to use the resistor; and one not using the resistor for
when the battery is below four volts.
I find that once the 9V battery is reading 3-4 volts
when using the resistor and LED, it can be used without
the resistor to drive the LED. At this point, the internal
resistance of the 9V battery is high enough to limit the
current, so no external resistor is needed.
Also, I’ve measured these currents at around 10 mA,
which is not even close to the 30 mA max current, or even
the recommended 20 mA for longer life for these LEDs.
Remember that it’s the heat generated by the LED that
eventually kills it. Two ways to limit the heat buildup are to
either drive the LED intermittently — as is done when using
the joule thief circuit — or limiting the DC current to below
I’ve had a couple fun
circuit ideas in the
back of my head for
years and started
playing with them,
changing things, and
them. They’re simple
and are a good
exercise in soldering
and circuit basics.
By Phillip Karras
Dead 9V battery
40 September/October 2018