Notes on lithium ion batteries, and small solar projects.
In my ever present quest to find new uses for those small little solar
panels that come out of solar powered garden lights, and to make use of
otherwise thrown away items.............
Has anyone else used those little lithium ion batteries out of cell phones,
as the energy storage for a small stand alone solar powered system? Was it
for an actual useful purpose, or just for entertainment purposes only?
Most people just ditch the battery when they ditch the phone, but i have
been playing around with around six different lithium ion cell batteries
It is easy to solder pigtails onto the battery terminals.
Most phone batteries are designed to be constant current charged until they
reach 4.2V per cell, then they are constant voltage charged until they drop
to a certain charge current and then the charge is terminated.
If you try to hold the battery voltage to 4.2V after the charge current
drops past a specific point, then you may start to overcharge the battery.
Some lithium batteries are designed to be continuously float charged, some
Even on batteries that are not designed to be continuously float charged at
4.2V, you hold the charge voltage to 4.1V then the charging drops off at
about 80% charge, but you can leave the charge voltage on the battery
constantly, without worrying about overcharging it. You won't get full
capacity out of it, but it won't accidentally over charge.
Operating it with a 4.1V charging level, allows you to use a simple fixed
voltage charge controller on the PV/battery system. All you have to do is,
size the PV panel so that it won't exceed the max charging current of the
battery. Then you will inherently have a limited current/limited voltage
The lower designed limit is 3V. You should have a low voltage cutout that
pulls the load before voltage drops below that level.
The battery packs have a safety system comprised of a PC board that
disconnects the battery from the system if it tries to charge the battery
past 4.3V. And it will disconnect the battery from the system if it tries to
discharge the battery below 2.3V. Even though you don't want to rely on the
internal protection system as your primary method of charge control, it will
stop things from going crazy if the charge controller you built, takes a
System I have had running for a while is a 7Voc panel with a shorted current
of 180mA charging a 980mAh lithium ion battery out of a Motorola mobile
phone. It has a shunt charge controller that clamps the charge voltage to a
maximum of 4.1V. It's driving a little Coby AM/FM/SW radio.
The only tricky part is building a low voltage cutout. Trying to make a
circuit that only pulls a milliamp or less takes a bit of thought. The
quickest method i thought of was to take a mosfet, and a high gain
transistor to shut off, or turn on the mosfet gate using high value
resistors, and a zener diode. The lowest current value for the LVC that I
could obtain was about 1uA in the "on" mode.
The radio pulls about 12mA on AM/SW, and 15mA on FM. It uses a single IC
receiver design. The IC operates from 6V+ down to 1.5V. The current draw
doesn't change much with voltage. Current draw goes up with increasing
Computed isolation is about 65 hours, or almost 3 days, if the radio
operates continuously on FM.
Any small radio with around 10 to 20mA current draw would do fine.
The radio has managed to come to a halt because of the dreary weather we
have had for the past several days.
It would make a nice source for background music in a shop or something, and
if you didn't use it continuously, it would make a nice little emergency
radio instillation, to get information during power outages and stuff.
Running the battery like that, you should be able to get a couple thousand
cycles out of it, before the radio starts having an objectionably short run
time, during cloudy weather. That should equate to about three or to five
years of relatively constant operation.
The reason I have brought this up is because I am seeing cell phone
batteries going for 2 to 3 dollars on ebay, with about $ shipping and
handling. For seven or eight dollars, a small lead acid battery can't really
compete. Especially if you salvage it out of the mobile phone that you are
going to throw away anyway.
And because of it's technology, you don't have to worry about it not
reaching full charge every once in a while like you do with a small sealed
lead acid battery.
And the lithium ion batteries seem to be less temperamental under normal use
than nicads are.
And they have a good discharge/charge efficiency.
I have never really had any luck with nickel cadmium batteries.
And yes, I know they can go boom if you mistreat them......
There is plenty of lithium ion batteries out there, give them a thought,
next time you are looking for a battery for your small solar project. :-)