Using a cellular phone as a Li-Ion batteries charger

[linterno]

I only have two cheap batteries (I won't say the brand to avoid people making comments on the batteries and not on the procedure).

My cheap and bad quality charger (I won't mention the brand) always charged batteries to 4.13 to 4.15 volts, so I just thought if I could use a cellular phone as a charger and I did some tests.

I used an LG MG160a cell phone (I paid $ 10 dollar for it new in the box with a pre-paid plan). This LG phone measured 4.197 volts between the charging pins (those going to the original battery) open-circuit. I found out which pin was the positive and which the negative. Using a couple of alligator clips cables (cheap one with some resistance) I connected those to the phone positive and negative pins and to a protected 16340 Li-Ion battery. The battery was charged at 100mA, something bad for 18650 battery, but not bad for 16340 (measures using a Fluke 179 DMM). The batteries stoped charging at 4.196 volts (no increment in more than half an hour, so I disconnected) when connected to the phone. 4.194 volts when disconnected.

I have some other old unused cell phones with Li-Ion batteries but some no one of those provided a voltage closed to 4.2+ volts open circuit. All provided les than that. A Nokia 8890 provided 3.5.

I think this could be a free on hand solution for me (I know there are excellent chargers like Pila IBC, but I have to pay almost twice the price in US to get it to my country and I won't expend that amount to charge just 2 cheap batteries).

I am just wandering what risks am I running doing this?

Juan C.

 

[MrAl]

Hi there,


I have to say this is a very interesting idea, which shows some good thinking
on your part.
I also have to say that if you need an Li-ion charger, you can build one for
cheap (less than 5 dollars) if you dont need to put it into a case.
You would have to use a wall wart too though, perhaps from something
else you have around that doesnt need it all the time.

If you would like more details just yell.
I would also have to know if you have ever built anything electronic before.

 

[linterno]

Hi MrAl and thank you for your reply.

Yes, I have built some electronic circuits and, yes, I would like to have more details.

I ordered some MAX1811ESA+ and MAX4172ESA+ samples from Maxim. I want to build a charger with a interface to a computer to be able to get charging information including voltage an current. I am planning to use a microchip PIC12F675 to get the battery voltage and charging current and send it to the computer via a serial communication (RS232).

Respectfully,

Juan C.

 

[MrAl]

Hola Juan,


Oh ok, i thought your only resource was the cell phone for a charger so i was
attempting to help by recommending the LM317 and a few resistors, which
makes one of the simplest yet highly effective Li-ion chargers you can build.
I see you already picked out some IC's though and your going with some
PC interfacing too in order to monitor the charger process, so i guess you
are already on your way to creating a really good charger.

 

[Fallingwater]

This LG phone measured 4.197 volts between the charging pins (those going to the original battery) open-circuit.

Do not assume this means it'll keep this voltage.
I hacked a LiIon to my old Panasonic phone, and when I started charging it the voltage on the terminals was 4.2. All good, thought I. Then an hour or so later I touched the cell and it was slightly warm. A voltage reading showed almost 4.5 volts!!!

What I'd do in your place would be to open up the original battery, remove the bare LiIon cell inside and solder charging leads to the protection circuit. This should prevent any unexpected behaviour.

 

[linterno]

Thank you MrAl and FallingWater for your replies.

MrAl: I was thinking about to use 2 LM317. The first one for constant voltage and the second one for constant current, both being controller by a microcontroller.

FallingWater: You are right. I am a little paranoid about battery explosion, more about Li-Ion batteries so I do baby sitting when charging this kind of batteries.

 

[MrAl]

Hi again linterno,

Just a little tip...
Because the spec on charging Li-ion cells is a bit overstated,
you dont actually need a 'constant current' with voltage
regulation to charge a cell. What this actually means is
'current limit' with voltage regulation.
This allows the use of a single LM317 set up to regulate
voltage (to say 4.15v or so) with a power resistor on the
input to limit current. The idea is that when the cell tries
to draw a lot of current the LM317 tries to also, and the
power resistor on the input drops the voltage so that
the LM317 output current gets limited. The size of the
power resistor depends on the wall wart voltage supply
voltage level used to power the LM317.
I posted this circuit quite some time ago but i think you get
the idea. Of course if you want to go with another LM317
as current limit on the input that's ok too, as these devices
arent very expensive these days.

 

[hopkins]

Had a related Li ion problem.
Still can't find my camera Li-ion battery charger so mod'd an old unused
cellphone to do it. Soldered wires on phone to mate up to the camera battery.
works fine and now have a graphic display for charging rather than a colored
LED to indicate the state of charge. Measured 4.18volts at battery when phone says Full.

 

[MrAl]

Hi,

Oh yes, very interesting for sure.
I have some old phones laying around too, but unfortunately
they dont use Li-ion because they were made a long time
ago.

BTW, there is a whole thread on the LM317 as battery
charger on this site. I'll see if i can find it.

 

[TigerhawkT3]

I don't think I'd feel comfortable charging ordinary Li-Ions with a LiPo charger.

 

[SilverFox]

Hello Tigerhawk,

Most of the Li-Po chargers terminate at 4.200 volts because Li-Po chemistry is not very tolerant to higher voltages.

The most published specification for Li-Ion charging states that the charge should not exceed 4.2 volts with a plus or minus of 0.05 volts.

I would rather use a Li-Po charger on a Li-Ion cell than the other way around...

Tom

 

[TigerhawkT3]

So the charger I had been asking about in that thread (charges LiPos to 4.26V) really was bad? :hairpull: Man, I wish you had chimed in earlier so I could have returned it. Should I stop using it, or is it reasonably safe enough?

 

[MrAl]

Hi there,

You might want to check the accuracy of your meter too,
just to make sure you are getting accurate readings.
If you could compare to another meter that might help.

If it's ok, then that 4.26 is definitely not good.
The only fix i know of is to connect a very low ohmage
resistor in series, and take the cell off charge before
the current drops too low. This will drop the voltage,
but also slow the charging process. You'll have to decide
if it's not as good as you want it to be.

For example, if your final charge current is 50ma then you
need a 1 ohm resistor in series. This would slow the charging
down too so you'll have to test to see if this still works
good enough for you.
If your final charge current is 100ma, then you need a
0.5 ohm resistor in series. This still slows the charge down
but not as much as with 1 ohm.

Another idea is to build a floating 0.1v voltage source
and connect it in series with the cell. This would lower
the voltage but not affect the charge time. Of course
this is harder to build, requiring a wall wart and regulator
ic chip, some resistors and a few small capacitors.
The wall wart has to be able to handle the full charge
current.
It might actually be easier to just build your own charger.