# Mobile charger to charge LiIon batteries, Usng Voltage Divider to drop voltage

Discussion in 'Power Electronics' started by khankll, May 15, 2012.

1. ### khankll

110
0
Feb 6, 2011
hii have a lot of mobile phone charges that say "output 5v .7A" i want to use it to charge a liion battery..both prismatic and cylindrical 18650.i have few question..

What does the figures in output mean ? does it means its a CC/CV output ?

Can i use a Voltage Divider Circuit to Drop the extra voltages and get 4.2volts and then connect those 4.2 volts to the output of protected 18650 cell or unprotected liion cell ( saftey against over charge is gauranteed by firstly the fact that when the battery voltage equals the charger output voltage no more current flows and secondly i will be constantly observing it..) while reading about Voltage divider network in the book "Electic Circuits" by Nilsson it mentioned that the as long as the Load Resistance is very very greater then R2 ( wheare as r1 and r2 are the two series resistors and r2 is the resistor accros which i will be geting output ) ..the output voltage remains fairly constant.. ? so in my case the load is cell what should be its resistance ?

Thanks

25,448
2,809
Jan 21, 2010

3. ### khankll

110
0
Feb 6, 2011

regarding whether the output of charger is CC/CV i m still a bit doubt full i tied few resistors in series.. and the current was lowering then the mentioned output so should it be safe to say its not CC ?

now i need to chose suitable resistors and go and test it..
here is some of my calculations ..i scanned the page please see it..

what should be the ideal resistor value/rating ... considering that i will be charging the battery at say 600 mA .. so Ic in my circuit will be 600mA and hopefully at least 600mA will also be flowing through R1.. what should be the current through R2 ?
and i am also attaching the pge from nilsson book about vdr

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Last edited: May 16, 2012
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
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Jan 21, 2010
As I said above, it's DEFINITELY NOT constant current. It IS constant voltage.

I recommend you don't do it this way.

It is very hard to say because this is NOT the way to do it. It WILL result in overcharging the battery and that is a bad thing.

Actually there are three ways it could work:

1) it overcharges the battery and the battery is damaged
2) you draw too much current and the power supply fails
3) there is a race to see if 1 or 2 happens first.

You need a current limited, fixed voltage (and that voltage is about 4.2V) charger (and one which preferably removes the charge current after if falls below a certain value. And you can't do that with just resistors.

5. ### Audioguru

3,040
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Sep 24, 2016
The "mobile phone charger" is not a charging circuit. It is simply the power supply for the charging circuit that is inside the phone. You should use a lithium-ion charging IC in between the power supply and the battery.

6. ### BobK

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1,686
Jan 5, 2010
You cannot do it either a constant current or a constant voltage. A LiIon charger must start with a constant current, then change over to constant voltage, watching the current and stopping when the current becomes small enough. As A.G. said, you need a chip to do this right.

Bob

7. ### Audioguru

3,040
678
Sep 24, 2016
I cheat and charge two basic (no protection circuits) 18650 Lithium ion cells in series without a charger circuit and without a balanced charger in my portable little vacuum cleaner. My charger uses an LM317 voltage regulator set to 8.40V and its wall wart limits the charging current. I disconnect it after a few hours.

8. ### BobK

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1,686
Jan 5, 2010
This is best done in a fireproof bag just in case the LM317 fails shorted.

Bob