Li-Ion Battery Charger Circuit Using IC 555




The following post explains a straightforward yet precise Li-Ion battery charger circuit using the ubiquitous IC 555.

A Li-ion battery as we all know needs to be charged under controlled conditions, if it's charged with ordinary means could lead to damage or even explosion of the battery.

Basically Li-ion batteries don't like over charging their cells. Once the cells reach the upper threshold, the charging voltage should be cut off.

The following Li-Ion battery charger circuit very efficiently follows the above conditions such that the connected battery is never allowed to exceed its over charge limit.

When the IC 555 is used as a comparator, its pin#2 and pin#6 become effective sensing inputs for detecting the lower and the upper voltage threshold limits depending upon the setting of the relevant presets.

Pin#2 monitors the low voltage threshold level, and triggers the output to a high logic in case the level drops below the set limit.

Conversely, pin#6 monitors the upper voltage threshold and reverts the output to low on detecting a voltage level higher than the set high detection limit.

Basically the upper cut off and lower switch ON actions must be set with the help of the relevant presets satisfying the standard specs of the IC as well as the connected battery.

The preset concerning pin#2 must be set such that the lower limit corresponds to 1/3rd of the Vcc, and similarly preset associated with pin#6 must be set such that the upper cut off limit corresponds to 2/3rd of Vcc, as per the standard rules of the IC 555.

The entire functioning of the proposed Li-Ion charger circuit using IC 555 takes place as explained in the following discussion:

Let's Assume a fully discharged li-ion battery (at around 3.4V) is connected at the output of the below shown circuit.

Assuming the lower threshold to be set somewhere above the 3.4V level, pin#2 immediately senses the low voltage situation and pulls the output high at pin#3.

The high at pin#3 activates the transistor which switches ON the input power to the connected battery.

The battery now gradually begins charging.

As soon as the battery reaches full charge (@4.2V), assuming the upper cut off threshold at pin#6 to be set at around 4.2v, the level is sensed at pin#6 which immediately reverts the output to low.

The low output instantly switches off the transistor which means the charging input is now inhibited or cut off to the battery.

The inclusion of a transistor stage provides the facility of charging higher current Li-Ion cells also.

The transformer must be selected with voltage not exceeding 6V, and current rating 1/5th of battery AH rating.



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    7 Discussions


    2 years ago

    Guys, stop messing around. This particular scheme is an adjustable electromagnet scheme that was posted here

    You won't be able to charge any lithium cells with it as a full charge of a lithium cell is 4.2V as OP says, this has to be 2/3 of Vcc or supply voltage of that chip. Now if you do your math then you will find out that 1/3 of Vcc would be only 2.1V. Any electronic device will be dead before battery voltage drops below 3V and the "charger" would still not charge the cell.


    3 years ago on Introduction

    Hi, Ashan

    I have 3.7 volts li-ion battery,how should i decide the lower and upper cut off voltage...


    3 years ago on Introduction

    The three pins are as follows:

    One pin is for (-), one is for (+) and the other is (T). the (T) is for temp control and the other two are for power respectively.

    This function is for the charging control to avoid cell phone burn out or fire due to damaged old batteries. Inside there may be more than one actual sensor and one actual pin (t) for output. The (T) pin is for protection and they do crystallize and heat up but are not bad just do not work. There is a de-crystallizer called desulfator which does the job, and be careful to inspect if battery is in good shape to begin. Sacrifice a old ESB cord and use pin # 1 (positive), pin # 4 (negative), respectively for a mistake will permanently+ damage the lithium. Jolt pin # 1 and pin # 4 until it clears the crystals and your battery should be refurbished.

    If you connect directly to normally charge then make sure you have a battery charger that is voltage controlled to turn off.

    Good luck



    4 years ago on Introduction

    can you make one for 3 pin 1 cell liion battery? Many mbile phones (incl. Samsung) have 3pins when 1 cell

    wood doctor

    4 years ago on Introduction

    what about making a 18v lion charger ? would appreiate some idea;s. have a nice day.


    4 years ago

    is this only for a single cell? or could it be used with a 6 cell li-ion batterypack?