The following post explains a straightforward yet precise automatic Li-Ion battery charger circuit using the ubiquitous IC 555.
Charging Li-ion Battery Can be Critical
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.
How the Circuit Works
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.
If you feel that the above design is much complex you could try the following design which looks much simpler:
How to Set up the Circuit
Connect a fully charged battery across the shown points and adjust the preset such that the relay just deactivates from N/C to N/O position....do this without connecting any charging DC input to the circuit.
Once this is done you can assume the circuit to be set and usable for an automatic battery supply cut off when fully charged.
During actual charging, make sure the charging input current is always lower than the battery AH rating, meaning if suppose the battery AH is 900mAH, the input should not be more than 500mA.
The battery should be removed as soon as the relay switches OFF to prevent self discharging of the battery via the 1K preset.
IC1 = IC555
All resistors are 1/4 watt CFR