Want to charge a high mAh rated Li-Ion battery with high current with an auto shut off facility at full charge level. The following simple circuit can be sued for implementing this.
You can make sure a lithium cell is not overly charged (more than 4.2 V) by simply employing a TL431 shunt regulator.
This means when the cell has reached its maximum voltage, the TL431 will start pushing every ampere of current flowing through the limiting resistor towards ground, in order to shunt it and prevent the current from going to the Li-ion cell.
The 4.7 Ω resistor restricts the current to stay within the range of the shunt capacity of the TL431.
For higher current regulation, as depicted in the above figure, a additional PNP output transistor is added to the TL431 using the Sziklai pair configuration.
The maximum shunt current is the 100 mA that the TL431 can govern multiplied by the gain of the PNP transistor.
When the TL431’s peak current rating becomes void of the limiting factor, the only problem is the maximum current that an accurately described USB charger can deliver which is around 500 mA.
Once the battery is at its lowest voltage, the current limiting resistor functions at 4.5 Ω. However, the most optimum value will be 4.7 Ω.
The transistor utilized here is a 2SB857 (T2) that was readily available when collecting the components. This is quite a common device with a 50-V, 4-A and a hfe(min) of 60, rating. You can try any other similar as per the high current demands of the cell
This shows just about any TO220 PNP with similar higher current ratings can do the job. The one we used in our construction was not fitted with a heatsink. Nevertheless, without any load (maximum shunt current), the transistor got a little warm but not overly hot.
Because the PNP transistor is doing most of the high current charging operations or the Li-ion cell, and also its collector is returned to the negative rail, resistor R4, which is a current sensing resistor, can be included in the collector lead to activate a second transistor T1.
This action BJT activates the ‘battery full’ indicator, LED1. This would have been extra difficult with the only TL431 because including the current sensing resistor in the anode lead would initiate a variable offset voltage functioning against its reference point.
The current sensing resistor in the collector lead is comprised in the feedback loop and contains an extremely minimal effect on regulation. P1 sets the ‘battery full’ voltage precisely.
Using LM338
If your Li-ion is higher than 5000 mAh current, then probably an LM338 based charger would be more favorable, as explained below:
The proven Li-ion Charger circuit is showcased to charge any Li-ion battery upto 5 AH with the proven IC2, or for 10AH batteries if IC2 is correctly replaced with a LM396
The design consists of two essential levels, the IC2 voltage regulator stage and the IC1 over charge cut-off stage.
IC2 is connected in its normal voltage regulator form, where P1 performs as the control knob allowing it to be adjusted to create the essential charging voltage across the associated Li-ion battery at the output.
IC1 pin3 is the sensing input of the IC and is ended with a preset P2 for supporting the over charge voltage level adjustment.
The preset P2 is adjusted such that when the battery reaches its full charge value, the voltage at pin3 just evolves into greater than pin2, contributing to a quick high at pin6 of the IC.
The moment this occurs the high from pin6 latches on to pin3 with a lasting high via R3, D2, freezing the circuit in that position.
The above high is usually supplied at the base of the BC547 which right away grounds the ADJ pin of IC2 forcing it to close down its output voltage consequently cutting off the voltage to the Li-ion battery.
The Red LED now illuminates suggesting the full charge level and the cut off circumstances of the circuit..
Parts List fro the suggested high current li-ion battery charger circuit
R1, R5 = 4K7
R2 = 240 Ohms
P1, P2 = 10 K Presets
R3, R4 = 10K
D1, D5 = 6A4 diode
D2 = 1N4148
D3, D4 = 4.7Vzener diode 1/2 watt
IC1 = 741 opamp for 12V input, LM321 for 24V input
IC2 = LM338
How to Start up the circuit.
In the beginning do not connect any battery at the output, and rotate P2 in order that its slider touches the ground end, in other words adjust P2 to make pin3 to zero or ground level.
Feed the input voltage, adjust P1 to get the preferred level of voltage across the output where the battery really should be attached, the green LED will be illuminated in such a position.
Now cautiously move P2 upwards until the the red LED just illuminates and latches in that position, avoid moving P2 any further, make sure with green LED shutting of in accordance with red LED illumination.
The circuit is focused now for the preferred high current Li-ion charging from a car battery or any 12/24V source..