Simple Auto Turn-off Battery Charger

Let us design a battery charger using 555 timer IC and some simple equipment. If we want to keep the charger always plugged in and use the system when the charging in progress this circuit will be vital. For this circuit to be simple let us choose an AC-DC adapter available in household. I am using an adapter of a TV card providing 5.25 volt DC and I shall use a 3.7 volt cell-phone Lithium-ion battery.

First of all we have to design a constant current source. It is simple to make with pnp transistor and a resistor.

Here current is limited by R1-6.8 Ω resistor. To get constant current we need constant voltage, which is provided by the voltage difference across three diodes and emiter-base junction (say 3 х 0.7 – 0.7 = 1.4 V).

So, Current through the resistor is,

1.4/6.8 = 205.88 mA

Now we need a triggering mechanism to automatically turn off or turn on the pnp Transistor.

Now we include a npn transistor which can cut off or turn on Q1 and the other end of R3 will go to pin no.3 of 555 timer IC. Let design control circuit.

Now we have got 2 comparator and  a flip-flop which are very vital for this design. First connect the pin 8 and 4 to the positive supply terminal. Then inverting input of Q1 set to (2 х 5.6/3 =) 3.73 V (app.) and the non-inverting input of Q2 set to (5.6/3 =) 1.867 V (app.). Let decide when to start charging the battery and when to stop charging. For cell-phone battery we can stop charging when it charged to approximately 3.7 V and we will consider this 3.69 V for toggle the output and let 2.9 V be the lower limit after which the battery will begin to charge automatically. Let us make simple voltage divider for getting upper threshold and lower threshold voltage. For lower threshold voltage when battery voltage become 2.9 Volt we need 1.867 Volt at pin no. 2 and for upper threshold voltage we need 3.73 volt at pin no. 6. But it is impossible because the highest voltage can be provided by the battery is 3.7 V. So we have to change the supply voltage. We can easily use a zener diode or regulator IC to make the supply voltage 5 volt.

For 5 volt power supply we have,

UTP = 2 х 5/3 = 3.33 V (app.)

LTP = 5/3 = 1.667 V (app.)

now it is possible to get 3.33 volt at pin no. 6 and 1.667 V at pin no. 2.

For UTP we have,

Choosing b = 1k and b = 10k we will get approximate value of 3.335 volt.

For LTP we have,

Choosing y = 47K and x = 56k we will get approximate value of 3.066 volt.

Now the control circuit can be included with the previous circuit.

Here is a rough Vero board layout.

Now have a look at another circuit diagram to make it clear how to modify this circuit for other type of battery with different voltage. let choose a 6 Volt lead-acid battery.

A variable resistor can be used instead of R4 and R5 and also can be used another instead of R2 and R1. To charge more than one battery from this circuit use the following circuit diagram-