DIY Mobile Phone Battery Charger

This battery charger circuit is for Sony Ericsson standard batteries, Li-Polymer type, with a nominal voltage of 3.6V and a capacity of 900mAh. But you can also charge Nokia and Samsung batteries as well.

Charging lithium cells is fairly straightforward. You generate a well regulated voltage and ensure the charging current is sufficiently low so as not to cause overheating problems. Also, when the correct terminal voltage as been attained, it is important to shut down the charging process at that point.

The circuit consists of three main sections: 

• A 5V DC constant voltage supply, which serves as the charging source for the mobile phone battery. Mains voltage is transformed via T1. After rectification and smoothing, via BR1/C1, about 14V DC is delivered to IC2, which is a 1A 5V regulator. The regulator output delivers the charging current to the mobile phone battery via current limiting resistor R1 and via relay contacts RLA2. Fuse FS1 provides protection in the event of excess current flowing for whatever reason.

• A 6V constant voltage reference source, which provides the ‘setpoint’ for the switched charge controller. A means of producing a reference voltage equal to the fully charged battery open circuitvoltage is required. (4.15V in the case). This is achieved by utilisingan LM317 device (IC3), which is an adjustable 3-terminal positivevoltage regulator capable of supplying in excess of 100mA over an output voltage range of 1.25V to 37V.This voltage regulator is exceptionally easy to use and requires only two external resistors to set the output voltage. Thus, resistors R2 and R3 set the LM317 output at about 6V, and this potential is applied acrosss a pot (VR1), the wiper (moving contact) picks off 4.15V and applies it to the inverting input of IC1.

•  A voltage comparator device, with an associated 12V relay switching and latching the circuit. Op amp IC1 is an LM358N, and is configured as a voltage comparator. The non-inverting input is connected directly to the +V ternminal of the mobile phone being charged via the 10kΩ resistor R5. When a discharged battery is connected into the circuit, the non-inverting input of IC1 will be lower than the inverting input, and IC1 output will be at, or near to 0V. Transistor TR1 will be switched off and relay RLA will be de-energised. Under this condition, relay contacts RLA1 will permit energising of LED1 (charging), and relay contacts RLA2 will permit the charging current to flow from IC2 via R1 into battery B1. As the battery accumulates charge, eventually, the battery voltage will slighty exceed the setpoint voltage (4.15V) and the output of comparater IC1 immediately swings fully positive and switches TR1 on, which in turn energises relay RLA. Relay contacts RLA1 now latches the relay, thus ensuring that no over-charging can take placeand contacts RLA2 energises LED2, the ‘charge complete’ indicator. The battery is then disconnected from the charger, which is reset by removing the mains supply ready for the next charging cycle.

Schematic Diagram

Component Required

Component	Value / Part Number
R1	10 ohm
R2, R7	270 ohm
R3,R8	1k ohm
R4	2k ohm
R5	10k ohm
R6	47k ohm
R9	470 ohm
VR1	2k pot
C1	100uF/25v Electrolyte
C2	150nF
C3	470nF
C4	100nF
C5	1uF
IC1	LM358N
IC2	LM7805
IC3	LM317
D1	1N4148 or 1N4007
RLA	12v two pole relay
TR1	BC300
T1	12v sec transformer
BR1	Bridge Rectifier
FS1	250mA fuse
LED1	Red LED
LED2	Green LED