How to Make an Inductive Li-Ion Battery Charger Circuit

Charging batteries through inductive charging is one of the applications that’s becoming very popular and getting appreciated by the uses. Here we’ll study the concept and one example circuit diagram related to the subject.


Any electrical system which involves wire networks or cables can be very messy and cumbersome. Today the world is getting hi-tech and the electrical systems are also transiting into better and hassle free versions for providing more convenience to us. Inductive power transfer is one such interesting concept which facilitates power transfer without the use of wires, or rather wirelessly.

As the name refers to, inductive power transfer is a process through which a certain magnitude of power is transferred from one fixed place to another through the air without using conductors, just as radio signals or cell phone signals are transmitted.

However the concept isn’t that easy as it sounds to be, because with radios and cell phones the transmitted power is merely in few watts and thus becomes quite feasible, but transferring  power (wirelessly) so that it can be used for powering high current devices is entirely a different ball game.

 Here we are talking about several watts or probably several hundreds of watts that needs to be carried without any dissipation, from point to the other without using wires, an issue difficult to implement.

However researchers are trying their best to find appropriate set ups which may become just suitable for implementing the above concept successfully.

The following points outline the concept, and help us to know how the above procedure actually takes place:

Induction as we all know is a process through which electrical power is transferred from one position to the other without incorporating direct connections. The best example is our regular electrical transformers, where an input AC is applied at one of its windings and an induced power is received at the other winding through magnetic inductions.

However the distance between the two windings inside a transformer is very small and therefore the actions take place very conveniently and efficiently. When the procedure needs to be implemented at greater distances the task gets a bit complicated.

By evaluating the induction concept we find that there are basically two obstacles that make the power transfer difficult and inefficient, especially as the distance between the inducting destinations are increased.

The first hurdle is the frequency and the second hurdle is the generated eddy currents in the winding core. 

The two parameters are inversely proportionate and therefore are directly dependant on each other. Another factor that hampers the proceedings, is the winding core material, which in turn directly affects the above two parameters.

  By carefully dimensioning these factors in the most efficient way, the distance between the inducting devices can be considerably stretched.

For transferring power in the above discussed method, we firstly require an AC, meaning the power which needs to be transferred must be a pulsating current. This frequency of the current when applied to a winding generates eddy currents, which are reverse currents opposing the applied current.

Generation of more eddy current means less efficiency and more power loss through core heating. However as the frequency is increased, generation of eddy currents is reduced proportionately. 

Also, if a ferrite material is used in place of the conventional iron stampings as the core of the winding helps to further reduce the eddy currents.

Therefore for implanting the above concept in the most efficient way we need to make the source power high in frequency, in the order of many kilohertz and use an input induction system that’s made up of ferrite as the core. Hopefully, this solves the issue to great extents; at least for the making the proposed project of an inductive charging circuit for Li-ion batteries.

About the Circuit

WARNING - THE CIRCUIT IS NOT ISOLATED FROM AC MAINS AND SO IS EXTREMELY DANGEROUS IF TOUCHED IN POWERED CONDITION.

The circuit is devised by me, but has not been verified practically, so I would advise the readers to take a note of this. The circuit can be understood with the following points:

Referring to the figure we see two units, one is the base or the transmitting module and the other one is the receiver module.

As discussed in the above paragraph, the core material of the base winding is a ferrite E-core which is relatively larger in size.

The bobbin that’s fitted inside the E-core has a single stage, neatly wound with 100 turns of 24 SWG super enameled copper wire. A center tap is extracted from the winding from its 50^th winding turn.

The above coil or transformer is connected to an oscillator circuit consisting of the transistor T1, preset P1 and the corresponding resistor and capacitor.

The preset is used for increasing the frequency through the winding up to optimal levels and needs to be experimented some.

A DC voltage is fed to the circuit for initiating the required oscillations, which is derived directly by rectifying and filtering the AC mains.

On applying the DC, the circuit begins oscillating and the oscillations from the inductor being high in frequency escapes into the air to a considerable distance and needs to be grabbed back for the proposed inductive reception.

The receiving unit also incorporates an inductor consisting of air cored 50 turns of 21 SWG super enameled copper wire, which becomes a kind of antenna for anticipating the released power waves from the base circuit.

Capacitor C3 is a variable capacitor, the one used in radio for tuning may be tried. It's used for trimming the reception until the resonating point is reached and L2 gets optimally tuned with the transmitting waves.

This instantly raises the output voltage from L2 and becomes optimally suitable for the charging requirements.

D6 and C4 are the rectifying components which finally converts the AC signals into pure DC.

When brought to a considerable proximity, the inductions from the lower base unit is induced inside the receiving coil, the induced frequency is suitably rectified and filtered inside the receiver circuit and is used for charging the connected Li-Ion battery.

CAUTION: THE WHOLE IDEA IS BASED ON MY ASSUMPTIONS ONLY; READERS DISCRETION IS STRICTLY ADVISED WHILE EMPLOYING THE DISCUSSED CONCEPT AND THE CIRCUIT.

Parts List

The following parts would be required for making this inductive battery charging circuit:

R1 = 470 Ohms,

R2 = 10K, 1Watt,

C1 = 0.47uF/400V, non polar,

C2 = 2uF/400V, non polar
C3 = Variable Gang Condenser,

C4 = 10uF/50V,

D1---D5= 1N4007,

D6 = Equal to Battery voltage, 1watt

T1 = UTC BU508 AFI

L1 = 100 turns, 25 SWG, center tap, over largest possible ferrite E-core

L2 = 50 piled turns, 20 SWG, 2 inches diameter, air cored

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