Lithium-ion battery management

Hi everyone, this blog is about the battery management circuit of a typical Lithium-ion battery. If you want to make a simple Lithium-ion battery charger or a handy wireless stuff with charging enabled, you can use this very simple circuit.

Lithium-ion batteries are nice tiny shaped and sized and can be used very easily for making a battery based device or a remote control handy stuff without wires all over the place. You can get a normal phone lithium-ion battery, very easily in nearby mobile stores and use them in your project.
For the traditional Ni-Ca battery charging circuit we had to detect the temperature of the battery to know when to stop charging it. But if you charge the Li-ion battery with low current, you will not have to worry about the heat sensing to determine when to disconnect the charging. Some of the available Li-ion batteries do have a built in temperature sensor which sense the cell but that’s just for the protection of the battery not to detect that the cell is finished charging.
To charge them is really easy, there is only one thing to look at and thats the maximum charging voltage, its mostly either 4.1 or 4.2 volts. 4.2 Volts is most common in most of the Li-ion batteries. That’s also why most of the cellular modules have their VBAT or VDD 4 Volts.

Also the life of Lithium ion battery is directly proportional to the maximum voltage you charge them at, but don’t worry about it as our battery management chip handles it all.

Charging management IC selection

For your charging management IC selection, make sure you pick the right voltage chip, same as 4.1/4.2 Volts selectable. In the below circuit I have used a Microchip’s MCP73832 IC.
The Li-ion battery charging is mostly a two step process. Constant Current Mode and the Constant Voltage mode there is one more mode that is the Preconditioning mode but it is kept optional. The chip handles it all, but let me show you the characteristics as because it’s interesting.

The Y-axis has got the Volts and the X-axis has got the Time. The Green Curve is the battery voltage and the blue curve is the charging curent.
Now this is important if the battery is a 50mAH . Then it should be charged to the maximum of half of its ratings or 25mAmps. So here in this curve the 100%C is the 25mA for 50mAH battery. During a time period chip pushes the constant current and there after reduces exponentially for a period of constant voltage.

The Basic Charging Circuit :

Here I am showing you a basic charging circuit consists of a Microchip’s MCP73832 Chip and a few passive components. The MCP73832 has a maximum charging current of 500mAmps. There are various modules of MCP73832 chips these are AC/DC/AT1/AD. AC model is the most common of them all but refer datasheet for more info on other types.

The above design is just the basic design of Li-Ion charging, but issues arises when load is applied to the battery.
In preconditioning mode and Constant current modes the chip limit the current, suppose it is programmed to limit the current to 50mAmps and the load needs 45mAmps then the battery will get only  5mAmps of current to charge, and if the load asks for 60mA then it will rather start the battery to discharge. So if the load is attached then charging will never seem to finish. But no need to worry about it because we have a load sharing circuit too, that will just increase 3 more components.

The Load Sharing Circuitry:

The circuit disconnects the battery charging circuit and gets direct supply from the mains input. Below is the circuitry of load sharing based Li-ion battery charging.

The P-channel MOSFET works as a switch here. It behaves as open circuit when input supply is ON and works as a close circuit of battery and MCU when the supply is OFF.
The Schottky Diode is there for the isolation of both the live lines, the supply line and the backup line.The Reason behind using the MOSFET and the Schottky diode is that they both works on the quickest switching, that prevents the power loss.
There is some calculations needed to select the value of R2. Diode acts as a resistance say Rd

Rd = Vout / Id             
=> Rd = 21K

It is recommended for the MOSFET’s gate to have a voltage of 1V, we name it as “Vtar

R2 = Vtar* Rd/(Vtar)
R2 = 1*21K/(4.2-1)
R2 = 6.5K

The R2 Shouldn’t exceed 100K value.

So, the next time you design a product with a rechargeable solution, use Li-ion battery and this very simple circuitry consisting very low cost and easily available IC and easily available low cost Li-ion batteries.
Hope you enjoyed the blog.