Special Charging Protocol for Lithium-Ion Cells
Charging Lithium-Ion cells necessitates a distinctly different protocol compared to NiCd or NiMH cells, and strict adherence to this protocol is essential. Over the past year, we have published two articles detailing the charging methods for these cells. This time, we are introducing a new IC from Linear Technology (www.linear.com), which, although potentially challenging to acquire due to its novelty, offers exceptional advantages. This incredibly compact IC can be permanently integrated into the cell, making it suitable for both permanent integration and ordinary charging applications.
The device is engineered to charge one cell at a time, delivering a consistent current of 500 mA. Upon connecting a new cell and applying power (in any order), the charging process commences. Initially, the cell’s temperature is examined using the NTC. Charging will only initiate if the cell’s temperature falls within the range of 0 to 50 °C. Particularly, when Lithium-Ion cells have been deeply discharged, they require delicate charging, beginning at a mere 50 mA, as long as the cell voltage remains below 2.49 V.
Precise Charging Process with Voltage Control
In this charging protocol, once the cell voltage surpasses a specific threshold, the charge current rises to 500 mA until the cell reaches its maximum voltage of 4.1 V (or 4.2 V, depending on the type). The voltage is then stabilized, leading to a gradual decrease in the charge current until the cell is fully charged. When the charge current reduces to 50 mA, the charging halts, completing the cycle. Additionally, the IC incorporates a timer that terminates the charging process after a designated time, even if the current hasn’t dropped below 50 mA.
LED D1 indicates the charging phases brightly during active charging, dimly when charging ceases due to the current falling below 50 mA, and it turns off when the timer stops the process. Once charging concludes, the supply is no longer necessary. The charger circuit can remain connected to the cell, drawing only about 5 to 7 μA, ensuring the cell won’t discharge quickly. A new charge cycle begins when an empty cell is connected and power is applied. It also initiates automatically when the cell voltage drops below 3.88 V (or 3.98 V) as long as power is supplied.
Modifying Charge Current and Temperature Protection
T1, a P-channel MOSFET, can be any power type or even replaced by a PNP Darlington with its emitter connected to R4. For accurate temperature measurement, NTC R5 should be mounted as close to the cell as possible. If the specific NTC used isn’t readily available, a fixed 10 k resistor can replace it, although temperature protection won’t function.
Low-current LEDs are suitable for D1 and D2. D3 can be any 1 A Schottky diode or an ordinary diode like the 1N4001 if a slightly larger voltage drop isn’t a concern. It’s crucial to note that Lithium-Ion cells must never be charged at voltages exceeding 4.1 V (or 4.2 V) to avoid potential explosions. The cell’s voltage threshold should be stated on the cell itself, or reference the information provided by the manufacturer. The LTC4050 IC comes in two versions with ‘-4.1’ or ‘-4.2’ as suffixes, available only in an SMD package (MS10).