Lithium-Ion cells require a totally different charging protocol to that for NiCd or NiMH cells, a protocol that has to be followed precisely. During the last year, we have already published two articles regarding the charging of this type of cell. This time we’re using a new IC (so it may still be difficult to obtain!) made by Linear Technology (www.linear.com), which is very small and can, therefore, be built into the cell permanently, but is also suitable for use as an ‘ordinary’ charger. It is designed to charge one cell at a time, at a current of 500 mA. When a new cell is connected and power is applied (in any order), the charging process begins. First, the temperature of the cell is checked with the help of the NTC. The charging will only start if the temperature is between 0 and 50 °C. When Lithium-Ion cells have been discharged too deeply they should at first be charged very gently, at a current of only 50 mA, as long as the cell voltage is below 2.49 V.
Above that voltage, the charge current is increased to a nominal 500 mA, until the maximum voltage of 4.1 V (or 4.2 V, depending on the type) has been reached. The cell voltage is now held at this level, causing the charge current to gradually decrease until the cell is fully charged. When the charge current has reduced to 50 mA, the charging stops and the cycle is complete. As an extra safety measure, the IC also contains a timer that stops the charging process after a specific time, even if the current hasn’t yet fallen below 50 mA. The phases described above are indicated by LED D1. During the charging of the cell, it will light up brightly. When the charging stops due to the current having fallen below 50 mA, it is lit dimly. And when the timer stops the charging process, the LED will be off. When the charging process has completed, the supply is obviously no longer required. The charger circuit itself can be left connected to the cell since it only draws about 5 to 7 μA, so there is no need to worry that the charger would quickly discharge the cell. A new charge cycle will begin when an empty cell is connected and power is applied. A new cycle will also automatically start (as long as power is applied) when the cell voltage drops below 3.88 V (3.98 V). The charge current can be modified by adjusting R3 and R4 according to the following formula: I = (2.47/R3) × (800/R4). The maximum charge time is determined by C2; the formula used here is: time = (C2 × 3 hours) / 0.1 μF. The timer doesn’t start until the cell voltage reaches 4 V. LED D2 is lit when the voltage supplied to the charger is high enough.
T1 is a P-channel MOSFET, which can be virtually any power type. It could even be replaced by a PNP Darlington, with its emitter connected to R4. NTC R5 should be mounted as closely as possible to the cell so that the cell temperature is measured accurately. It won’t be easy to find the NTC used in this circuit, but the accuracy of the 0 en 50°C temperature limits aren’t that important. Since its resistance at 25°C is 10 k, it could even be replaced by a fixed 10 k resistor. Obviously, the temperature protection will then no longer function. For D1 and D2 you should use low-current (also known as high efficiency) LEDs. D3 can be any 1 A Schottky diode, or an ordinary diode such as the 1N4001 if it doesn’t matter that there is a slightly bigger voltage drop. There is one final point, which most of you probably know: Lithium-Ion cells may absolutely never be charged at voltages greater than 4.1 V (4.2 V) because they could explode under those circumstances. It should be stated on the cell whether it is a 4.1 V or 4.2 V type, otherwise, you will have to refer to information provided by the manufacturer. The LTC4050 comes in two versions, with ‘-4.1’ or ‘-4.2’ as a suffix. The IC is only available in an SMD package (MS10).