Li-ion Battery Charger with TL431

Li-ion Battery Charger with TL431

After a simple treatment can be omitted from the initial involvement thermistor to compensate for temperature dependence. After treatment (see Figure 1) works not as a transistor T4 voltage follower, but as a switch. If charging voltage is present, the transistor T4 is closed and the divider resistors R4 and R6 does not pass any current. Base-emitter transition T1 is polarized in the reverse, based on the zero voltage, so neither transistor T1 does not pass overhead stream. After connecting the charging voltage of at least 4.9 V is based on T1 will stress about 0.5 to 0.6 V greater than the battery voltage nabíjeného. When fully charged battery (4.2 V) is based on T1 approximately 4.7 V. Because the R input voltage stabilizer TL431 is approximately 2.5 V, the transistor T4 opened voltage around 2.2 V. At this voltage the transistor T4 open a channel is switched resistance of singles, more than tens of ohms and its resistance in the divider practically apply. If charged periodically discharged battery, the voltage on the basis of T1 and T4 is less need not be fully open. It does not matter, because at this stage the battery is charging in the same mode of constant current and voltage stabilization does not apply. For proper function of the charger must have transistor T4 low threshold voltage. Good matches transistor BSS138 (SMD) or BS108 for classical mounting. Both types have a threshold voltage of typically about 1 V. The device charging current with transistors T2 and T3 has been described in detail here and I will not repeat it here.

Charging voltage adjustment is no longer dependent on the threshold voltage of T4 and the thermistor resistance. Temperature stability is determined practically only the stability of IC1. It was therefore removed from involvement trimmer and charging voltage is set to fixed resistors R4 and R6. If you omit (shorts) resistor R5, the final charging voltage typically slightly larger than 4.2 V. The integration of resistor R5 can adjust the voltage to size. Just when you recharge the battery for 90 to 95%, we recommend setting the charging voltage from 4.1 to 4.15 V. The lower charging voltage is very significantly increases the number of charge cycles and prolonged battery life.