Using TEA1100 integrated circuit manufactured by Philips can be built a NiMH, NiCd battery charger.
When the battery is fully charged, TEA1100 goes to maintenance charge mode. In this way, one tenth of the normal charge time flow 1/2 of normal charge current (battery is in pulse charging representing 1/20 of normal charging current). The intermittent charge current may changed using formula:
Intermitent = 1.25 - Rsense / 10 * Rsunt - Rn.
Current is determined from the voltage drop on sunt resistor and is switched through transistor pin 2 of IC1.
The voltage at pin 7 must be between 0.385 V and 3.85 V. The voltage divider R4-R5 makes the battery voltage to match the ranges. The value of these resistors is given by:
R4 / (R4 + R5) * n * 1.8
R4 / (R4 + R5)
R4 / (R4 + R5) * n * 1.1> 0.385;
R4 / (R4 + R5)> 0.35 * n
Charge current, Ic, is given by: Ic = 1.25 Rsense / Rsunt - Rref (where Rsense is R10, R8 Rsunt is in parallel with R9, and Rref is R2).
TO - overcoming the time (time - out) - is a protective feature of the circuit. If, in a certain range can not detect any maxim, IC stop charging at a set time. This constant time is given by: TO = 226 • Baskets * 0.93 * Rref where Baskets = C3.
When no battery is connected to terminals of K1, or when the battery is charging pulse, D1 light at a constant pulse, when the voltage at pin 7 decreases, the LED lights continuously.
Voltage at pin 12 should be 5.65 - 11.5 V