The monitor is particularly intended as an aid for fast charging of NiCd batteries. Most commercially available fast chargers have no temperature sensor, although the temperature is an important factor in the fast charging of NiCd batteries.
According to manufacturers’ data sheets, temperatures for various states of a NiCd battery should roughly be: (a) 30 °C when half charged: (b) 37 °C when fully charged; and (c) 48 C when 20% over-charged.
The monitor is connected in series with the battery and the charger. The temperature is assessed by a couple of NTC (negative temperature coefficient) resistors. The output of these devices is compared with reference levels in ICib-ICid! The output of comparator IC id goes low when the temper-ature—measured by R13 and R14-rises above 25 °C, whereupon D7 lights. The output of IC], goes low when the temperature reaches 38 °C, whereupon D8 lights. This signals that the battery is fully charged. Finally, the output of ICib goes low when the temperature reaches 45 °C, whereupon D9 lights. This is an alarm signal calling attention to the battery being over-charged. At the same time, T1 is switched on, whereupon relay Rey is energized and its contact disconnects the charging current. The charging current can be reconnected only after the temperature has dropped to well below 45 °C.
Diodes D1-D3 ensure that only one of the three LEDs can light at any one time. The circuit is calibrated with P1 to make certain that the LEDs light at the correct temperatures.
When the monitor is connected to the charger, it is essential that the charging current, originally flowing via Kl, flows via K2 and K3. The batteries must, therefore, be connected to K2 and K3. The terminals of the NTC resistors are also coupled to these connectors.
The NTC resistors must be coupled to the batteries in a manner that ensures goc6 contact. It is good practice to fit them permanently to the batteries.
The supply for the monitor may be taken from the charger, either via the 12 V pin or the L pin. In the latter case: wire link JP1 must be short-circuited. The monitor draws a current of about 15 mA when the relay is not energized.
Variations in the supply voltage do not affect the monitor since the NTC resistors are connected in a bridge arrangement. As -shown, the monitor is intended for use with 12 V batteries. It can be modified for use with 6 V batteries merely by using a 6 V relay instead of a 12 V type.