OUCC Proceedings 11 (1983)

Graham Naylor

We present here a simple design for a constant current charger. Two circuits are given: the first will charge Ni-Cds and lead acids at three possible charging rates (0.7, 1 or 1.7 amps, others being possible by the use of different resistors) whilst the second is a design for the new sealed Ni-Cd from Speleo-technics, requiring a charging rate of only 0.7 amps.

Lead acid / Ni-Cd charger

A 9 V (rms) transformer is required, which should be rated at about 20 VA or greater per number of cells the charger is being built to charge (e.g. for a 5-cell charger a 9 V, 100 VA transformer is required). Two switches (per cell) are used to select the charging rate. A tri-colour LED is used to indicate the charging rate: red and green indicating the two normal charging rates (0.7 and 1 amps) and a mixture (orange/yellow) indicates the higher charging rate (1.7 amps). This LED is available from Radio Spares (part no. 587-771).  

FX2 charger 

A 4.5 V transformer is required with a rating of only about 5 VA per cell. No switches are required and a fixed charging rate of 0.7 A is given with a simple LED to indicate that charging is proceeding.