Battery-Charging Indicator For Mains Adaptor


Although you may well be
the proud owner of the very latest NiCd battery charger, you may still
come across the odd ‘incompatible’ battery, for example, one having a
rare voltage or requiring a much higher charging current than can be
supplied by your off-the-shelf charger. In these cases, many of you will
resort to an adjustable mains adaptor (say, a 500-mA type) because that
is probably the cheapest way of providing the direct voltage required
to charge the battery. Not fast and not very efficient, this ‘rustic’
charging system works, although subject to the following restrictions:

Circuit diagram:

Battery Charging Indicator Circuit

Battery-Charging Indicator Circuit Diagram

  • You should have some idea of the charging current. In case you use
    an adaptor which is adjustable but of the unregulated, low output
    current type, you can adjust the current by adjusting the output
    voltage.
  • You have to know if the current actually flows through the
    battery. A current-detecting indicator is therefore much to be preferred
    over a voltage indicator.
  • To prevent you from forgetting all about the charging cycle, the
    indicator should be visible from wherever you pass by frequently. Using
    the circuit shown here, the LED lights when
    the baseemitter potential of the transistor exceeds about 0.2 V. Using a
    resistor of 1 ? as suggested this happens at a current of about 200 mA,
    or about 40 mA if R1 is changed to 4.7?. The voltage drop caused by
    this indicator can never exceed the base-emitter voltage (UBE) of the transistor, or about 0.7V. Even if the current through R1 continues to increase beyond the level at which UBE
    = 0.7 V, the base of the transistor will ‘absorb’ the excess current.
    The TO-220 style BU406 transistor suggested here is capable of accepting
    base currents up to 4A. Using this charging indicator you have overcome
    the restrictions 2 and 3 mentioned above.

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