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'Battery Charging Techniques - Reply'
1997\08\06@095417 by ang (Chee Foon Tiang)

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NiMH batteries requires constant current charge.
Typical fast charge is at 1C, where C is the capacity of the battery.

e.g If you have a 800mAH 3-cell NiMH battery, you charge them at
   a constant 800mA

End of charge condition:

i.   -dV    - small drop in battery voltage at full charge
ii.  0dV    - consequtive zero increment in battery voltage over a
             period of time at full charge
iii. dT/dt  - Sharp increase in the rate of change of temperature
             at full charge, typically 0.8 C / min.
iv.  timer  - fail safe timer cut-off
v.   max V  - fail safe absolute maximum voltage cut-off
vi.  max T  - fail safe absolute maximum (charger) temperature
             cut-off

Any one of the above would be satisfactory, but typically used
in combination. Fail-safe cut-off are required in commercial
charger to prevent possible fire hazard.

Normally 8-bit ADC would be sufficient but do note that the
the resolution, with a Vref of 5V, is around 20mV / bit.
And the -dV drop is in that region.

Furthermore, NiMH have a smaller -dV drop as compared to NiCD and
typical commercial charger uses a combination of at least two
end-of-charge detection.

Hope this helps,

Peter Tiang

====================================================================

    Guys,

    I know this is off topic, so please repsond via email.

    I need to implement a battery charger for NiMH cells, the
battery has
    three cells.

    The battery needs to be fast charged in about 1 hour, the
obvious
    thinsg I need to know are how do I determine when the battery
has been
    charged. What time period did people take a measurement of the
battery
    voltage ... 30secs, 1min maybe 2mins? Do I need to wait that
long for
    the battery voltage to have change enough to be detected by an
8bit
    ADC. That of course raises the question as to whether 8bit ADC
is
    enough ... I may be able to get a 10bit one.

    I am in the process of buidling a test rig with GPIB equipment
to
    collate the data I need, but if anybody would care to enlighten
me
    form their own experiences I would be very grateful.

    kind regards,
    Scott.

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