'Battery Charging Techniques - Reply'
ang (Chee Foon Tiang)
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
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
Hope this helps,
I know this is off topic, so please repsond via email.
I need to implement a battery charger for NiMH cells, the
The battery needs to be fast charged in about 1 hour, the
thinsg I need to know are how do I determine when the battery
charged. What time period did people take a measurement of the
voltage ... 30secs, 1min maybe 2mins? Do I need to wait that
the battery voltage to have change enough to be detected by an
ADC. That of course raises the question as to whether 8bit ADC
enough ... I may be able to get a 10bit one.
I am in the process of buidling a test rig with GPIB equipment
collate the data I need, but if anybody would care to enlighten
form their own experiences I would be very grateful.
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