Linear Technology has an interesting charger for Lithium Ions
(actually, they have several interesting chagers - I've now designed
two of them into products). One I haven't used yet, but looks
interesting, is the LTC4052-4.2 . This is a "pulse charger" that uses
a current limited "wall wart" power supply. The charger just
switches, connecting the wall wart to the battery and disconnecting
it. The wall wart becomes a "constant current" charger. As the
battery approaches 4.2V, the duty cycle of the switching is
decreased, decreasing the average current going into the battery. The
chip has an output indicating when the average charge current has
fallen to 10% of maximum (I'm guessing they're detecting 10% duty
cycle) for a "near end of charge" indication. Charging is terminated
by a timer.
     So, looking at NiMH, I'm thinking of doing something similar under
PIC control. I'd use a current limited wall wart and switch it on and
off the battery, watching the battery voltage (turning the charger on
at some voltage, then off when the battery reached some higher
voltage). Depending on the hysteresis, it seems that this could
handle the entire charging process. As the battery self-discharges,
the charger would pulse the battery again for a short period of time.
We could also do a charger terminate based on the 10% duty cycle.
     Thoughts?

Harold


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