> Does anyone have any more info on this type of charger? My NiMH
charger
> has
> just gone bang (literally) and I'd like to build a replacement. Reflex
> charging looks quite interesting, but how do you detect end-of-charge
on a
> NiMH? What sort of timing and currents are used (e.g. how long to
charge,
> what wait time, how long to discharge, charge/discharge current...)?
> I'm aiming for a charge time of around four hours for four 1.2V
1600mAh
> NiMHs.
> I might also see about implementing charge balancing (make sure all
the
> batteries end up at the same voltage).

NIMH cells are quite complicated to charge, if you really do it right.

Discharging before charge is not needed, and just spends cell life.

Typical fast charge is around 1C, with a range of 0.5C to 2C. Outside
this range, the end of charge signals will become less distinct.

Check the cell specs carefully. 
If they are cheap Chinese cells, I'd advise against anything faster than
0.5C. And I'd suggest a face shield and gloves.

I've done this, using the AVR.
Implementing the reverse (discharge) burp is pretty easy, you can use a
mosfet and big 0.2-ish ohm resistor, or use a series R/C with discharge
R across the C (safer in development)

End of charge is detected in a number of ways.

1: Final backup, a timer that simply counts to say 120% normal charge
time.
2: A pipo over temperature cutout made into the battery pack.
3: Absolute temperature of > 40C
4: Rate of rise of temperature > 1C/min
5: Absolute voltage > 1.6V/Cell
7: Negative delta V of a few mV/Sec

Take your voltage measurements at the end of the burp, or pause, where
they are more stable.

Feeding the cells with constant current is a big help, it exaggerates
the voltage and temperature curves.

I did a monitoring system using a PMD1208LS USB DAQ to monitor the
battery and charging system during development.  Good batteries give a
sharp thermal spike at end of charge. The cheap ones have a very mushy,
or even more or less linear curve of temperature over time. Those cells
I would store in a metal can, if I were you.

Good resources, though somewhat conflicting:

McGraw-Hill's handbook of batteries
Sanyo and Panasonic data sheets.
Data sheets from your cell manufacturer.
Data sheets on charge ICs from Galaxy power.
Data sheets from the Cadex web site.




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