Re: How to charge a 9V Ni-MH

sdeyoreo@xxxxxxxxxxx wrote:
On Mon, 31 Dec 2007 00:01:24 GMT, ehsjr <ehsjr@xxxxxxxxxxxxxxxx>
wrote:


sdeyoreo@xxxxxxxxxxx wrote:

How to make a 9V Ni-MH charger?

Battery is 9V, 150mAh. I read the charging current should be C/10 or
15 mA. Charging voltage should be ~ 1.41 v per cell. Also read that
NiMH is 1.2v/cell. The package for the battery says fully charged will
read 8.4 v. So that's 7 cells, 8.4/1.2. So charge at 7*1.41v = 9.9v @
15mA?

No, it's 6 cells at ~ 1.4 each.


So, what do I use for a current limit resistor? (9.9-8.4)/15mA or
100 ohms?

If you decide to use a resistor to limit the current
to 15 mA, you *must* use a regulated voltage source.
I prefer a regulated current charger (shown below) to
charge the cells.

This is a charger circuit that will hold the current
to very close to 15 mA. The input voltage must be
at least 3 volts higher than the full charge voltage
of 8.4, so 12 volts is shown, because the LM317 chip
needs close to 3 volts of "headroom", minimum, to
operate properly. You can use any 12 volt "wall wart"
with this circuit:

-----
+12 ---|LM317|---+
----- |
| [83R]
| |
+------+------- To NiMh +

Gnd --------------------- To NiMh -

Change the 83 ohms to 167 ohms if you want to use
the C/20 that Henry mentioned.

Ed

Thanks. Am I reading your schematic right?: + of battery is going to
ADJ of LM317 & 83ohms between ADJ and OUT of LM317 ?

No, the + from the battery goes to the LM317 *Vin*,
not adj. 83 ohms between out and adj is correct.

Ed
.

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