How to charge a 9V Ni-MH

snipped-for-privacy@hotmail.com schrieb:

If you don'*t cancel charging after 14 hours, go with C/20.

- Henry

Ok. I'll probly leave on overnight. Am I right with the resistor value and charging voltage?

It needs to be about 15 volts or more - assuming it is filtered DC.

Then you pick a resistor value that gives about 10mA current flow with the voltage difference above 9.9 volts.

........ Phil

snipped-for-privacy@hotmail.com schrieb:

I wrote "if you don't cancel after 14 hours" -> meaning: constant-current charge without final voltage control.

For over night just set it to C/10. I don't know what's the correct termination voltage for 9V blocks is. And before I write wrong values please ask Wikipedia or Google. Or "robert" in the german electronics group. He is an expert for batteries of all kind.

- Henry

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

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

"ehsjr"

If tested under typical load conditions, a 7 cell NiMH pack will read close to 1.2 volts per cell.

Most nominal 9 volt batteries sold nowadays use 7 cells.

....... Phil

You charge with constant current. To do that, you use a source of voltage appreciably higher than the battery voltage, and a resistor to limit the current.

ehsjr schrieb:

Looks good.

- Henry

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

He can add a 9.1V zener reference diode and automatically taper it off to float charge at full charge of 1.4V/cell, like so, the series diode prevents battery discharge on loss of line power. At 15mA, the 1.7V will be enough headroom for the '317: View in a fixed-width font such as Courier. . . . . . ----- . +12 -+----|LM317|-----. . | ----- | . | | [82R] . | | | 1N4148 . | '--------+---|>|----> To NiMh + . | | . | | . +|10u | . === | 1N5239B . |16WVDC -/ 9.1V . | ^ . | | . | | . | | . | | . Gnd -+----------------+----------> To NiMh - . . . . . . .

The characteristic is sharper with the bigger diodes, and it is more likely to withstand a momentary reverse battery connection with a wimpy

View in a fixed-width font such as Courier.

. . . . . . ----- . +12 -+----|LM317|-----. . | ----- | . | | [82R] . | | | 1N4001 . | '--------+------|>|-------> To NiMh + . | | . | | . +|10u | . === | . |16WVDC | 1N4739 . | -/ 9.1V, 1W . | ^ . | | . | | . | | . Gnd -+----------------+----------------> To NiMh - . .

I doubt that any off-the-shelf zener is accurate enough for long battery life.

I'd use a TL431 (or CMOS equivalent) and a pot.

...Jim Thompson

Well, as long as we're talking Radio Shack, adding an IRF510 MOSFET like shown will take care of the momentary reverse battery hazard, limiting the current to 15mA. View in a fixed-width font such as Courier.

. . . . . . ----- . +12 -+----|LM317|-----. . | ----- | . | | [82R] . | | | 1N4001 . | '--------+------------+--|>|----> To NiMh + . | | | . | | | . +|10u | | . === | | . |16WVDC | 1N4739 | . | -/ 9.1V, 1W | . | ^ |G . | | - . | | --- IRF510 . | | v | . Gnd -+----------------+-----------' '--------> To NiMh - . S D .

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

Ed

That's a nice addition. I do that a little differently stealing current with a TL431. I set the TL431 for the desired end-of-charge voltage and have it light an LED with the stolen current. When the LED comes on, you know the charge is complete. The zener can be used the same way, of course, but the TL431 is crisper and allows you set to different end-of-charge voltages. The R for setting the charge rate is computed based on C/10 + Ipot. A resistance can be placed in parallel with the LED & 1K if you want to steal more current.

• ---[LM317]--[R]--+ | | +----------+--->|---+---- to batt + | | [1K] | | | [LED] / | \ [TL431]---->/ | \ | | Gnd ---------------+--------+---- To batt -

For simple, you can't beat the 1 chip + 1 R solution. For nice, the taper idea is great. Thanks for bringing it up.

Ed

At 15mA, the 1.7V will

You don't want that. The TL431 set for Vbatt=8.4 makes the difference junction 8.4+Vdiode=9V say, then the TL431+VF,LED =2.5+2=4.5V, making the 431 saturation current (9-4.5)/1K=4.5mA, leaving 10.5mA through the battery; you can't reduce it more than that. You would want to allow a saturation current of say 20mA to keep the '431 linear, changing that 1K to 4.5V/0.02=220R. But even then, you still don't know the battery taper current at 8.4V.

Sure you can, just use a regulated plug-in supply and single R. He doesn't even need a circuit board if he puts the R inline with the (+) wire.

The taper idea is essential if you want both a 12-hour charge and plug-in and forget it capability.

You can always adjust this one in, state of charging indicators added: View in a fixed-width font such as Courier.

. . . ----- . +15 -+--|LM317|---. ~~ 1N4001 . | ----- | .--|>|-----|>|----------> To NiMh + . | | [82R] | . | | | | . | '------+------+ . | | . | | ~~ IRF510 D--> To NiMh . | '--|>|-+-------------G . | | S . | |1N4739 | . | |9.1V | . | -/ | . | ^ | . | | | . +| | | . === | | . 10u| | | . | | 50R | . | '----/\\/\\/\\ --- . | ^ . | | . | | . Gnd -+---------------------------------' . .

Yep. Poor man's equivalent to a TL431 ;-)

...Jim Thompson

I think you missed a line in my post: "A resistance can be placed in parallel with the LED & 1K if you want to steal more current. "

The TL431 is linear under 1 mA Ik (about 400 uA), per the datasheet.

It's the parallel R that allows you to set the taper rate. You set the taper current rate to what you want with the parallel R mentioned in the post, stealing the "excess". Using your numbers, there is 10.5 mA left for the battery when there is 9 volts before the diode. If you wanted a taper rate of C/50 you'd need to steal 7.5 mA. So a parallel R across the LED and its R would be 867 ohms. Of course in practice, an 866 would be close enough, or even an 820, yielding about C = 1/58

I *knew* that was coming. :-)

I was drawing a comparison between the 1 chip + 1 R versus the 1 chip + 1 R + TL431 circuit.

Yes!

Ed