How to protect NiMH against over discharge?

[Cemoi]

I plan to build a front bike light using a Cree XR-E, powered by two AA NiMH in series and a Micropuck driver.
I've tried it on a test circuit board and it works as advertised: 360 mA output, 80% efficiency. But it continues to work when the battery voltage gets very low: still 100 mA in the LED when the total input voltage is down to 1.0 V.
This means that the NiMH batteries can get damaged while the light output is still quite good.

So I'm wondering if I can add something in the circuit, which would act as an automatic switch when the input voltage gets below e.g. 2.0V.

 

[kuksul08]

I'd also like to know about this, and what is the minimum voltage NiMH's should be discharged to, to be safe?

 

[kuksul08]

This may be an option actually

http://homepages.paradise.net.nz/bhabbott/lvw.html

 

[balou]

kuksul08: 1V seems about right for most applications. For high drain, you might want to set the low voltage cutoff at 0.9v

See here for discharge graphs:
http://www.candlepowerforums.com/vb/showthread.php?t=79302

Another low voltage cutoff option might be an Atmel microcontroller with builtin DAC and a MOSFET switching the light on/off. Then you would also get PWM brightness control for free, just add a few lines of code..

 

[mpf]

Another low voltage cutoff option might be an Atmel microcontroller with builtin DAC and a MOSFET switching the light on/off. Then you would also get PWM brightness control for free, just add a few lines of code..

for the few line of code see http://www.candlepowerforums.com/vb/showthread.php?t=201383
This is a complete tutorial for beginners using atmel for led control (no smd required)

 

[Cemoi]

This may be an option actually
http://homepages.paradise.net.nz/bhabbott/lvw.html

Thanks, but this will not protect my batteries if e.g. I forget to switch out my light when parking my bike. I need an automatic feature that switches off the light as soon as the input voltage reaches 1.0V per cell.

 

[Cemoi]

This is a complete tutorial for beginners using atmel for led control (no smd required)

Sorry mpf, this is definitely not for beginners. I have a decent background in electricity, I am still a beginner at electronics having built only a few basic LED circuits, but your page is all Greek to me.

Isn't there an easier way to achieve this?
How do the protected Li-ion protection circuit work? I need a similar (very compact) circuit, but which strips at 2.0V.

 

[Steve K]

So I'm wondering if I can add something in the circuit, which would act as an automatic switch when the input voltage gets below e.g. 2.0V.

How about a NCP303 low voltage detector to detect the low battery voltage (might also need a resistor divider network at the input to the NCP303), and then have it drive a mosfet that is wired in series with the Buckpuck input? Seems fairly simple to me....

Steve K.

 

[balou]

How do the protected Li-ion protection circuit work? I need a similar (very compact) circuit, but which strips at 2.0V.

Microcontrollers too

 

[linterno]

You could use an Atmel attiny13V or a Microchip PIC12F683 driving a MOSFET enabling Brown-out detect (BOD). When Brown-out is detected the device will reset and the MOSFET will be turned off.

For PIC devices the reset voltage it 2 to 2.2 volts; For attiny13V can be 1.8 volts.

 

[Cemoi]

How about a NCP303 low voltage detector to detect the low battery voltage

I checked the spec sheet for the NCP303, many pages not really designed for nubes.

For PIC devices the reset voltage it 2 to 2.2 volts

This would do, as when a NiMH cell goes down to 1.1V there is not much power left in it anyway.

A circuit sketch for any or both of the two above options would be most welcome:thanks:

 

[odessit]

This would do, as when a NiMH cell goes down to 1.1V there is not much power left in it anyway.

I agree with balou, it greatly depends on load.
At high load I would think 0.9V is the minimum.
At low load 1.1V will go for quite some time...

Also, my own testing does not agree with the SolverFox' graphs that balou posted.
http://www.candlepowerforums.com/vb/showthread.php?t=212883
At 0.5A load - I was right around 1.1V and lower for Eneloop and Sanyo 2700. Something just occurred to me - SilverFox probably used batteries hot off the charger, where in my case the batteries were "aged". I posted a questin about this in my thread.