Circuit to prevent over-discharge?

[Dodge]

I have a DeWalt xenon snake light which takes the same 14.4v batteries as my power tools. It's a useful light, as you can prop it at all sorts of angles. But it's all too easy to leave on, and I'm sure I've killed a couple of nicad packs this way.

Is there a simple circuit I could build, which would cut the power at a preset voltage? (Probably about 11v I guess). Could I use a zener diode for that?

Not sure what current it's drawing at the moment, but I have plans for a 4xCree upgrade, so it would be useful if any such circuit could handle at least 1A.

Sorry if this is a FAQ, couldn't find anything with a search.

 

[Dodge]

Come on, you're a clever lot, surely someone knows the answer to this? Or is it too basic to bother with?

 

[Stregone]

I've wondered the same thing, but for stuff like pencil sharpeners and other relatively high drain devices you would use AAs in that are too low tech to have a built in cut off.

 

[koala]

MAX8211/12 http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1273

Please note that the MAX821x has an operating voltage of 2.0V-16.5V and absolute max is 18V. A 14.4V battery pack fresh off the charger can be as high as 17.4(1.45V per cell). Once it settles the voltage will drop. If you don't wish to risk it then use a low power signal diode to drop the voltage going in to the MAX821x, hopefully the diode will not use too much current. The MAX821x alone sips current at <20uA so it's negligible.

You'll only need R1 and R2. R3 is not required if you don't need hysteresis feature. For cut off voltage of <11V, use 39Kohm(R1) and 330Kohm(R2) + 3K3(in series with R2, optional). Without the optional 3K3 in series with R2, the cut off voltage is about 10.88V.

A 14.4 nicd/nimh battery pack consist of 12cells. That's 10.8V cut off for 0.9V per cell, 11V seems all right for a medium power load.

http://www.maxim-ic.com/appnotes.cfm/appnote_number/926

Read the above application note carefully. 5 or perhaps 4 additional parts is required to complete the circuit. I think it makes sense to have a reset switch if your load is extremely high otherwise just keep it closed, and let the chip do the job. Finally, choose your load driver wisely.

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