Draining NiMH by using LED in flashlight?

Is it true you an use an LED flashlight to drain an NiMH rechargeable AA cell to a suitable voltage in order to condition the cell?

ISTR that NiMHs should nto be drained to below 0.9 volts. Would an LED used in a typical flashlight stop drawing current once the voltage fell below

0.9 volts?
Reply to
david
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yes, actually, it'll stop higher than that, depending on the LED you are using.. Make sure you have a ballast resistor in series with it, don't connect it directly.. The ballast R depends on the voltage your NiMHs are..

Jamie

Reply to
Jamie

Most LED flashlights have "usual white LEDs", which are blue ones with a phosphor that converts much but not all of the blue light to a broadband yellowish light. The yellowish light combines with the unconverted part of the blue light to form white or whitish light.

These LEDs do not significantly conduct at any voltage normally produced by a single NiMH cell. An LED flashlight that uses a single NiMH cell has a boost converter circuit. Chances are fairly high that the boost converter circuit will not shut off when the cell voltage decreases to .9 volt.

On the other hand, there are many 3-cell white LED flashlights. If you put in 3 equally-charged NiMH cells, chances are "reasonably good" that they are "discharged safely" once the flashlight runs them down to being "noticeably dim" or "almost uselessly dim". Don't run the cells down to "completely uselessly dim" or "dim as a glowing cigarette" or "completely out".

One more thing - if an NiMH or NiCd cell, after removing it from the device that discharged it, still has voltage at least .5 volt and in the proper polarity, then in my experience any overdischarge damage is very negligible. My experience is that "overdischarge damage" mostly occurs from either "discharging past zero" ("reverse charge") or prolonged storage with zero charge.

--
 - Don Klipstein (don@misty.com)
Reply to
Don Klipstein

Thanks for a great reply. I'm simply trying to condition new NimH cells to have max capcity by discharging and then recharging them for a few cycles.

What you write sounds like I would need to keep checking the flashlight brightness to see how the discharge was progressing.

I was hoping to just put the NiMH cells into a flashlight and leave them to discharge until the flashlight completely stopped working. I kinda hoped that would stop automatically at about the right voltage for the cell to benefit from the discharge. Are you saying I would discharge the cells too much if I left the flashlight to go "completely out"?

Reply to
david

If that's all you want to do, simply discharge the single cells to zero with a resistor. The only reason to discharge NiMH or NiCd down to a terminal (your .9V/cell) voltage is if they're in a multi-cell battery. Then, you want to ensure that you don't reverse any cells. With an individual cell this cannot happen.

Reply to
krw

If the flashlight is a 1-cell one, brightness is not a good indicator, except that damage to only 1 cell is probably extremely minimal if severe discharge is caught within half a day.

If the flashlight is a usual 3-cell one with no semiconductors other than the LEDs, you don't have to monitor it continuously. Just check it every couple hours. If its current derain is lower so that "normal runtime" is more than a day, then I think it's OK to let it run while you sleep or go to work. As the decreasing voltage dims the LEDs, the current draw greatly decreases.

A hint: The usual 3-cell LED flashlights make great "emergency flashlights", since they keep going and going and glowing when the batteries get weak. That is unlike incandescent flashlights pooping out more rapidly because incandescent lamps have efficiency and electrical resistance decreasing as voltage decreases. BUT - emergency flashlights should have non-rechargeable batteries. Rechargeable batteries have greater self-discharge than disposable batteries do, so rechargeables could leave you in the dark months down the road.

--
 - Don Klipstein (don@misty.com)
Reply to
Don Klipstein

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