Draining NiMH by using LED in flashlight?

[david]

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?

 

[Jamie]

david wrote:

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?
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

 

[david]

On 19:48 8 May 2011, Don Klipstein wrote:

In article <Qgzxp.4437$BG2.367@newsfe08.iad>, Jamie wrote:
david wrote:
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 shouldn't 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?

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..

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.

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"?

 

[Don Klipstein]

In article <Qgzxp.4437$BG2.367@newsfe08.iad>, Jamie wrote:

david wrote:
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 shouldn't 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?

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..

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)

 

[krw@att.bizzzzzzzzzzzz]

On Sun, 08 May 2011 20:47:35 +0100, david <d_brown@msn.com> wrote:

On 19:48 8 May 2011, Don Klipstein wrote:

In article <Qgzxp.4437$BG2.367@newsfe08.iad>, Jamie wrote:
david wrote:
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 shouldn't 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?

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..

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.

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.

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.

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"?

 

[Don Klipstein]

In article <Xns9EDFD384DC54D8D59A@178.22.82.31>, david wrote:

On 19:48 8 May 2011, Don Klipstein wrote:

In article <Qgzxp.4437$BG2.367@newsfe08.iad>, Jamie wrote:
david wrote:
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 shouldn't 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?

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..
SNIP some of wha5t I said to edit for space
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.

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"?

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)