C
Chris Carlen
- Jan 1, 1970
- 0
Hi:
I have been working a bit with NiMH AA cells lately in powering various
gadgets such as LED flashlights, HID flashlights, and a photoflash unit.
I have been using the highly regarded Sanyo and Energizer 2500mAh cells.
I use a LaCrosse BC-900 to pre-condition and charge the cells.
http://www.greenbatteries.com/bcalbachandr.html
The Energizer and Sanyo cells seem to be fairly well matched when
conditioned well, with a spread of perhaps 2.4 to 2.6 Ah.
However, when used in series packs of 4 to 8 cells, always it is the
case that one or two cells collapse before the other cells are depleted.
They never all go down together. Thus, the evidence of cell depletion
as perceived in the operation of the device is very ambiguous, making it
hard to tell when to turn it off to avoid cell damage.
Of course I have no way to be sure if a cell has experienced reverse
voltage while installed in most devices. If I have the chance, I
quickly remove the cells and measure with a DMM when the device appears
to be running low on power. What I find is one or two cells with a very
low voltage of 0.9V or so, rapidly rising. Thus, I suspect that these
cells likely had reversed voltage when current was flowing, and are now
recovering their positive voltage due to material diffusion.
Inserting the cells in the BC-900 on discharge mode reveals that sure
enough the low cells have effectively zero capacity and the other cells
might have 5-10% or so of remaining capacity. Just enough to keep the
gadget going long enough to reverse the voltage on the weaker cells for
a considerable time.
Now I am fortunate enough to have test instruments, know a little about
the need to quickly turn off a gadget once declining power begins to be
apparent, and I have one of the most sophisticated non-laboratory grade
battery chargers on the market (which I think is still rather deficient
in many quality and feature aspects).
The best things about the BC-900 is it allows one to condition new
cells, and measure their capacities. I would never be satisfied with a
charger that just said "has a conditioning cycle with 'done' indicator
LED" because one really needs a quantitative measure to be certain the
cells have been conditioned. Even the BC-900 typically fails to fully
condition on the first run. A second run usually does the trick. This
can take 2-3 days.
New cells take quite a few cycles to approach full capacity. This
coupled with the cell voltage reversal problem leads me to suspect that
the average consumer might have mostly disappointing experiences with
NiMH cells sold in stores. Especially since the chargers sold with them
don't provide diagnostic info nor have the ability to condition cells
with multiple cycles before first use. Also, the capacities are VERY
poorly matched on the first few cycles. Finally, unless they buy very
good cells, many cheap cells out their never have well balanced
capacities even if one attempts to condition them. (The batteries that
come with the charger are a case in point--chuck them and buy
Energizers.) So if the new cells are used in devices rather than first
in a conditioning cycle in a charger (with *independent* cell charging
channels) then the cells are likely to experience the most prolonged and
damaging voltage reversals in the first few uses!
I have noticed that the rate of self-discharge is very slightly faster
on the cells which I suspect have reversed. Though, they only reversed
for a very brief time. If consumers leave their gadgets running until
they basically "don't go anymore" ie, deep discharge the packs, then
they likely are damaging their cells quite a bit, leading to a
continually degenerating performance of the pack.
My experiences with NiMH cells have convinced me that they take a lot of
care and effort to use effectively. Ideally a device engineered to use
them will shut down before cells collapse. But this is very difficult
to ensure unless circuitry can actually monitor every cell
independently. Of course, no designer would spare the expense of such a
scheme. Thus I doubt that consumers are ever realizing the full
potential of these cells, which truly have remarkable capacities, and
the current to make high-drain devices perform very well. But without
the proper care which I suspect most folks never give these cells, they
are likely to seem like junk, with very poor lifetimes and performance
in gadgets.
Any thoughts?
P.S. I have also recently tried rechargable Li-ion CR123 shaped cells.
Just a pair of them in an LED flashlight. Sure enough one cell
collapses before the other and now that cell has slightly higher
self-discharge rate than the other.
It seems cells need to have a diode built in to limit reverse voltage to
a non-damaging or at least a very minimally damaging level (maybe a
Shottky).
--
Good day!
________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
[email protected]
NOTE, delete texts: "RemoveThis" and
"BOGUS" from email address to reply.
I have been working a bit with NiMH AA cells lately in powering various
gadgets such as LED flashlights, HID flashlights, and a photoflash unit.
I have been using the highly regarded Sanyo and Energizer 2500mAh cells.
I use a LaCrosse BC-900 to pre-condition and charge the cells.
http://www.greenbatteries.com/bcalbachandr.html
The Energizer and Sanyo cells seem to be fairly well matched when
conditioned well, with a spread of perhaps 2.4 to 2.6 Ah.
However, when used in series packs of 4 to 8 cells, always it is the
case that one or two cells collapse before the other cells are depleted.
They never all go down together. Thus, the evidence of cell depletion
as perceived in the operation of the device is very ambiguous, making it
hard to tell when to turn it off to avoid cell damage.
Of course I have no way to be sure if a cell has experienced reverse
voltage while installed in most devices. If I have the chance, I
quickly remove the cells and measure with a DMM when the device appears
to be running low on power. What I find is one or two cells with a very
low voltage of 0.9V or so, rapidly rising. Thus, I suspect that these
cells likely had reversed voltage when current was flowing, and are now
recovering their positive voltage due to material diffusion.
Inserting the cells in the BC-900 on discharge mode reveals that sure
enough the low cells have effectively zero capacity and the other cells
might have 5-10% or so of remaining capacity. Just enough to keep the
gadget going long enough to reverse the voltage on the weaker cells for
a considerable time.
Now I am fortunate enough to have test instruments, know a little about
the need to quickly turn off a gadget once declining power begins to be
apparent, and I have one of the most sophisticated non-laboratory grade
battery chargers on the market (which I think is still rather deficient
in many quality and feature aspects).
The best things about the BC-900 is it allows one to condition new
cells, and measure their capacities. I would never be satisfied with a
charger that just said "has a conditioning cycle with 'done' indicator
LED" because one really needs a quantitative measure to be certain the
cells have been conditioned. Even the BC-900 typically fails to fully
condition on the first run. A second run usually does the trick. This
can take 2-3 days.
New cells take quite a few cycles to approach full capacity. This
coupled with the cell voltage reversal problem leads me to suspect that
the average consumer might have mostly disappointing experiences with
NiMH cells sold in stores. Especially since the chargers sold with them
don't provide diagnostic info nor have the ability to condition cells
with multiple cycles before first use. Also, the capacities are VERY
poorly matched on the first few cycles. Finally, unless they buy very
good cells, many cheap cells out their never have well balanced
capacities even if one attempts to condition them. (The batteries that
come with the charger are a case in point--chuck them and buy
Energizers.) So if the new cells are used in devices rather than first
in a conditioning cycle in a charger (with *independent* cell charging
channels) then the cells are likely to experience the most prolonged and
damaging voltage reversals in the first few uses!
I have noticed that the rate of self-discharge is very slightly faster
on the cells which I suspect have reversed. Though, they only reversed
for a very brief time. If consumers leave their gadgets running until
they basically "don't go anymore" ie, deep discharge the packs, then
they likely are damaging their cells quite a bit, leading to a
continually degenerating performance of the pack.
My experiences with NiMH cells have convinced me that they take a lot of
care and effort to use effectively. Ideally a device engineered to use
them will shut down before cells collapse. But this is very difficult
to ensure unless circuitry can actually monitor every cell
independently. Of course, no designer would spare the expense of such a
scheme. Thus I doubt that consumers are ever realizing the full
potential of these cells, which truly have remarkable capacities, and
the current to make high-drain devices perform very well. But without
the proper care which I suspect most folks never give these cells, they
are likely to seem like junk, with very poor lifetimes and performance
in gadgets.
Any thoughts?
P.S. I have also recently tried rechargable Li-ion CR123 shaped cells.
Just a pair of them in an LED flashlight. Sure enough one cell
collapses before the other and now that cell has slightly higher
self-discharge rate than the other.
It seems cells need to have a diode built in to limit reverse voltage to
a non-damaging or at least a very minimally damaging level (maybe a
Shottky).
--
Good day!
________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
[email protected]
NOTE, delete texts: "RemoveThis" and
"BOGUS" from email address to reply.
