NIMH battery charging.

[Chuck Olson]

What is the group's opinion of the chargers that Radio Shack sells at
this time?

And for that matter their batteries also.

Thanks

TMT

Here's a very good place to get the latest on chargers and batteries
http://candlepowerforums.com/vb/forumdisplay.php?f=9

Chuck

 

[Too_Many_Tools]

Thanks for the lead.

TMT

 

[[email protected]]

What does a smart charger do when compared to just charging NIMH cells
at ten percent of their capacity, (as I do nicads now) ? What are the
characteristics of the charge cycle with the different types of
chargers and can a circuit to do this be built to handle batteries of
different voltages and capacities? Lenny,.

 

[Dave D]

Thanks for the lead.

While we are on the subject, what is the shelf life of NiMH batteries
if they are left in their original containers?

If one buys a package and puts it on the shelf for let's say five
years, will the battery work as if you had charged it when you first
bought it?

Theoretically, yes. In practice, it's possible for them to deteriorate.

Dave

 

[imbsysop]

What does a smart charger do when compared to just charging NIMH cells
at ten percent of their capacity, (as I do nicads now) ?

you mean 10% under their max. capacity ? How do you
define/test/measure those 10% ?
IIRC and per specs. one needs an 120% capacity charge to have a cell
fully charged

What are the

characteristics of the charge cycle with the different types of
chargers and can a circuit to do this be built to handle batteries of
different voltages and capacities? Lenny,.

it detects the -dV/dt notch that occurs when batteries are fully
charged .. I think Maxim (sp?) has a number of IC's that can be used
to build a home made charger with such characteristics .. it may be
worth the effort for the fun of building I don't think it will save
you a dime compared to commercially available units ..
FWIW

 

[Nostrobino]

you mean 10% under their max. capacity ? How do you
define/test/measure those 10% ?
IIRC and per specs. one needs an 120% capacity charge to have a cell
fully charged

He said charging *at* ten percent of capacity, e.g. charging 2000mAh cells
at 200mA. That's the long-established rate for standard (as opposed to fast)
charging of such cells, I believe.

N.

 

[imbsysop]

He said charging *at* ten percent of capacity, e.g. charging 2000mAh cells
at 200mA. That's the long-established rate for standard (as opposed to fast)
charging of such cells, I believe.

sorry for my bad reading ..

caveat of that method is that it is time controlled and one never
knows the discharge state of the batteries when charging starts so the
"timer run" for "full" is impossible to define .. with a -dV/dt
charger nothing to worry about anymore. have been using one for 4
years now and my NiMh's still feel like brand new ..
FWIW

 

[Ken Weitzel]

imbsysop wrote:

you mean 10% under their max. capacity ? How do you
define/test/measure those 10% ?
IIRC and per specs. one needs an 120% capacity charge to have a cell
fully charged

Hi...

Suspect that you'll find that 120% isn't sufficient. You'll need
about 140%

Ken

 

[Nostrobino]

sorry for my bad reading ..

caveat of that method is that it is time controlled and one never
knows the discharge state of the batteries when charging starts so the
"timer run" for "full" is impossible to define ..

Yes, good point.

with a -dV/dt
charger nothing to worry about anymore. have been using one for 4
years now and my NiMh's still feel like brand new ..
FWIW

You're way ahead of me on this and I'm afraid I don't know what -dV/dt
means. Is that something related or similar to the "FLEX negative pulse"
charging that Maha advertises (which is also Greek to me)?

N.

 

[imbsysop]

imbsysop wrote:



Hi...

Suspect that you'll find that 120% isn't sufficient. You'll need
about 140%

well I was just reporting a "general" statement .. I know that it
should be 140% as charger manufacturers as well as charger IC
manufacturers use an 1.4 loss factor when charging so they actually
dimension their chargers for an 140% overcharge

 

[imbsysop]

You're way ahead of me on this and I'm afraid I don't know what -dV/dt
means. Is that something related or similar to the "FLEX negative pulse"
charging that Maha advertises (which is also Greek to me)?

whoops .. yes it is probably the same thing the Maha guys mean ..

what is refered to is that, at "full" charge NIMH's and NiCd's exhibit
a short steep voltage raise followed rather quickly by a sudden small
voltage drop. This is commonly refered to as a -dV/dt aka a negative
voltage increase per time .. modern chargers detect this dip and act
accordingly, mostly switching to so called "trickle" charging ..
HTH

 

[ASAAR]

whoops .. yes it is probably the same thing the Maha guys mean ..

what is refered to is that, at "full" charge NIMH's and NiCd's exhibit
a short steep voltage raise followed rather quickly by a sudden small
voltage drop. This is commonly refered to as a -dV/dt aka a negative
voltage increase per time .. modern chargers detect this dip and act
accordingly, mostly switching to so called "trickle" charging ..

I don't think so. dV/dt just refers to monitoring the changing
slope of voltage vs. time. That could be done with any type of
charging circuit, even an extremely simple one based on little more
than a capacitor and a diode, and maybe a current limiting resistor.
Charging using pulses of current (used by my old, but poorly
designed RayOVac renewable alkaline/NiCad charger) is supposed to be
a better method. Possibly more efficient. But adding negative
pulses (slightly discharging between positive charging pulses)
supposedly leaves the chemical structure of the battery in a better
state. i.e., the batteries retain their fullest capacity for a
greater number of charge cycles. Whether that's completely true or
an oversimplification I can't say, as I don't recall reading it in a
white paper or fact sheet from a battery manufacturer's website.

 

[Nostrobino]

I don't think so. dV/dt just refers to monitoring the changing
slope of voltage vs. time. That could be done with any type of
charging circuit, even an extremely simple one based on little more
than a capacitor and a diode, and maybe a current limiting resistor.
Charging using pulses of current (used by my old, but poorly
designed RayOVac renewable alkaline/NiCad charger) is supposed to be
a better method. Possibly more efficient. But adding negative
pulses (slightly discharging between positive charging pulses)
supposedly leaves the chemical structure of the battery in a better
state. i.e., the batteries retain their fullest capacity for a
greater number of charge cycles. Whether that's completely true or
an oversimplification I can't say, as I don't recall reading it in a
white paper or fact sheet from a battery manufacturer's website.

Dang, this is an interesting discussion. (Defined as "any discussion that
leaves me knowing more coming out than I did going in.")

N.

 

[imbsysop]

I don't think so. dV/dt just refers to monitoring the changing
slope of voltage vs. time. That could be done with any type of
charging circuit, ..

I think you are largely missing the point .. the formula describes the
mathematical differential function (integration) that describes the cell
behaviour at full charge .. the charger doesn't particpate in this except
for detecting this "endpoint" by working as a dt integrator.

 

[ASAAR]

I think you are largely missing the point .. the formula describes the
mathematical differential function (integration) that describes the cell
behaviour at full charge .. the charger doesn't particpate in this except
for detecting this "endpoint" by working as a dt integrator.

I'm fully aware of the formula and what it represents, at least
mathematically. But this is not the proper forum to discuss
integral calculus and derivatives. But your point that it was
"probably the same thing the Maha guys mean ..", and was also
referring to FLEX negative pulse charging seems to be incorrect.
Are you saying that all of the smart chargers that monitor "-dV/dt"
use pulse charging circuits? I thought they were used only by some
of them. And that not all of those also utilize negative charge
pulses. If I'm mistaken could you provide some documenting links?

 

[John Henderson]

But adding negative pulses (slightly discharging between
positive charging pulses) supposedly leaves the chemical
structure of the battery in a better state. i.e., the
batteries retain their fullest capacity for a greater number
of charge cycles.

As I understand it, PCR (periodic current reversal) has a
beneficial effect on the physical structure of a metallic
cathode as it's formed, rather than a chemical effect.

In fact, I believe PCR is required in electroplating, otherwise
the plated material is too spongy to be useful. In a battery,
this can mean dendrite formation.

My own electroplating experiments (without PCR) when I was
young produced very soft deposits.

John

 

[ASAAR]

As I understand it, PCR (periodic current reversal) has a
beneficial effect on the physical structure of a metallic
cathode as it's formed, rather than a chemical effect.

Right. That's why I added the word "structure". If I thought it
only affected the chemical soup (or paste?) I would have worded it
differently. I don't know if the PCR's benefit is related to NiCad
dendrite formation, but I assumed it would either prevent something
like that, or the opposite, keeping the cathode from being eroded in
a manner similar (but chemically, not physically) to the way halogen
lamps redeposit vaporized metal back onto the filament.

 

[imbsysop]

I'm fully aware of the formula and what it represents, at least
mathematically. But this is not the proper forum to discuss
integral calculus and derivatives. But your point that it was
"probably the same thing the Maha guys mean ..", and was also
referring to FLEX negative pulse charging seems to be incorrect.
Are you saying that all of the smart chargers that monitor "-dV/dt"
use pulse charging circuits? I thought they were used only by some
of them. And that not all of those also utilize negative charge
pulses. If I'm mistaken could you provide some documenting links?

I agree on your points .. I was not aware of the "newer"(?) negative
pulse charging systems that probably do a better job in preserving the
microcrystaline structure of the "internal chemical battery soup" ...
as to "smart chargers" is there any other method that can be
considered "intelligent" than monitoring the voltage dip ? If a
charger doesn't use this method, I for one would not qualify it as an
"intelligent charger"

 

[imbsysop]

As I understand it, PCR (periodic current reversal) has a
beneficial effect on the physical structure of a metallic
cathode as it's formed, rather than a chemical effect.

In fact, I believe PCR is required in electroplating, otherwise
the plated material is too spongy to be useful. In a battery,
this can mean dendrite formation.

My own electroplating experiments (without PCR) when I was
young produced very soft deposits.

Well in a previous life I used to work in Nikkel electroplating but we
always used "straight" DC .. Our biggest enemy in deposits quality was
actually copper which made the deposits brittle. But times may
have changed
FWIW

 

[Chuck Olson]

On Tue, 27 Sep 2005 21:53:02 +0200, imbsysop wrote:

I'm fully aware of the formula and what it represents, at least
mathematically. But this is not the proper forum to discuss
integral calculus and derivatives. But your point that it was
"probably the same thing the Maha guys mean ..", and was also
referring to FLEX negative pulse charging seems to be incorrect.
Are you saying that all of the smart chargers that monitor "-dV/dt"
use pulse charging circuits? I thought they were used only by some
of them. And that not all of those also utilize negative charge
pulses. If I'm mistaken could you provide some documenting links?

A year or so ago, I bought the MH-C401FS battery charger with "FLEX NEGATIVE
pulse charging" feature. I wanted to find out what their charging procedure
was that produced such good results. I have a chart recorder that will trace
a line on a paper graph at an adjustable speed of paper movement. The
process I wrote in my notes is as follows:

"As soon as a battery is inserted in the holder, the controller switches
1000 mA charging current through a cycle of 1/4 sec on, followed by 3/4 sec
off, and it does this for 66 seconds before settling down to a nearly
continuous charge rate. I say nearly continuous because the charging is
switched off for 9.6 milliseconds four times per second. This continues
until about 50 minutes into the charging process when the charging current
is turned off for about 1 minute, and then continues for another 50 minutes
or until the charging process is stopped by the detection of a slight drop
in battery voltage, at which time the green light is lit and the charging
switches to a trickle charge by turning on the 1000 mA for only 1/4 second
every 4 seconds. So that's what happens to the current - - but I have no
idea when the microprocessor looks at battery voltage."

The chart recorder was set to display zero current at chart center so that
any reversal of current that might be described as "Flex Negative charging
current" would be displayed as a negative value. The charging current never
went negative! But this run was done on a set of new cells, so maybe the
"Flex" part didn't need to be executed. I can imagine applying a momentary
resistive load would result in a "negative" pulse of charging current, so it
might happen, but I never saw it.

Chuck