Best Method to Slow Charge NiMH Batteries

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Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

Thank You in advance, John

 

[mike]

Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

Thank You in advance, John

What does it say on the manufacturer's technical datasheet or charging
application note?

If they were NiCd, you'd have no problem.
The consensus appears to be that NiMH don't tolerate long-term overcharge.
Opinions vary on short-term overcharge. I expect you're doing about
the best you can do with simple schemes.

The difficulty is not in the equation. The difficulty is in determining
full discharge. How do you determine that?
Do you measure the voltage? under what conditions? OR do you just
charge it when the performance of your device drops? OR at random?
The range between 1.1V and 0.9V is huge depending on the load current
you use to test. And the device probably quits long before that.
I know because I have a computer hooked to a programmable power supply
and load fixture and I've tested it on a many different cells with widely
varying history.

One example application is the emergency flashlight.
You can't just charge it when it goes dark. Even in the best of conditions,
when it's starting to dim, you know you should charge it IMMEDIATELY...
but you've only got one more thing to do...so you do it.
More batteries are damaged from overdischarge than anything else.
For it's intended use, the emergency flashlight needs to be at FULL
charge all the time. So does your cell phone and your car battery and
your electric drill and...
Even low-drain applications are not immune. Do you want your alarm
clock to go dead at 4AM on the morning of your big presentation.

Bottom line is that you expect your device to perform for the entire
duration of the need...even when you have no idea how long that is.
That requirement often leads to systematic overcharge.

But:
I expect you're doing about the best you can do with simple schemes.

If you've done something like solder on the cells, you can expect
some variations in self-discharge and capacity. Some amount of overcharge
tends to keep them equalized.

Are we having fun yet?

 

[William Sommerwerck]

In the dark ages, I've charged NiCd and NiMH batteries with a timer.

It's a good way *IF* you know the capacity of the battery and the
state of charge. Capacity is easy as it's in the spec sheet. However,
a given battery can have radically different capacity values at
different charge/discharge rates. To make the numbers look huge,
manufacturers use a slow discharge rate. The battery also has a
charging efficiency, where it takes more coulombs to charge the
battery, than it can deliver. I use 75% for most batteries. Once you
know all that, and the state of charge, you can calculate how long
you need to charge the battery. If the battery gets warm at the end,
you're overcharging and have probably killed the battery.

This is one of those rare cases where I have to disagree with Mr Lieberman.
How warm is warm? "Getting warm" is an indication that charging is complete,
or near-complete.

Many NiMH manufacturers claim their cells need to be "slammed" to get maximum
charge. I'm reluctant to do this. However, I've charged NiMH cells in those
"15-minute" chargers, and though the cells got hot, they were not destroyed.
(I no longer do this. I have about 20 cells of 2500mAh and higher capacity, so
I never need a quick charge.)

The best way to charge batteries is with a charger that lets you set the
charge rate, and watches for the signs that indicate the cell is charged, such
as the MAHA C9000. I generally charge at 0.3C or 0.4C, which is considered on
the low side for NiMH cells. Contrary to what Battery University claims, I've
never had problems with the cells overheating, or the charger failing to halt
at around 1.42 to 1.45 volts. (Note that his gripes are principally directed
at "consumer" chargers.)

The C9000 displays the battery voltage, so I can choose to stop charger at
whatever voltage I feel comfortable with. I'm not stupid enough to let any
charger operate without checking it occasionally. If you like, stick a timer
on the charger to shut it off.

 

[William Sommerwerck]

<http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg>
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?

No. It means hitting it hard with a high charge current.

I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge.

I've never had that problem -- as far as I can tell. I've had cells that sat
around for the better part of year deliver 20 or 30 flashes, with rapid
recycling.

Most of my cells have been close to rated capacity. In one case, a cell was
about 20% low, and MAHA replaced it.


<http://www.mahaenergy.com/mh-c9000/>
Nice charger/analyzer. I dunno the break-in and battery-forming
features. Stressing the battery doesn't seem like a good way to
increase battery life though it might produce rated capacity earlier.
However, it's a much better charger than the common consumer junk
chargers, and would probably be a good charger for the OP.

Agreed. Thomas sometimes sold the C9000 for ~ $40, but those days seem over. I
have two, so I don't have to wait if a lot of cells need charging.

 

[William Sommerwerck]

"Jeff Liebermann" wrote in message

No. It means hitting it hard with a high charge current.

Yuck. Why would I want to do that? If there were dendrites in NiMH
batteries like in NiCd, then perhaps it would make sense. Besides if
such a high current blast was necessary to produce a proper battery,
or to produce bigger numbers on the data sheet, the manufacturers
would already be doing it.

Where did you see this recommendation? I couldn't find anything with
Google under "NiMH slamming" variations, except under National
Institute of Mental Health. I did find this blurb that recommends
against the practice:
<http://forums.dpreview.com/forums/post/25581821>


"Slam" is my choice of words.

I can't give you any references, but I've repeatedly seen in the documentation
for NiMH cells, that hitting them hard is necessary to get a "full" charge.
You are repeatedly warned NOT to charge them below 0.3C, and higher values are
preferable.

Perhaps my cameras and GPS require a higher minimum state of charge
than your flash? When I used to leave my conventional NiMH batteries
in the camera for a few months, it would usually be good for one or
two photos before complaining. I always kept a spare set of charged
NiMH with the camera in case I run out. They were also only good for
a few shots.

Most of my flashes are 500-series Canons, which are hardly wimpy li'l
critters.


<http://www.stefanv.com/electronics/sanyo_eneloop.html>
Scroll down to the two pink graphs near the bottom of the article. The
conventional battery is down to half capacity in 75 days, and 1/4
capacity in 150 days.

That's not what I would consider "rapid" self-discharge -- other than,
perhaps, compared to a lead-acid battery.



Incidentally, this article:
<http://www.stefanv.com/electronics/using_nimh.html>
covers quite a bit of ground on dealing with NiMH batteries, including
trickle charging, and a home made NiMH computah controlled charger:
<http://www.stefanv.com/electronics/battman2.html>
and a simplified USB powered charger:

Most of my cells have been close to rated capacity. In one case, a cell was
about 20% low, and MAHA replaced it.

I didn't know that Maha made batteries:
<http://www.mahaenergy.com/batteries/>

I don't know if they make them, but they definitely sell them under the
PowerEx brand. Thomas sells four for about $12. Considering that a charged
2500mAh cell gives about as much runtime as an alkaline cell, it doesn't take
long to amortize the cost.

Thomas sometimes sold the C9000 for ~ $40, but those days seem over.
I have two, so I don't have to wait if a lot of cells need charging.

I'm finding prices from $50 to $140 (including tax and shipping).
<http://www.ebay.com/sch/i.html?_nkw=maha mh-c9000&_sop=15>

The eBay prices are nuts.

I was wrong. Thomas's holiday deal is the charger, a generic carrying case,
four Immedion AA cells and a plastic case for the latter, for about $52. Given
the cost of the "accessories", the charger nets at $40. This would be a very
good time to grab a C9000 from Thomas Distributing.

 

[mike]

Yuck. Why would I want to do that? If there were dendrites in NiMH
batteries like in NiCd, then perhaps it would make sense. Besides if
such a high current blast was necessary to produce a proper battery,
or to produce bigger numbers on the data sheet, the manufacturers
would already be doing it.

Where did you see this recommendation? I couldn't find anything with
Google under "NiMH slamming" variations, except under National
Institute of Mental Health. I did find this blurb that recommends
against the practice:


Perhaps my cameras and GPS require a higher minimum state of charge
than your flash? When I used to leave my conventional NiMH batteries
in the camera for a few months, it would usually be good for one or
two photos before complaining. I always kept a spare set of charged
NiMH with the camera in case I run out. They were also only good for
a few shots.

<http://www.stefanv.com/electronics/sanyo_eneloop.html>
Scroll down to the two pink graphs near the bottom of the article. The
conventional battery is down to half capacity in 75 days, and 1/4
capacity in 150 days.

Incidentally, this article:
<http://www.stefanv.com/electronics/using_nimh.html>
covers quite a bit of ground on dealing with NiMH batteries, including
trickle charging, and a home made NiMH computah controlled charger:
<http://www.stefanv.com/electronics/battman2.html>
and a simplified USB powered charger:


I didn't know that Maha made batteries:
<http://www.mahaenergy.com/batteries/>
$4.25/ea for AA in a 4 pack is not cheap. Somewhat more expensive
than Sanyo Eneloop. I dunno.

There are other low self discharge NiMH batteries available.
Duracell Pre Charged
<http://www.duracell.com/en-US/product/rechargeable-staycharged-batteries.jspx>
Rayovac Hybrid or Platinum:
<http://www.rayovac.com/Products/Rec...argers/Everyday-Use-best-battery-charger.aspx>
Kodak Pre-Charged



I'm finding prices from $50 to $140 (including tax and shipping).
<http://www.ebay.com/sch/i.html?_nkw=maha mh-c9000&_sop=15>

+1 on the c9000

I've had good results with the older
Enloop charger...the one with the single led that can
charge an odd number of cells.

There are two categories of dendrites.
Dendrites that have already shorted need to be vaporized with
short-duration
high-current. That clears the short and blasts a bigger hole in the
separator.
Causes higher self-discharge and faster dendrite regrowth.
Back in the day when NiCds were expensive and shorted ones were free,
it made a lot of sense for hobby projects.

The second class is those that have not yet pierced the separator.
For those, I'm a fan of burp charging. Same process you'd use
for plating when you want a smooth surface.
I did some experiments, but I had nowhere enough data to draw a
conclusion. And I concluded that the cost of powering the computer
and the programmable power supply and load probably exceeded the benefit.

The biggest problem with charging batteries is determining when to
stop.

I had a friend who did radio repair for the city.
Back in the day, police radios were terminated by a thermal switch.
That worked surprisingly well, but only because the battery was
predictably depleted during the shift.
If you stuck a charged radio into the charger, you did some damage.
He had a lot of failed batteries from people who left the radio off
all day and stuck it back in the charger.

There's so much variability in temperature, thermal time constant,
voltage gradient, etc. that it's best to measure change than
absolute value.
I'm a fan of 0deltaV termination with timer and temperature fail-safe
for NiMH cells.

 

[William Sommerwerck]

Most of my flashes are 500-series Canons, which are hardly

I'm not familiar with the flash, but I suspect is does not have
the continuous high current load of a camera with it's focus
and aperture motors grinding away for many minutes.

Most DSLRs use lithium-ion batteries. I've never measured the current drain,
but I doubt it approaches what's required to charge an electronic flash from a
dead start. And unless you're using the focus-track feature, auto-focus
operation is intermittent.

--> discussion of rechargeable versus disposable batteries snipped <--

I use rechargeables on any device where I can easily swap the batteries.
Disposables are okay for certain kinds of toys and LED flashlights. (Most take
AAA cells, and I have a huge box of Polaroid AAA alkalines I got cheap.)

 

[William Sommerwerck]

MAHA has told me it will never provide a C or D adapter for the C9000. Not
wanting to figure out how to reliably attach wires to the charger's terminals,
I charge my C NiMH cells * in an Eveready charger with a NiCd / NiMH switch on
it. I monitor the charge with my hand. When the cell gets a bit warm, I pull
it.

* I use C cells in a potato-masher flash and a classic Sony FM radio.

Also known as "negative pulse charging" or "reflex charging".
There are commercial chargers that do this.
Pulse-Power:

"Negative Pulse Charging Myths and Facts"

I'm not convinced, but then I haven't tried one of these chargers.

This principle isn't new. The basic system dates back to the late 60s.
Motorola made a "pulse-discharge" charger for Honeywell flashes. The claim was
not only that it could quickly recharge the power pack, but that it would
often "heal" a badly performing or even "dead" pack.

"Modern Photography" gave the system a rave review, confirming that it worked
as claimed. Several years later I asked Burt Keppler what happened to the
system. He said Honeywell pulled it because there were too many instances of
the packs exploding. When I asked him why he never reported this in "Modern",
he had no answer.

 

[mike]

Sigh. I've got an itchy mouse finger hovering over the "buy" button.


I've done my share of NiCd cells directly across a 12v car battery for
a few fractions of a second. No explosions, and a rather minimal
success rate. The ones that worked didn't last very long.

You need better control than that. I've had better luck discharging
a big cap into the cell.

Also known as "negative pulse charging" or "reflex charging". There
are commercial chargers that do this.
Pulse-Power:
<http://www.fm2way.com/batterycharger/technica.htm>


Yep. Kinda like filling the gas tank with no gas gauge or automatic
pump shutoff.


I know it well. The Motorola HT600 and similar "brick" radios. As an
added bonus, Motorola may have designed the charger to destroy the
battery packs.
<http://www.repeater-builder.com/motorola/genesis/ht600-charger-mod.html>
They also have to be modified to work with NiMH packs:
<http://www.repeater-builder.com/motorola/genesis/nimh-charger-mod.html>

However, my favorite was a Kenwood something HT, that had small a red
plastic window in the bottom of the battery pack. The charger had a
corresponding window with a IR photo transistor in the charger base.
This was suppose to measure the cell temperature and stop charging
when it became warm. Unfortunately, a little dirt on either window
turned the charger into a battery overcharger.


For NiMH, that's a problem. Trickle charging an NiMH battery makes
the drop in terminal voltage very difficult to see.

Agreed. Trickle charging is for sissies. ;-) And only makes sense when
you have no reasonable termination system...and then, it still makes no
sense.

That's also my complaint about the C9000.
If your cells have slightly high internal resistance, it refuses
to charge them at rated current. If you crank the current down till
they charge, you risk failure of charge termination.

It's in the low

 

[Adrian Tuddenham]

[...]

<http://www.buchmann.ca/article18-page1.asp>
[..]
Overcharge could occur
even though the NiMH battery feels cool to the touch."

Where does the excess energy go if it doesn't finish up as heat in the
battery?

 

[mike]

[...]

<http://www.buchmann.ca/article18-page1.asp>
[..]
Overcharge could occur
even though the NiMH battery feels cool to the touch."

Where does the excess energy go if it doesn't finish up as heat in the
battery?

Cool to the touch is not the same as exactly at ambient.