While discharging, battery voltage increases?

I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

Or maybe I'm reading the analyzer voltage that produces the 500
milliamps through the battery? And that's the voltage required to
produce the 500 milliamps? As expected, several other batteries show
the voltage levels decreasing as they discharge, but this one is
consistently unusual.

I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.

Thanks.

"John Dope"

I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V.


** So it is completely flat in one minute ??????????

Wot asinine drivel is this brain dead cretin on about now ??





....... Phil

"Bob******"
"John Dope"
I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

Or maybe I'm reading the analyzer voltage that produces the 500
milliamps through the battery? And that's the voltage required to
produce the 500 milliamps? As expected, several other batteries show
the voltage levels decreasing as they discharge, but this one is
consistently unusual.

I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.



It's probably not applying a constant load to the battery.


** Correct - but can you also explain the unusual behaviour of just one
particular cell?

And the wrong voltage reading.

1.05 volts = a flat cell.



....... Phil

On Fri, 12 Dec 2008 02:12:34 GMT, John Doe wrote:


I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

Or maybe I'm reading the analyzer voltage that produces the 500
milliamps through the battery? And that's the voltage required to
produce the 500 milliamps? As expected, several other batteries show
the voltage levels decreasing as they discharge, but this one is
consistently unusual.

I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.

Thanks.

Could be the analyzer. I've never had much luck with anything from Maha,
especially their batteries. But it could depend too on what you might have been
doing during testing. If you interrupted the discharge at any point, a battery
will slightly recover some of its charge.

Think of any rechargeable battery (or any battery for that matter), as a bucket
filled with some porous sediment. Now dig a hole out of the center to the bottom
of the bucket. Fill the well that you dug in the center of that bucket with
water. (The water in this analogy is the electricity you are putting into the
battery.)

As you fill it with water it will top off (full voltage), but the water from the
central well will leach into the surrounding porous sediment. Eventually you'll
be able to add no more water as the sediment around the well becomes saturated.
The "battery" is now fully charged.

In an analogous way, overfill it too fast or for too long and some of that
porous sediment will be washed over the side, become too mushy to work properly
and the well closes up or fills in with surrounding sediment, or it might even
distort the bucket. I.e. overcharging/overheating for too long = bad, needed
chemistry inside will boil off through the battery's vents, or destroy the
laminations and/or chemistry inside.

Now comes the time to draw off some current from it. As you draw water out of
that central well the main amount of water is still contained in all that porous
sediment around it. Draw off the water too quickly and you can run the well dry,
then the water slowly trickles in from the main reservoir around the well. In
very low amperage batteries that well is very narrow and the sediment is very
tightly packed so the water won't drain back-in to refresh that well as fast.
You can only draw off a small amount at a time at a continuous rate (the
available amperage).

If a battery was manufactured poorly and there are some contaminants in that
sediment that cause it to be more clay-like, less porous, it will refresh that
well even slower.

If you stop drawing off water from the well, the well will slowly refill to the
water level in the surrounding sediment. This is why a battery will increase in
voltage after you let it rest a bit after a discharge. If you ever get the
"Change Batteries" warning and you have no more batteries with you but you MUST
take a few more photos, wait a couple minutes. You'll be able to shoot off a few
more pictures and then get the warning again. That central well in your AA
buckets are slowly refilling from the porous sediment around it while you wait.

To confirm if its the charger/analyzer or the battery:

Try putting the "bad" battery into a different slot in the analyzer compared to
a known good one in the same slot that the "bad" battery was in. If the known
good one acts as the "bad" one then you know the analyzer is bad. Or it might be
something as simple as finger-grease on a battery terminal or charger terminal.
Get an ballpoint-ink eraser and clean all contacts with that. A ballpoint-ink
eraser has a fine abrasive in it that acts as a very good electrical-contact
cleaner. Just be sure to dust out all the bits of left-over eraser.

If the bad one still acts bad in a new slot then it was just a manufacturing
error in chemistry or internal laminations in that battery alone. **** happens.

If you had interrupted the testing at some point and it caused the peak in
voltage reading, then see above for why this happens. If the contacts were dirty
than just moving the charger/analyzer might have been enough to temporarily
interrupt the current flow and cause the rise in voltage for a moment.

0.03v (3 hundredths of a volt) is not a lot to worry about too. For perspective,
your own body will show anywhere from 0.1 to 0.5v of AC voltage in it at all
times from surrounding radio transmissions and AC power-line sources in your
home. Got a sensitive oscilloscope? Put it on a high sweep-rate and a 0.5v
scale. Hold one probe in each hand and watch the way cool and complex Lissajoux
patterns of AC voltage that are coursing through your body from all the
radio-waves you are receiving. Move the probes to different areas of your body
to change the patterns and voltage readings. The ear's lobes and folds seem to
be a particularly changeable area for a wide variety of voltages and patterns. I
suspect that the convoluted shapes of cartilage are acting as antenna-lengths
for certain frequencies. (I also suspect that this is how acupuncture-fads seem
to be able to pick up voltage fluctuations there so much. You can even do this
with a simple digital multimeter set on 0.5 VAC scale. Hold one probe grounded
in your hand and touch the other probe to parts of your ear. I've gotten some AC
voltage readings as high as 0.9 VAC when testing this in the past. Much can
depend on where I'm standing in the house.)

But I digress....

You might have dropped that battery at some point or it received a knock to its
side and distorted the laminations inside a bit. It'll refill the well from its
porous sediment a little differently now. There might be a slightly more porous
section of its sediment that allows it to refill the well a little more rapidly
when the water level gets that low. Might have been nothing more than a little
unknown micron-thin contaminant on some layer when they were assembling the
battery layers.

0.03v is really not something to worry about.

On 2008-12-12, John Doe wrote:

I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

change of temperature could do that possibly?

Phil Allison wrote:
"Bob******"
"John Dope"
I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

Or maybe I'm reading the analyzer voltage that produces the 500
milliamps through the battery? And that's the voltage required to
produce the 500 milliamps? As expected, several other batteries show
the voltage levels decreasing as they discharge, but this one is
consistently unusual.

I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.


It's probably not applying a constant load to the battery.

** Correct - but can you also explain the unusual behaviour of just one
particular cell?

And the wrong voltage reading.

1.05 volts = a flat cell.

But there is something odd happens with matched sets of NiMH batteries
in some applications. I have several sets for a Pentax istD digicam.
What I have noticed is that one cell always appears weaker than all the
others by a significant margin and the camera fails early with "low
battery" when NiMH cells are used ending up with 3 good ones and one
flat. The first one to go flat kills it. The odd thing is that I suspect
the flat is almost always in the same battery slot - as if there is some
extra burden on that specific cell. Using flash heavily or excessive
cold generally tips it over the edge more quickly.

I have taken to putting dots on the failing cells but there is no
obvious pattern - it isn't same cell failing every time due to higher
self discharge or internal factors. I have noted the position where the
cell most likely to fail resides and swap it out on failure. This
usually works and has better odds than perm 4 from 8 with 2 duds.

More curious still single use Duracells with higher terminal voltage
which I use when the NiMH have all died always seem to discharge evenly.
I would be interested if anyone can explain this.

Regards,
Martin Brown
** Posted from http://www.teranews.com **

John Doe wrote:
I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

Or maybe I'm reading the analyzer voltage that produces the 500
milliamps through the battery? And that's the voltage required to
produce the 500 milliamps? As expected, several other batteries show
the voltage levels decreasing as they discharge, but this one is
consistently unusual.

I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.

Thanks.

If the analyzer is applying only a light load, that is possible- the
battery heats up a bit, and the light load voltage is temperature
dependent. But 500 mA is not a light load, so that is surprising if it
REALLY is discharging at that rate.

On Fri, 12 Dec 2008 06:09:53 -0800 (PST), Martin Brown wrote:

But there is something odd happens with matched sets of NiMH batteries
in some digicam applications. I have several sets for a Pentax istD
digicam. What I have noticed is that one cell always appears weaker
than all the others by a significant margin and the camera fails early
with "low battery" when NiMH cells are used ending up with 3 good ones
and one flat. The first one to go flat kills it. The odd thing is that
I suspect the flat is almost always in the same battery slot - as if
there is some extra burden on that specific cell. Using flash heavily
or excessive cold generally tips it over the edge more quickly.

I've noticed that as well, but it's to be expected when the
batteries are used too long between recharges. Cells in a matched
set don't all have the same capacity. They're just placed in sets
that have tighter tolerances, but one of them will *always* be have
less capacity than the rest, and this 'weaker' cell will be the
first to die.


More curious still single use Duracells with higher terminal voltage
which I use when the NiMH have all died always seem to discharge
evenly. I would be interested if anyone can explain this.

Alkaline and NiMH batteries may have large differences in their
measured voltage curves as they're used, but both really discharge
fairly evenly. The big difference is that alkalines don't have
voltages that decline precipitously when they're nearly exhausted.
If you measure individual voltages of alkaline cells as they're
used, there may be differences, but they're fairly close, and they
remain usable at much lower voltages than rechargeables. If
measured, you'll probably find voltages clustered around 1.1v, 0.9v,
0.7v and 0.5v. Using the last as an example, with 4 AA cells you
might measure 0.53v, 0.50v, 0.48v and 0.47v under load. Cameras are
different in that they tend to require slightly more than 1.0 or 1.1
volts per cell to keep operating. When cameras exhaust their AA
alkaline cells, they're far from being really exhausted, will
probably show more than 1.2 volts if measured without a load, and
can continue to be used for hours, sometimes many dozens of hours in
devices that don't require high loads, such as analog radios (at low
or medium volume), LED lights, etc.

NiMH cells also discharge uniformly, but at the point were they're
nearly exhausted (somewhere between 1.1v and 1.0v) the first to
become completely exhausted will show a very rapid drop in voltage
as it plunges from 1.0v to 0.0volts. The remaining 3 NiMH cells
will still be pumping out more than 3.0 volts and you'd really want
the device they're powering to shut down at this point, because if
it doesn't, the depleted cell will start to become reverse charged,
damaging or killing it. This is true even though the 3 remaining
cells may have less than 1% of their capacity remaining. Most
digital cameras require more than 3.0volts to operate, so they won't
tend to kill NiMH cells, but if you don't remove them for recharging
until the camera shuts down, one of the cells is pretty much
guaranteed to appear near dead in a battery tester. The harm in
doing this repeatedly depends on the individual camera, ie, at how
low the voltage of the complete battery set has to go before it
shuts down. At the point that digital cameras shut down when
alkalines are used, they'll have a good deal of unused capacity, and
can probably provide a little more than 1.1 or 1.2 volts under light
loads to other devices for a long time.

It's possible that one or two cells may be loaded more heavily in
certain slots, and if that's the case it might be to provide a lower
voltage for clock or memory chips at very low currents. In this
case the effect would be noticeable only if the devices are used
very intermittently, where the batteries would last quite a while
before needing to be replaced, depending on design anywhere from a
couple of months to a year or more. This can be ruled out if you
open the case and check the battery compartment to see if it has no
more than two wires connecting the device to the battery pack.

John Doe wrote:
I'm discharging a nickel metal hydride (NiMH) AA battery at 500
milliamps. After the first minute or so it shows 1.05 V. Between then
and three hours later, the voltage steadily rises to 1.08 V. Is there
something wrong with the battery analyzer, or can battery voltage
increase very slightly as the battery discharges?

Or maybe I'm reading the analyzer voltage that produces the 500
milliamps through the battery? And that's the voltage required to
produce the 500 milliamps? As expected, several other batteries show
the voltage levels decreasing as they discharge, but this one is
consistently unusual.

I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.

Thanks.

I have an .mp3 player that uses a single AAA. I notice the charge
indicator on the device starts of full, shows partial charge after usage
and will return to show a full charge when next turned on. Never really
gave the matter much thought.
Dave Cohen

patrick chilton wrote:

snip detailed explanation

To confirm if its the charger/analyzer or the battery:

Try putting the "bad" battery into a different slot in the
analyzer compared to a known good one in the same slot that the
"bad" battery was in. If the known good one acts as the "bad" one
then you know the analyzer is bad.

0.03v is really not something to worry about.

I was partly curious. After a charge/discharge/charge cycle, I
switched the two batteries and started discharging them again at 500
milliamps, paying closer attention. The bad battery in the other
slot rose from about .96 V slowly and steadily upwards. Later, after
it reaches 1.15 V or whatever and is significantly discharged, it
probably starts going back downwards (as I recall from the last
time). Still, it charged to a higher capacity than the other same
type battery. I just threw it away.

Batteries are not nearly consistent as I would have guessed.



--
thanks to the replies