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Sci.Electronics.Basics -> # of batteries to cause polarity reversal
There are 23 messages in this thread.
You are currently looking at messages 1 to 20.
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If I have two NiMH batteries in series under a load, and one is weaker
than the other, is it possible for the stronger one to force the weaker
one to reverse polarity?
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mrdarrett@gmail.com wrote:
> If I have two NiMH batteries in series under a load, and one is weaker
> than the other, is it possible for the stronger one to force the weaker
> one to reverse polarity?
Update to above:
I noticed that my son's motorized bubble blower, which uses six AA
batteries (I use NiMHs), frequently causes one or two cells to reverse
polarity.
A monstrosity I built (16 NiMH AAs in series) to provide power to my
laptop's 18.5VDC input, *always* fails by causing one or more cells
(sometimes four at a time!) to reverse polarity.
So, would it be better to power devices with multiple AAs wired in
PARALLEL, with a step-up converter to raise the voltage, to prevent
cell reversal altogether?
Michael
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Author: Don BruderDate: 15:17 16-01-07
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In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
mrdarrett@gmail.com wrote:
> mrdarrett@gmail.com wrote:
> > If I have two NiMH batteries in series under a load, and one is weaker
> > than the other, is it possible for the stronger one to force the weaker
> > one to reverse polarity?
>
>
> Update to above:
>
> I noticed that my son's motorized bubble blower, which uses six AA
> batteries (I use NiMHs), frequently causes one or two cells to reverse
> polarity.
>
> A monstrosity I built (16 NiMH AAs in series) to provide power to my
> laptop's 18.5VDC input, *always* fails by causing one or more cells
> (sometimes four at a time!) to reverse polarity.
>
> So, would it be better to power devices with multiple AAs wired in
> PARALLEL, with a step-up converter to raise the voltage, to prevent
> cell reversal altogether?
>
> Michael
>
Actually, it would be better to pay attention to how far down you're
drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
result of discharging the cell too deep - there is no other cause. Avoid
droppng a cell below the critical point (I've forgotten what that value
is for an NiMH cell), and you'll *NEVER* reverse one.
--
Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd>; for more info
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Author: James ArthurDate: 17:10 16-01-07
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Don Bruder wrote:
> In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
> mrdarrett@gmail.com wrote:
>
> > mrdarrett@gmail.com wrote:
> > > If I have two NiMH batteries in series under a load, and one is weaker
> > > than the other, is it possible for the stronger one to force the weaker
> > > one to reverse polarity?
> >
> >
> > Update to above:
> >
> > I noticed that my son's motorized bubble blower, which uses six AA
> > batteries (I use NiMHs), frequently causes one or two cells to reverse
> > polarity.
> >
> > A monstrosity I built (16 NiMH AAs in series) to provide power to my
> > laptop's 18.5VDC input, *always* fails by causing one or more cells
> > (sometimes four at a time!) to reverse polarity.
> >
> > So, would it be better to power devices with multiple AAs wired in
> > PARALLEL, with a step-up converter to raise the voltage, to prevent
> > cell reversal altogether?
> >
> > Michael
> >
>
> Actually, it would be better to pay attention to how far down you're
> drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
> result of discharging the cell too deep - there is no other cause. Avoid
> droppng a cell below the critical point (I've forgotten what that value
> is for an NiMH cell), and you'll *NEVER* reverse one.
According to DuraCell's OEM data, it's okay to go as low as 0v across
an NiMH cell. Any reverse voltage risks permanently damaging the cell.
Best,
James Arthur
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Don Bruder wrote:
> In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
> mrdarrett@gmail.com wrote:
>
> > mrdarrett@gmail.com wrote:
> > > If I have two NiMH batteries in series under a load, and one is weaker
> > > than the other, is it possible for the stronger one to force the weaker
> > > one to reverse polarity?
> >
> >
> > Update to above:
> >
> > I noticed that my son's motorized bubble blower, which uses six AA
> > batteries (I use NiMHs), frequently causes one or two cells to reverse
> > polarity.
> >
> > A monstrosity I built (16 NiMH AAs in series) to provide power to my
> > laptop's 18.5VDC input, *always* fails by causing one or more cells
> > (sometimes four at a time!) to reverse polarity.
> >
> > So, would it be better to power devices with multiple AAs wired in
> > PARALLEL, with a step-up converter to raise the voltage, to prevent
> > cell reversal altogether?
> >
> > Michael
> >
>
> Actually, it would be better to pay attention to how far down you're
> drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
> result of discharging the cell too deep - there is no other cause. Avoid
> droppng a cell below the critical point (I've forgotten what that value
> is for an NiMH cell), and you'll *NEVER* reverse one.
>
> --
> Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my
whitelist,
> or the subject of the message doesn't contain the exact text
"PopperAndShadow"
> somewhere, any message sent to this address will go in the garbage without my
> ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd>; for more
info
Are there circuits which exist which periodically poll the voltage
across each cell, and can signal the load to stop drawing current?
Michael
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Author: Don BoweyDate: 18:00 16-01-07
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On 1/16/07 2:36 PM, in article
1168986995.554131.108740@a75g2000cwd.googlegroups.com, "mrdarrett@gmail.com"
<mrdarrett@gmail.com> wrote:
>
> Don Bruder wrote:
>> In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
>> mrdarrett@gmail.com wrote:
>>
>>> mrdarrett@gmail.com wrote:
>>>> If I have two NiMH batteries in series under a load, and one is weaker
>>>> than the other, is it possible for the stronger one to force the weaker
>>>> one to reverse polarity?
>>>
>>>
>>> Update to above:
>>>
>>> I noticed that my son's motorized bubble blower, which uses six AA
>>> batteries (I use NiMHs), frequently causes one or two cells to reverse
>>> polarity.
>>>
>>> A monstrosity I built (16 NiMH AAs in series) to provide power to my
>>> laptop's 18.5VDC input, *always* fails by causing one or more cells
>>> (sometimes four at a time!) to reverse polarity.
>>>
>>> So, would it be better to power devices with multiple AAs wired in
>>> PARALLEL, with a step-up converter to raise the voltage, to prevent
>>> cell reversal altogether?
>>>
>>> Michael
>>>
>>
>> Actually, it would be better to pay attention to how far down you're
>> drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
>> result of discharging the cell too deep - there is no other cause. Avoid
>> droppng a cell below the critical point (I've forgotten what that value
>> is for an NiMH cell), and you'll *NEVER* reverse one.
>>
>> --
>> Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my
>> whitelist,
>> or the subject of the message doesn't contain the exact text
>> "PopperAndShadow"
>> somewhere, any message sent to this address will go in the garbage without my
>> ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd>; for more
>> info
>
>
> Are there circuits which exist which periodically poll the voltage
> across each cell, and can signal the load to stop drawing current?
>
> Michael
>
Modern laptop batteries have circuitry built in to monitor and control
discharge and charge, but even they can fail.
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Author: Don BruderDate: 19:09 16-01-07
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Author: rebelDate: 19:14 16-01-07
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On 16 Jan 2007 11:41:35 -0800, mrdarrett@gmail.com wrote:
>
>mrdarrett@gmail.com wrote:
>> If I have two NiMH batteries in series under a load, and one is weaker
>> than the other, is it possible for the stronger one to force the weaker
>> one to reverse polarity?
>
>
>Update to above:
>
>I noticed that my son's motorized bubble blower, which uses six AA
>batteries (I use NiMHs), frequently causes one or two cells to reverse
>polarity.
>
>A monstrosity I built (16 NiMH AAs in series) to provide power to my
>laptop's 18.5VDC input, *always* fails by causing one or more cells
>(sometimes four at a time!) to reverse polarity.
>
>So, would it be better to power devices with multiple AAs wired in
>PARALLEL, with a step-up converter to raise the voltage, to prevent
>cell reversal altogether?
No. Consider the effect of switch losses with such a low input voltage. The
efficiency of switchmode boost converters suffers at low Vin.
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Author: Jim ThompsonDate: 19:29 16-01-07
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On Tue, 16 Jan 2007 16:09:10 -0800, Don Bruder <dakidd@sonic.net>
wrote:
>In article <1168986995.554131.108740@a75g2000cwd.googlegroups.com>,
> mrdarrett@gmail.com wrote:
>
>> Are there circuits which exist which periodically poll the voltage
>> across each cell, and can signal the load to stop drawing current?
>
>Yes.
But I don't know of any you can buy after-market.
They're usually designed into LiIon charge control systems that check
all cells and restrict both charging and discharging.
If you'll recall a post of mine some time back that showed how to
control current flow direction using back-to-back PMOS power-FETs...
that, plus a controller that sequentially samples each cell and
compares to a reference, and you have a charge/discharge controller.
I designed my first one maybe 15 years ago ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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Jim Thompson wrote:
> On Tue, 16 Jan 2007 16:09:10 -0800, Don Bruder <dakidd@sonic.net>
> wrote:
>
> >In article <1168986995.554131.108740@a75g2000cwd.googlegroups.com>,
> > mrdarrett@gmail.com wrote:
> >
> >> Are there circuits which exist which periodically poll the voltage
> >> across each cell, and can signal the load to stop drawing current?
> >
> >Yes.
>
> But I don't know of any you can buy after-market.
>
> They're usually designed into LiIon charge control systems that check
> all cells and restrict both charging and discharging.
>
> If you'll recall a post of mine some time back that showed how to
> control current flow direction using back-to-back PMOS power-FETs...
> that, plus a controller that sequentially samples each cell and
> compares to a reference, and you have a charge/discharge controller.
>
> I designed my first one maybe 15 years ago ;-)
>
> ...Jim Thompson
> --
> | James E.Thompson, P.E. | mens |
> | Analog Innovations, Inc. | et |
> | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
> | Phoenix, Arizona Voice:(480)460-2350 | |
> | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
> | http://www.analog-innovations.com | 1962 |
>
> I love to cook with wine. Sometimes I even put it in the food.
Yep, I did a quick google search, and found a few interesting related
patents on google.com/patents.
Not something that one could easily just pop into a bubble blower toy,
though... ;-(
Michael
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Author: Henry KieferDate: 21:25 16-01-07
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Author: JamieDate: 21:27 16-01-07
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mrdarrett@gmail.com wrote:
> Don Bruder wrote:
>
>>In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
>> mrdarrett@gmail.com wrote:
>>
>>
>>>mrdarrett@gmail.com wrote:
>>>
>>>>If I have two NiMH batteries in series under a load, and one is weaker
>>>>than the other, is it possible for the stronger one to force the weaker
>>>>one to reverse polarity?
>>>
>>>
>>>Update to above:
>>>
>>>I noticed that my son's motorized bubble blower, which uses six AA
>>>batteries (I use NiMHs), frequently causes one or two cells to reverse
>>>polarity.
>>>
>>>A monstrosity I built (16 NiMH AAs in series) to provide power to my
>>>laptop's 18.5VDC input, *always* fails by causing one or more cells
>>>(sometimes four at a time!) to reverse polarity.
>>>
>>>So, would it be better to power devices with multiple AAs wired in
>>>PARALLEL, with a step-up converter to raise the voltage, to prevent
>>>cell reversal altogether?
>>>
>>>Michael
>>>
>>
>>Actually, it would be better to pay attention to how far down you're
>>drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
>>result of discharging the cell too deep - there is no other cause. Avoid
>>droppng a cell below the critical point (I've forgotten what that value
>>is for an NiMH cell), and you'll *NEVER* reverse one.
>>
>>--
>>Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my
whitelist,
>>or the subject of the message doesn't contain the exact text
"PopperAndShadow"
>>somewhere, any message sent to this address will go in the garbage without my
>>ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd>; for more
info
>
>
>
> Are there circuits which exist which periodically poll the voltage
> across each cell, and can signal the load to stop drawing current?
>
> Michael
>
many cell phones and laptops tap into the multiple cells to monitor the
condition of the pack. if one cell is out of spec, it refuses to charge
because it could cause damage. At least, that is the way I understand it.
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
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Author: jasenDate: 02:49 17-01-07
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On 2007-01-16, mrdarrett@gmail.com <mrdarrett@gmail.com> wrote:
>
> mrdarrett@gmail.com wrote:
>> If I have two NiMH batteries in series under a load, and one is weaker
>> than the other, is it possible for the stronger one to force the weaker
>> one to reverse polarity?
>
>
> Update to above:
>
> I noticed that my son's motorized bubble blower, which uses six AA
> batteries (I use NiMHs), frequently causes one or two cells to reverse
> polarity.
>
> A monstrosity I built (16 NiMH AAs in series) to provide power to my
> laptop's 18.5VDC input, *always* fails by causing one or more cells
> (sometimes four at a time!) to reverse polarity.
>
here's a plan for something that detects the voltage of the weakest
cell.
| | | |
gnd -+---||----+---||----+---||----+---||----+--
| | | | | | | | |
| | | | |
+->|-+->|-+->|-+->|-+->|-+->|-+->|-+->|-+
| | | |
___ === === === ===
| | | | | |
|OSC|---[R]--+---+---------+---------+---------+
|___| | .--------.
| `-|envelope|
gnd |detector|
`--------'
when the envelope drops to two diode drops pk-pk one of the cells is
depleted to 0V. so if you stop at around 3 diode drops... you'll
have got about 99% of the available energy.
Bye.
Jasen
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Author: DidiDate: 10:57 17-01-07
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> A monstrosity I built (16 NiMH AAs in series) to provide power to my
> laptop's 18.5VDC input, *always* fails by causing one or more cells
> (sometimes four at a time!) to reverse polarity.
It should not be that bad, this is abnormal. If you have all 16 cells
of the same model and age, the reason for such behaviour is probably
heat - some get much hotter than others, NiMH are very heat dependent.
Mounting them all on a metal plate should improve things; and the
overall mechanical design must ensure there are no hot-spots of
the powered or whatever circuitry significantly thermally coupled
to some of the batteries.
> So, would it be better to power devices with multiple AAs wired in
> PARALLEL, with a step-up converter to raise the voltage, to prevent
> cell reversal altogether?
No, you would have the same problem. Some cells would drop faster
than others if they are heated up externally, then they will begin to
draw current from the others and get even hotter etc. More or less
same picture, only adding the switching losses and complexity...
Dimiter
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
------------------------------------------------------
mrdarrett@gmail.com wrote:
> mrdarrett@gmail.com wrote:
> > If I have two NiMH batteries in series under a load, and one is weaker
> > than the other, is it possible for the stronger one to force the weaker
> > one to reverse polarity?
>
>
> Update to above:
>
> I noticed that my son's motorized bubble blower, which uses six AA
> batteries (I use NiMHs), frequently causes one or two cells to reverse
> polarity.
>
> A monstrosity I built (16 NiMH AAs in series) to provide power to my
> laptop's 18.5VDC input, *always* fails by causing one or more cells
> (sometimes four at a time!) to reverse polarity.
>
> So, would it be better to power devices with multiple AAs wired in
> PARALLEL, with a step-up converter to raise the voltage, to prevent
> cell reversal altogether?
>
> Michael
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Author: ehsjrDate: 11:52 18-01-07
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James Arthur wrote:
> Don Bruder wrote:
>
>>In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
>> mrdarrett@gmail.com wrote:
>>
>>
>>>mrdarrett@gmail.com wrote:
>>>
>>>>If I have two NiMH batteries in series under a load, and one is weaker
>>>>than the other, is it possible for the stronger one to force the weaker
>>>>one to reverse polarity?
>>>
>>>
>>>Update to above:
>>>
>>>I noticed that my son's motorized bubble blower, which uses six AA
>>>batteries (I use NiMHs), frequently causes one or two cells to reverse
>>>polarity.
>>>
>>>A monstrosity I built (16 NiMH AAs in series) to provide power to my
>>>laptop's 18.5VDC input, *always* fails by causing one or more cells
>>>(sometimes four at a time!) to reverse polarity.
>>>
>>>So, would it be better to power devices with multiple AAs wired in
>>>PARALLEL, with a step-up converter to raise the voltage, to prevent
>>>cell reversal altogether?
>>>
>>>Michael
>>>
>>
>>Actually, it would be better to pay attention to how far down you're
>>drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
>>result of discharging the cell too deep - there is no other cause. Avoid
>>droppng a cell below the critical point (I've forgotten what that value
>>is for an NiMH cell), and you'll *NEVER* reverse one.
>
>
> According to DuraCell's OEM data, it's okay to go as low as 0v across
> an NiMH cell. Any reverse voltage risks permanently damaging the cell.
>
> Best,
> James Arthur
>
Cite please. It is not what I found on the
Duracell site - quite the opposite.
Here's what I found:
http://www.duracell.com/oem/Pdf/others/nimh_5.pdf
If you can't get there directly, go to
http://www.duracell.com/oem/rechargeable/Nickel/polarity.asp
and click on Download PDF
"5.3 Capacity: Effect of Discharge Rate and Temperature"
"... The delivered capacity can be increased by continuing
the discharge to lower end voltages.
*However, the battery should not be discharged to too low*
*a cut-off voltage (less than 0.9 volts per cell) as the*
*cells may be damaged (see Section 5.6).*
*The recommended cutoff voltage for nickel-metal hydride*
*batteries is 1.0 volt per cell.* ..."
(emphasis mine)
Section 5.6 states that cell polarity reversal can occur
when series connected cells are overdischarged and states:
"Device designers can help prevent overdischarge by
designing a cutoff voltage for device operation of
1.0 volt per cell."
It *may* be ok to allow a single cell to drop to 0
volts if there is only 1 cell in the circuit, but
that is *never* ok if the cell is in series with
another cell or cells.
Ed
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Didi wrote:
> > A monstrosity I built (16 NiMH AAs in series) to provide power to my
> > laptop's 18.5VDC input, *always* fails by causing one or more cells
> > (sometimes four at a time!) to reverse polarity.
>
> It should not be that bad, this is abnormal. If you have all 16 cells
> of the same model and age, the reason for such behaviour is probably
> heat - some get much hotter than others, NiMH are very heat dependent.
> Mounting them all on a metal plate should improve things; and the
> overall mechanical design must ensure there are no hot-spots of
> the powered or whatever circuitry significantly thermally coupled
> to some of the batteries.
>
> > So, would it be better to power devices with multiple AAs wired in
> > PARALLEL, with a step-up converter to raise the voltage, to prevent
> > cell reversal altogether?
>
> No, you would have the same problem. Some cells would drop faster
> than others if they are heated up externally, then they will begin to
> draw current from the others and get even hotter etc. More or less
> same picture, only adding the switching losses and complexity...
>
> Dimiter
>
> ------------------------------------------------------
> Dimiter Popoff Transgalactic Instruments
>
> http://www.tgi-sci.com
> ------------------------------------------------------
Ok, thanks to everyone who replied.
Over the years I've collected some 30 or so NiMH AAs. They're all
shuffled; no easy way to tell which I bought when. I did mark the ones
which die rapidly though - they're the ones that reversed polarity.
Not much need to use the 16 battery pack since I finally "refilled" my
laptop battery. www.batteryrefill.com, about 1/3 the cost of buying a
new oem battery. (works for 4 hours continuously, too.)
Michael
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Author: James ArthurDate: 15:22 18-01-07
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ehsjr wrote:
> James Arthur wrote:
> > Don Bruder wrote:
> >
> >>In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
> >> mrdarrett@gmail.com wrote:
> >>
> >>
> >>>mrdarrett@gmail.com wrote:
> >>>
> >>>>If I have two NiMH batteries in series under a load, and one is
weaker
> >>>>than the other, is it possible for the stronger one to force the
weaker
> >>>>one to reverse polarity?
> >>>
> >>>
> >>>Update to above:
> >>>
> >>>I noticed that my son's motorized bubble blower, which uses six AA
> >>>batteries (I use NiMHs), frequently causes one or two cells to reverse
> >>>polarity.
> >>>
> >>>A monstrosity I built (16 NiMH AAs in series) to provide power to my
> >>>laptop's 18.5VDC input, *always* fails by causing one or more cells
> >>>(sometimes four at a time!) to reverse polarity.
> >>>
> >>>So, would it be better to power devices with multiple AAs wired in
> >>>PARALLEL, with a step-up converter to raise the voltage, to prevent
> >>>cell reversal altogether?
> >>>
> >>>Michael
> >>>
> >>
> >>Actually, it would be better to pay attention to how far down you're
> >>drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
> >>result of discharging the cell too deep - there is no other cause. Avoid
> >>droppng a cell below the critical point (I've forgotten what that value
> >>is for an NiMH cell), and you'll *NEVER* reverse one.
> >
> >
> > According to DuraCell's OEM data, it's okay to go as low as 0v across
> > an NiMH cell. Any reverse voltage risks permanently damaging the cell.
> >
> > Best,
> > James Arthur
> >
>
> Cite please. It is not what I found on the
> Duracell site - quite the opposite.
>
> Here's what I found:
> http://www.duracell.com/oem/Pdf/others/nimh_5.pdf
>
> If you can't get there directly, go to
> http://www.duracell.com/oem/rechargeable/Nickel/polarity.asp
> and click on Download PDF
>
>
> "5.3 Capacity: Effect of Discharge Rate and Temperature"
>
> "... The delivered capacity can be increased by continuing
> the discharge to lower end voltages.
> *However, the battery should not be discharged to too low*
> *a cut-off voltage (less than 0.9 volts per cell) as the*
> *cells may be damaged (see Section 5.6).*
> *The recommended cutoff voltage for nickel-metal hydride*
> *batteries is 1.0 volt per cell.* ..."
>
> (emphasis mine)
>
> Section 5.6 states that cell polarity reversal can occur
> when series connected cells are overdischarged and states:
> "Device designers can help prevent overdischarge by
> designing a cutoff voltage for device operation of
> 1.0 volt per cell."
>
> It *may* be ok to allow a single cell to drop to 0
> volts if there is only 1 cell in the circuit, but
> that is *never* ok if the cell is in series with
> another cell or cells.
>
> Ed
My mistake, it's in the Energizer NiMH applications manual
(http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf, pg. 14)
"The typical voltage profile for a cell carried through a total
discharge involves a dual plateau voltage profile as indicated in
Figure 14. The voltage plateaus are caused by the discharge of first
the positive electrode and then the residual capacity in the negative.
At the point both electrodes are reversed, substantial hydrogen gas
evolution occurs, which may result in cell venting as well as
irreversible structural damage to the electrodes."
"[...] The key to avoiding harm to the cell is to terminate the
discharge at the point where essentially all capacity has been obtained
from the cell, but
[Figure 14. Nickel-Metal Hydride Cell Polarity Reversal
Voltage Profile]
prior to reaching the second plateau where damage may occur."
Figure 14 shows first plateau is 0v, the second is roughly -1.7v.
Best regards,
James Arthur
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Author: ehsjrDate: 01:12 19-01-07
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James Arthur wrote:
> ehsjr wrote:
>
>>James Arthur wrote:
>>
>>>Don Bruder wrote:
>>>
>>>
>>>>In article <1168976495.665174.195960@51g2000cwl.googlegroups.com>,
>>>>mrdarrett@gmail.com wrote:
>>>>
>>>>
>>>>
>>>>>mrdarrett@gmail.com wrote:
>>>>>
>>>>>
>>>>>>If I have two NiMH batteries in series under a load, and one is
weaker
>>>>>>than the other, is it possible for the stronger one to force the
weaker
>>>>>>one to reverse polarity?
>>>>>
>>>>>
>>>>>Update to above:
>>>>>
>>>>>I noticed that my son's motorized bubble blower, which uses six AA
>>>>>batteries (I use NiMHs), frequently causes one or two cells to
reverse
>>>>>polarity.
>>>>>
>>>>>A monstrosity I built (16 NiMH AAs in series) to provide power to my
>>>>>laptop's 18.5VDC input, *always* fails by causing one or more cells
>>>>>(sometimes four at a time!) to reverse polarity.
>>>>>
>>>>>So, would it be better to power devices with multiple AAs wired in
>>>>>PARALLEL, with a step-up converter to raise the voltage, to prevent
>>>>>cell reversal altogether?
>>>>>
>>>>>Michael
>>>>>
>>>>
>>>>Actually, it would be better to pay attention to how far down you're
>>>>drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
>>>>result of discharging the cell too deep - there is no other cause. Avoid
>>>>droppng a cell below the critical point (I've forgotten what that value
>>>>is for an NiMH cell), and you'll *NEVER* reverse one.
>>>
>>>
>>> According to DuraCell's OEM data, it's okay to go as low as 0v across
>>>an NiMH cell. Any reverse voltage risks permanently damaging the cell.
>>>
>>> Best,
>>> James Arthur
>>>
>>
>>Cite please. It is not what I found on the
>>Duracell site - quite the opposite.
>>
>>Here's what I found:
>>http://www.duracell.com/oem/Pdf/others/nimh_5.pdf
>>
>>If you can't get there directly, go to
>>http://www.duracell.com/oem/rechargeable/Nickel/polarity.asp
>>and click on Download PDF
>>
>>
>>"5.3 Capacity: Effect of Discharge Rate and Temperature"
>>
>>"... The delivered capacity can be increased by continuing
>>the discharge to lower end voltages.
>>*However, the battery should not be discharged to too low*
>>*a cut-off voltage (less than 0.9 volts per cell) as the*
>>*cells may be damaged (see Section 5.6).*
>>*The recommended cutoff voltage for nickel-metal hydride*
>>*batteries is 1.0 volt per cell.* ..."
>>
>>(emphasis mine)
>>
>>Section 5.6 states that cell polarity reversal can occur
>>when series connected cells are overdischarged and states:
>>"Device designers can help prevent overdischarge by
>>designing a cutoff voltage for device operation of
>>1.0 volt per cell."
>>
>>It *may* be ok to allow a single cell to drop to 0
>>volts if there is only 1 cell in the circuit, but
>>that is *never* ok if the cell is in series with
>>another cell or cells.
>>
>>Ed
>
>
> My mistake, it's in the Energizer NiMH applications manual
> (http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf, pg. 14)
>
> "The typical voltage profile for a cell carried through a total
> discharge involves a dual plateau voltage profile as indicated in
> Figure 14. The voltage plateaus are caused by the discharge of first
> the positive electrode and then the residual capacity in the negative.
> At the point both electrodes are reversed, substantial hydrogen gas
> evolution occurs, which may result in cell venting as well as
> irreversible structural damage to the electrodes."
>
> "[...] The key to avoiding harm to the cell is to terminate the
> discharge at the point where essentially all capacity has been obtained
> from the cell, but
>
> [Figure 14. Nickel-Metal Hydride Cell Polarity Reversal
> Voltage Profile]
>
> prior to reaching the second plateau where damage may occur."
>
> Figure 14 shows first plateau is 0v, the second is roughly -1.7v.
>
> Best regards,
> James Arthur
>
Thanks for the cite. You and I see it differently.
Figure 14 shows that *overdischarge* occurs when the
cell goes below ~ .9 volts. It shows one electrode
already going into polarity reversal at < ~.9 volts.
The text recommends .9 volts as an excellent discharge
termination point for most <1C discharge applications.
If you care to wade through the discourse on the site or
below, the *lowest* discharge voltage deemed acceptable
by the site is .75 volts. For cells in series,
discharging to 0 volts per cell is *never* acceptable.
They also recommend a different end of discharge
voltage that is ~ 75% of the "mid point voltage".
The lowest mid point voltage (defined on pg 11)
shown is over 1.1 volts, which at 75% is .825
volts. When you string cells in series, the
formula for EODV (end of discharge voltage) given
on the site is EODV = ((MPV-.15)*(N-1)) - .2
where N is the number of cells. Using worst case
of MPV = 1.1 and 2 cells in series we get
EODV = ((1.1 - .15) * 1 ) - .2 or .75 volts.
Ed
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Author: ehsjrDate: 01:23 19-01-07
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mrdarrett@gmail.com wrote:
> Jim Thompson wrote:
>
>>On Tue, 16 Jan 2007 16:09:10 -0800, Don Bruder <dakidd@sonic.net>
>>wrote:
>>
>>
>>>In article <1168986995.554131.108740@a75g2000cwd.googlegroups.com>,
>>>mrdarrett@gmail.com wrote:
>>>
>>>
>>>>Are there circuits which exist which periodically poll the voltage
>>>>across each cell, and can signal the load to stop drawing current?
>>>
>>>Yes.
>>
>>But I don't know of any you can buy after-market.
>>
>>They're usually designed into LiIon charge control systems that check
>>all cells and restrict both charging and discharging.
>>
>>If you'll recall a post of mine some time back that showed how to
>>control current flow direction using back-to-back PMOS power-FETs...
>>that, plus a controller that sequentially samples each cell and
>>compares to a reference, and you have a charge/discharge controller.
>>
>>I designed my first one maybe 15 years ago ;-)
>>
>> ...Jim Thompson
>>--
>>| James E.Thompson, P.E. | mens |
>>| Analog Innovations, Inc. | et |
>>| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
>>| Phoenix, Arizona Voice:(480)460-2350 | |
>>| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
>>| http://www.analog-innovations.com | 1962 |
>>
>>I love to cook with wine. Sometimes I even put it in the food.
>
>
>
> Yep, I did a quick google search, and found a few interesting related
> patents on google.com/patents.
>
> Not something that one could easily just pop into a bubble blower toy,
> though... ;-(
>
> Michael
>
About the best you could do practically is to
add a circuit that warns you (or shuts the toy off)
when the total voltage from the cells drops to
~ 5.4 volts. Then you can re-charge prior to
cell reversal damage.
Ed
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Author: James ArthurDate: 04:41 19-01-07
|
|
ehsjr wrote:
> James Arthur wrote:
> > ehsjr wrote:
> >
> >>James Arthur wrote:
> >>
> >>>Don Bruder wrote:
> >>>
> >>>
> >>>>In article
<1168976495.665174.195960@51g2000cwl.googlegroups.com>,
> >>>>mrdarrett@gmail.com wrote:
> >>>>
> >>>>
> >>>>
> >>>>>mrdarrett@gmail.com wrote:
> >>>>>
> >>>>>
> >>>>>>If I have two NiMH batteries in series under a load, and
one is weaker
> >>>>>>than the other, is it possible for the stronger one to
force the weaker
> >>>>>>one to reverse polarity?
> >>>>>
> >>>>>
> >>>>>Update to above:
> >>>>>
> >>>>>I noticed that my son's motorized bubble blower, which uses six
AA
> >>>>>batteries (I use NiMHs), frequently causes one or two cells to
reverse
> >>>>>polarity.
> >>>>>
> >>>>>A monstrosity I built (16 NiMH AAs in series) to provide power
to my
> >>>>>laptop's 18.5VDC input, *always* fails by causing one or more
cells
> >>>>>(sometimes four at a time!) to reverse polarity.
> >>>>>
> >>>>>So, would it be better to power devices with multiple AAs wired
in
> >>>>>PARALLEL, with a step-up converter to raise the voltage, to
prevent
> >>>>>cell reversal altogether?
> >>>>>
> >>>>>Michael
> >>>>>
> >>>>
> >>>>Actually, it would be better to pay attention to how far down
you're
> >>>>drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a
> >>>>result of discharging the cell too deep - there is no other cause.
Avoid
> >>>>droppng a cell below the critical point (I've forgotten what that
value
> >>>>is for an NiMH cell), and you'll *NEVER* reverse one.
> >>>
> >>>
> >>> According to DuraCell's OEM data, it's okay to go as low as 0v across
> >>>an NiMH cell. Any reverse voltage risks permanently damaging the cell.
> >>>
> >>> Best,
> >>> James Arthur
> >>>
> >>
> >>Cite please. It is not what I found on the
> >>Duracell site - quite the opposite.
> >>
> >>Here's what I found:
> >>http://www.duracell.com/oem/Pdf/others/nimh_5.pdf
> >>
> >>If you can't get there directly, go to
> >>http://www.duracell.com/oem/rechargeable/Nickel/polarity.asp
> >>and click on Download PDF
> >>
> >>
> >>"5.3 Capacity: Effect of Discharge Rate and Temperature"
> >>
> >>"... The delivered capacity can be increased by continuing
> >>the discharge to lower end voltages.
> >>*However, the battery should not be discharged to too low*
> >>*a cut-off voltage (less than 0.9 volts per cell) as the*
> >>*cells may be damaged (see Section 5.6).*
> >>*The recommended cutoff voltage for nickel-metal hydride*
> >>*batteries is 1.0 volt per cell.* ..."
> >>
> >>(emphasis mine)
> >>
> >>Section 5.6 states that cell polarity reversal can occur
> >>when series connected cells are overdischarged and states:
> >>"Device designers can help prevent overdischarge by
> >>designing a cutoff voltage for device operation of
> >>1.0 volt per cell."
> >>
> >>It *may* be ok to allow a single cell to drop to 0
> >>volts if there is only 1 cell in the circuit, but
> >>that is *never* ok if the cell is in series with
> >>another cell or cells.
> >>
> >>Ed
> >
> >
> > My mistake, it's in the Energizer NiMH applications manual
> > (http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf, pg. 14)
> >
> > "The typical voltage profile for a cell carried through a total
> > discharge involves a dual plateau voltage profile as indicated in
> > Figure 14. The voltage plateaus are caused by the discharge of first
> > the positive electrode and then the residual capacity in the negative.
> > At the point both electrodes are reversed, substantial hydrogen gas
> > evolution occurs, which may result in cell venting as well as
> > irreversible structural damage to the electrodes."
> >
> > "[...] The key to avoiding harm to the cell is to terminate the
> > discharge at the point where essentially all capacity has been obtained
> > from the cell, but
> >
> > [Figure 14. Nickel-Metal Hydride Cell Polarity Reversal
> > Voltage Profile]
> >
> > prior to reaching the second plateau where damage may occur."
> >
> > Figure 14 shows first plateau is 0v, the second is roughly -1.7v.
> >
> > Best regards,
> > James Arthur
> >
>
> Thanks for the cite. You and I see it differently.
> Figure 14 shows that *overdischarge* occurs when the
> cell goes below ~ .9 volts. It shows one electrode
> already going into polarity reversal at < ~.9 volts.
> The text recommends .9 volts as an excellent discharge
> termination point for most <1C discharge applications.
>
> If you care to wade through the discourse on the site or
> below, the *lowest* discharge voltage deemed acceptable
> by the site is .75 volts. For cells in series,
> discharging to 0 volts per cell is *never* acceptable.
>
> They also recommend a different end of discharge
> voltage that is ~ 75% of the "mid point voltage".
> The lowest mid point voltage (defined on pg 11)
> shown is over 1.1 volts, which at 75% is .825
> volts. When you string cells in series, the
> formula for EODV (end of discharge voltage) given
> on the site is EODV = ((MPV-.15)*(N-1)) - .2
> where N is the number of cells. Using worst case
> of MPV = 1.1 and 2 cells in series we get
> EODV = ((1.1 - .15) * 1 ) - .2 or .75 volts.
>
> Ed
Howdy Ed,
I understand both Energizer and Duracell's descriptions to mean that
actual reversal of polarity[3,4] is the damaging event. Since an NiMH
cell's voltage falls very rapidly once exhausted, both vendors sensibly
recommend higher discharge cutoffs to avoid the danger.
Since this is also going to s.e.b., I'll elaborate...
Echoing your statement above, the big problem is that with 4 cells in
series, you can't tell from a 3v terminal potential whether you've got
a) 4 cells x 0.75v (= good), or
b) 3 cells x 1.0v, and 1 zero-volt cell about to be reversed. (=
bad)
So, algebra demands a higher cutoff voltage. A 4v cutoff, for
example, would ensure that no single cell could ever be driven into
reverse. (In practice, Energizer's formula for a 4-cell pack
recommends a minimum cutoff of 2.95v, figuring that matched cells'
voltages will match somewhat, but I'd feel better choosing 4v here.)
In an abundance of caution, Energizer's Figure 14 graph labels all
voltages under 1.0 volts as being "overdischarge," however the text
that follows recommends a 0.9v cutoff, with lower cutoffs when cells
are heavily loaded.
Storage at zero volts appears perfectly fine[1,2] (though Energizer
warns against storage under load, even microamps).
Overall, I certainly agree with you: although AFAICT 0v is not
injurious, I wouldn't design a product that allowed any cell of a
series NiMH pack to discharge to 0v -- that risks wrecking the pack,
and, for a 1C discharge rate there's little energy to be gotten from
going below even 1.1V.
I hope the above clarifies my thinking on the subject.
Best regards,
James Arthur
~~~~~~~~~~~~~
[1] Duracell's section 5.8: _Self-Discharge and Charge Retention_
"Generally, long term storage of a nickel-metal hydride battery in
either a charged or discharged condition has no permanent effect on
capacity."
[2] Energizer pg. 25: _Capacity Recovery After Storage_
"In normal practice, stored cells will provide full capacity on the
first discharge after removal from storage
and charging with standard methods."
[3] Energizer, pg. 26: _Cell Reversal_
"Discharge of nickel-metal hydride batteries to the degree that some or
all of the cells go into reverse can
shorten cell life, especially if this overdischarge is repeated
routinely."
[4] Duracell, sect. 5.6 _Polarity Reversal During Overdischarge_
"The more cells that are connected in series, the greater the
possibility of a cell being fully discharged and driven into
overdischarge and polarity reversal. During reversal, hydrogen gas
evolves from the positive electrode. Hydrogen gas will be reabsorbed by
the negative electrode and eventually oxygen gas will evolve from the
negative electrode. Extended overdischarge will lead to elevated cell
pressure and opening of the safety vent within the nickel-metal hydride
cells."
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