# of batteries to cause polarity reversal

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?

Reply to
mrdarrett
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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

Reply to
mrdarrett

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.

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Reply to
Don Bruder

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

Reply to
James Arthur

Are there circuits which exist which periodically poll the voltage across each cell, and can signal the load to stop drawing current?

Michael

Reply to
mrdarrett

Modern laptop batteries have circuitry built in to monitor and control discharge and charge, but even they can fail.

Reply to
Don Bowey

No. Consider the effect of switch losses with such a low input voltage. The efficiency of switchmode boost converters suffers at low Vin.

Reply to
rebel

Yes.

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Reply to
Don Bruder

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

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Reply to
Jim Thompson

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

Reply to
mrdarrett

"Jim Thompson" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com... | On Tue, 16 Jan 2007 16:09:10 -0800, Don Bruder | wrote: | | >In article , | > snipped-for-privacy@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.

Once I designed one for lead-battery electrical vehicle. Patented and even works.

- Henry

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Reply to
Henry Kiefer

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.

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Reply to
Jamie

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

Reply to
jasen

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.

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

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Reply to
Didi

Cite please. It is not what I found on the Duracell site - quite the opposite.

Here's what I found:

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If you can't get there directly, go to

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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

Reply to
ehsjr

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.

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about 1/3 the cost of buying a new oem battery. (works for 4 hours continuously, too.)

Michael

Reply to
mrdarrett

My mistake, it's in the Energizer NiMH applications manual

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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

Reply to
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

Reply to
ehsjr

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

Reply to
ehsjr

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."
Reply to
James Arthur

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