new hybrid batteries and overcharging

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Have just bought the latest type of rechargeable *Hybrid* batteries from  
Maplin, made by Uniross, called Hybrio. (also Argos are now selling them  
made by Sanyo).

They are supposed to have the advantage over Nickel Metal Hydride in that  
they don't have the 'memory effect'.  Also the advantage of the Alkaline, in  
that they dont self discharge at quite a high rate when not being used.

The charger I have is an Energiser designed for Nickel Hydride batteries and  
which indicates when a battery is fully charged, by the charging light going  
off.  But when putting in the Hybrid batteries the light does not go off.

Is there an harm done to these hybrid batteries if they are overcharged?  

Re: new hybrid batteries and overcharging

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According to the manufacturer web pages I've read, overcharging *any*
NiMH cell significantly can shorten its life... they're somewhat less
tolerant to this than nickel-cadmium cells.  Also, it's somewhat more
difficult to detect the "full charge" state in a NiMH than it is in a
NiCd, especially at low charge rates.

Based on what I've read, there seem to be two charging schemes for
NiMH which the cells will tolerate fairly well:

-  Slow charge (0.1 C or so) with a timed cutoff after 12 to 16 hours.
   If you touch the cells during charging and find that they are
   significantly warm, then they're probably "full".  [They do warm up
   somewhat during the normal charging process, so judging whether
   they're warm enough to indicate full-charge is not always easy.]
-  Fast charge (0.5C to 1C, or in some cases even higher) with primary  
   cutoff based on temperature rise, secondary cutoff based on zero
   delta-V (i.e. the cell voltage stops rising when full-charge is
   reached) and a timed cutoff as a failsafe.
Intermediate rates (above .1C and below .5C) have some
disadvantages... this amount of current may not result in a rapid
temperature rise at full-charge (thus making full-charge harder to
detect reliably) but is high enough to affect the cell's lifetime if
you do end up overcharging the cell.

It sounds to me as if your Energizer charger has its full-charge
detection circuit tuned properly for this newer type of NiMH cell.  If
it's a slow (overnight) charger, you probably won't hurt the cells
significantly using it as long as you shut it down manually at the
proper time.  If it's a "quick" or "fast" charger, it may very well be
overheating the cells enough to reduce their lifetime.

If you plan to use a lot of NiMH cells, or to recharge them
frequently, it might very well be a good investment to buy a
high-quality charger specifically designed for reliable fast-charging
of such cells.  I like the Powerex MH-C9000 myself, as it's fast and
reliable and has a lot of useful features.

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Re: new hybrid batteries and overcharging
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How do you define "overcharging"? Switching to a trickle charge at the end
of the charge cycle is, technically, overcharging, but no one considers it

NiMH cells can tolerate huge charging currents. MAHA specifically states
that do not recommend charging at _less_ than 1/3 C, and permit charge rates
as high as 1.0 C!

Whether this applies only to their cells, or pretty much everyones, I don't
know. But NiMH cells don't appear to be particularly "delicate".

"Overcharging" probably means, as others have suggested, continuing the
charge past "negative delta V" and continuing to charge at a high rate to
the point where the cell badly overheats.

But, as I said in my "moody" missive, this is something you should ask the
manufacturer, as only it knows how its only cells respond to various
charging protocols.

The people in this group do not.

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Yes and no. NiMH chargers can use either a rise in temperature (which might
be hard to judge when the sensor is not part of the battery pack) or a drop
in voltage to signal "full charge". The latter is supposedly larger and more
distinct at higher charge rates.

I don't believe either of these apply to nicad charging.

Re: new hybrid batteries and overcharging

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Pretty much the way you do later, and the way that the manufacturers
seem to.  "Overcharging" is when one continues to force charging
current into the cell, once the cell's electrochemistry has reached
the point of saturation and no further useful electrochemical
conversion can be performed.

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Actually, some of the manufacturer data sheets I've read seem to
recommend against it.

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True.  That's one (not the only) definite advantage of NiMH cells -
they can be recharged very quickly.

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They're not particularly delicate in terms of their rate of charge
absorbtion _during_ proper charging.  As you say, they can eat a lot
of current.

They are, however, more easily damaged than NiCd cells by the
overheating which occurs if you continue to pump energy into them
after their electrochemistry has saturated.

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I read "overcharging" as any continued charging past the point of
"full".  High-rate and low-rate overcharging does affect NiMH cells
differently, as the latter doesn't heat up the cells very much.

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True.  Unfortunately, without further information about how the
specific charger operates and behaves, even the cell's manufacturer
probably won't be able to give a useful answer.

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Actually, both of them do, although NiMH and NiCd cells differ
somewhat in both of these respects.

During the normal charging cycle (when they're still accepting
charge), NiCd batteries do not heat up very much at all... the
electrochemical process in these batteries is said to be endothermic
during charge acceptance.  The cell's terminal voltage rises slowly
during this phase of charging.  Once the plates are fully charged up,
the electrochemical reaction changes, and a secondary reaction
develops which releases the energy as heat... and so the NiCd cell
heats up significantly.  As a result of the change and the heating,  
the cell's terminal voltage stops rising, and actually drops
significantly.  This reversal of the voltage curve with time isn't
hard to detect, and most NiCd fast-chargers seem to use a "negative
delta-V" detection circuit to determine that the cell has reached full
charge and to shut off the current (or drop it to a trickle).

NiMH cells behave a bit differently.  They do warm up somewhat during
the main phase of charging - the electrochemical reaction is
exothermic.  Like a NiCd, their terminal voltage rises slowly during
the charge cycle.  Also like a NiCd, when they reach full charge they
start dissipating most of the incoming charge energy as heat, and (in
a fast-charge scenario) they can get quite warm quite quickly.
However, the effect of this on their terminal voltage is a bit
different... it stops rising, but it doesn't begin to fall
significantly until you've gone pretty far past the full-charge point
and gotten them pretty hot... and the manufacturer data sheets I've
read say that this degree of overcharging will shorten their life

So, the manufacturer data sheets I've read recommend using the
temperature rise (absolute and/or delta-temperature-over-time)
directly, using a thermistor, as the primary means of detecting full
charge in a NiMH.  Zero-delta-V-over-time makes a good secondary
shutoff mechanism, and a timed shutoff for safety is also recommended.

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Re: new hybrid batteries and overcharging
Dave Platt wrote:

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I have two chargers designed to charge AA NiMH in three hours or less.  
I wish they sensed temperature, but it seems they work strictly by  
voltage changes.

The first one was designed for NiCds as well.  I don't recall any  
trouble with NiCds, and what you've written may explain it.  With NiMH,  
each charger has occasionally stayed on longer than expected, and I  
removed the cells because they felt hot.  I haven't seen any signs of  
damage from these incidents.

Before I bought my first NiMH cells, I looked at data published by an  
amateur photographer using several brands of cells and more than one  
charger.  Sometimes when he took pictures he would find that a set of  
cells hadn't taken a normal charge.  I think that's a drawback in  
charging more than one cell in series in a circuit designed to shut off  
when a cell is charged.  Even when each cell is charged in its own  
circuit, I think gas bubbles formed in a cell during charging may cause  
a voltage fluctuation that may shut off a charger.  I wonder if that  
happens more often with new cells.

Re: new hybrid batteries and overcharging
I only use Ray O Vac alkaline batteries.

Re: new hybrid batteries and overcharging

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Some of us are more financially conservative and environmentally savvy.
How do you dispose of your alkaline batteries? It's against the law to
just throw them into the trash here in California.

Re: new hybrid batteries and overcharging
I have a lot of Eveready rechargable batteries and the proper Eveready
battery gizmo recharger here.I am too lazy to fool around with that
stuff all the time, unlless a long power outage occurs.I can use one of
my inverters and charge those batteries up from one of my van batteries
if I need to.

Re: new hybrid batteries and overcharging
Thank gawd I don't live on the left coast or new england either! Those
people are Crazy!

Re: new hybrid batteries and overcharging

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Which goes counter to what the trash is for, too...

Then again, it's California, where *everything* has been shown to
cause cancer and must have a warning.

Re: new hybrid batteries and overcharging

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Nah, we just don't like to pollute our ground water.

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