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Quick charger vs Slow: Which last longer NiMH batteries?

 
 
ASAAR
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      01-01-2009
On Thu, 01 Jan 2009 13:26:38 -0800, Non-expert John Navas wrote:

>> The experts sat that it doesn't matter if the charger is properly
>>designed. The life of a NiMH battery is defined by a number of charges.

> . . .
>
> What "experts"? According to battery manufacturers, life can be
> affected by charging, particularly in cases of overheating and
> overcharging.


Heating is a normal part of the charge cycle. Overheating, enough
to cause batteries to vent and lose capacity or worse, is not by any
means normal, and is not due to good or poor charger design, but due
to defective batteries or chargers.


>> NiMH batteries are fairly robust. On the other hand, Lithium batteries
>> are light, but they are not robust. The can be destroyed by completely
>> discharging. Some brands of lithium batteries are much more fragile.

>
> NiMH batteries will also be destroyed by complete discharge. Look up
> "polarity reversal".


Don't you know the difference between complete discharge and
reverse charge? One can produce polarity reversal. The other
won't. Try putting a NiMH AA cell in a single AA cell flashlight.
Turn it on. How long do you think it will take for the cell to
achieve polarity reversal? The only thing destroyed here, once
again, is your credibility. [as if this wasn't expected! ]
Despite having relatively low self-discharge rates, Li-Ion batteries
can fail completely if they go too long without being charged. They
don't need a full charge, just a few minutes on the charger every 6
months or so is good enough. I didn't have to look it up. This was
part of the notes included with several after-market Li-Ion battery
packs, and later verified several years ago in the ng by a couple of
users who had put their cameras away in drawers for too many
consecutive months of non-use (and non-charge).

 
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ASAAR
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      01-02-2009
On Fri, 02 Jan 2009 10:27:10 -0800, John Navas wrote:

> 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 prior to reaching the second plateau where damage may
> occur. ...
>
> There's more. Suggest you read it this time.


You need to follow this advice even more. The real, significant
damage occurs when additional cells that still have capacity left
force the depleted cell beyond zero volts into negative territory
where the two voltage plateaus exist. At that time reversal already
occurred, and it doesn't happen to single cells. The authoritative
source you quoted from specifically talks of the cell being carried
"through", not "to" total discharge. The "potential for irreversal
harm to the cell" was also a clue missed by you, specifically the
word "potential". The two voltage plateaus are seen in Energizer's
chart, the first appearing at about -0.3 volts and the second at
about -1.8 volts. Multiple cells driving current through a depleted
cell are required to do this, and Energizer specifically warns of
the damage that might occur due to cell venting at this lower, -1.8
volt plateau. Your quote ended with this :

> 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 prior to reaching the second plateau where damage may
> occur. ...
>
> There's more. Suggest you read it this time.



The "more" that you didn't quote, and which immediately followed
"where damage may occur. ..." was this :


> Two issues complicate the selection of the proper voltage for
> discharge termination: high-rate discharges and multiple-cell
> effects in batteries.


Get it? Multiple-cell effects. Single cell devices won't
seriously damage NiMH AA cells. Even those that use two cells are
pretty safe, since with one "dead" cell contributing no voltage to
the help the current flow, and the other cell contributing probably
less than 1.1 volts (and some of that appearing only across the
device), there's no way that the dead cell will be forced as low as
-1.8 volts, as that would indicate that the "dead" cell was able to
reverse the current flow through the "good" cell, in effect charging
it, and this doesn't even begin to touch on what would be happening
to the device, but it would be akin to installing good batteries in
it, but backwards in their slots.

With multi-cell devices, cell reversal can happen without the user
being aware that this is happening, especially if more than 4 cells
are used. Most or all digital cameras will power down long before
this can occur. With single cell devices, even the most clueless
would have long seen a need to remove and recharge the cell in the
non-functioning device. And if not, little or no harm would occur
anyway.

Good tools are only at their most useful when in the right hands.
Good battery application manuals also need to be in the right hands
to be useful. Your hands don't qualify. Misunderstanding and
shortening quotes to eliminate context and create the opposite of
Energizer's intent demonstrates your lack of qualification. But
what else is new? Good thing for you that you won't read this
unless it's quoted by someone not already added to your large and
growing kill filter!

 
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