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Re: Batteries: autopsy and restoration

Subject: Re: Batteries: autopsy and restoration
From: Walter Knapp <>
Date: Fri, 14 Jun 2002 16:16:25 -0400
Klas Strandberg wrote:
> 
> Just remember: It is absolutelly superior anything else to use singe cells
> and slow-charge them individually. Next best is slow charge when the
> batteries are in series.

In the modern NIMH batteries this difference between fast and slow
charging is very small. They are, in fact, usually designed for fast
charge with slow charge as a option.

Note that what constitutes fast charge varies with the manufacturer.
Truly fastest charge is at a rate equal to 2x the mah capacity of the
battery in the case of NIMH or 3x in the case of Nicads. On that basis,
the 2200mAh NIMH C cells currently available would be fast charged at
4.4 amps in a half hour or slightly more! Most fast chargers don't do
near that much current. And many manufacturers don't recommend such a
fast rate. Panasonic, for instance on it's cells usually recommends a
fast charge of between 0.5x & 1x for it's various cells, still probably
2 hours or less to charge. They make a considerable variety of solder
tabbed etc. cells.

It does still pay to keep the batteries in sets, which will have the
same history. Of course reloading a battery pac with fresh cells will
constitute a set. Early failure in a set is much less likely than
bunches of loose batteries which are used in groups and charged in
groups but shuffled in between.

Individual cell chargers are getting to be much less common. Same with
slow charge. And individual cell chargers that can charge large numbers
of cells at once individually are even less common. Connecting a bunch
of cells in parallel across a individual cell charger will almost
certainly cause some cells to fail. So, if going the route of individual
cell charging you will probably have to have many more chargers. For
instance the Portadisc uses 8 AA cells. I use two modern computerized 4
cell chargers to fast charge it's batteries. Or if I'm being lazy and
have plenty of time I just plug it in to it's charger, which is kind of
a intermediate charger.

> Remember: The voltage from the charger is of no importance, as long as it
> gives correct current, 180 mA or 220 mA or whatever. NiCads etc need is
> current regulation, (mA) not voltage regulation. A lead-acid accu needs
> voltage regulation! That is something else.
> 
> A charger for 7,2 volts Nicads or NiMhy batteries may give 30 volts when you
> measure it, not connected to the batteries, but fall to some few volts more
> than the accus, when you connect it.

It's rating, what you see on the label, will be 7.2 volts regardless.
And you cannot get the necessary charging current unless the charger is
capable of putting out a voltage under load of somewhat over the final
voltage, or just at it. The voltage determines the current, so always matters.

Many early chargers and most slow chargers depended on having a voltage
output at the final fully charged voltage so that current would drop to
zero or near so when charging was complete. 

Modern chargers actively monitor the battery charging voltage and
current and regulate a preset curve of charging current. They do not
charge at a fixed current. Neither did the old ones for that matter. In
fact some of the newest chargers actually use a pulsed current to keep
gas bubble buildup down during charge. In this case they cycle rapidly
between charging and discharging all the time, with the charging part
being greater, of course.

One of the problems of going for a rote timed charge or rote current is
that cells as they age change capacity. Yes, when new it's rating and
actual capacity are probably close, but not for older cells. I have some
old Nicads that are down to less than half their original capacity even
after regeneration. I can still use them for some things and can keep
them in good shape with the computerized chargers I use now. In fact
some of these cells have run at reduced capacity for a number of years
even though they appeared to be completely dead under the old style,
slow charging routine. I keep any cell that the computerized charger can
regenerate and when I first started with the newer chargers I had quite
a pile of supposedly dead cells that suddenly were usable again. They
have all only been fast charged since with the new equipment.

Walt



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