In article ,
William Sommerwerck wrote:


A very low (e.g. C/100 or so) charge rate seems to be safe,
I gather, but it's also rather ineffective for NiMH cells -- they
have a poor "charge acceptance" at low charge rates, and
most of the power just turns into heat.

Maha says roughly the same thing. The recommend charging at at least 0.3C,
though it's not clear whether the reason is to push the cell towards a
significant delta-V at full charge, or to charge more "efficiently".

I think it's the former. The higher charging current leads to a more
rapid flattening-out of the voltage-vs.-time curve, and also leads to
a more rapid rise in temperature at the full-charge point. Both of
these make it easier for a good charger's circuitry to detect the
full-charge point and shut down the current flow at the right time.

I'm
bothered by the temperature rise, and tend to charge at 0.2C, which (based
on many years using nicads), seems downright excessive!

I believe that Powerex actually *increased* their minimum-charge-rate
recommendation for the MH-C9000 - it was originally .2C and now they
recommend .3C or above. This change, plus some firmware changes in
the more recent versions, seems to have eliminated the problem of "the
charger never shuts off, and the batteries get really hot" problem
that early revs of this charger were prone to.

I've been charging my various low-discharge AA cells in a C9000 at .5C
or so (the charger's 1000 mAh default rate) and they don't seem to
much, if any, warmer at charge-shutdown time than if I stick them in
an old-style .1C slow-charger and let 'em fill up. The C9000 appears
to detect the full-charge state quite reliably... I have yet to run
into a single hot-battery problem.

At this point, I prefer to stick with one of two charge regimes - a
fairly fast charge (.5C) using voltage-and-temperature cutoff, or a
slow one (.1C or below) using a timed cutoff.

I might add that 2500mAh NiMH cells pretty much mark the end of the alkaline
cell, except for "intermittent", low-drain applications, especially where
frequently swapping the cells would be incovenient (eg, remote controls).
Such cells can provide at least the same run-time as a throw-away alkaline.
And, yes, I've checked and confirmed this.

I can well believe it.

The major niches I see for alkaline AAs are the ones you suggest (e.g.
remote controls) and long-storage-time emergency backup applications
(e.g. I keep a partial "brick" of AA alkalines with my amateur-radio
"go-kit", so I'd have a couple of days of on-the-air time prior to
having to drag out a recharger).

For my wife's digital cameras (and my own) I've settled on a
combination of low-discharge-rate NiMH for day-to-day use, and lithium
AA disposables for travelling (they're light, really long lasting, and
it saves the trouble of taking an overseas-voltage-qualified charger
along).

I wish I'd one 20 years ago.

Yah. I've been rather dissatisfied for years with commercial NiCd/NiMH
chargers, and toyed with the idea of designing my own with a boatload
of flexibility and control, good full-charge detection, reconditioning
circuitry, etc. - an ideal application for a small microcontroller.
Never got around to doing it... and now there's no need to go to the
effort!

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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