Another factor is something that I understand is true about
NiMH cells in general (not just the low-self-discharge type):
charge acceptance is quite poor at low charge rates.

I've heard this as well. Specifically, at "low" charge rates (which which
would actually be on the "high" side for nicads), NiMH cells are less likely
to enter the negative delta-V region that indicates end of charge.

I think there are two effects here - how much of the energy going into
the battery is being lost as heat, and how much of an effect the heat
change has on the voltage. NiMH cells differ from NiCd in both
respects.

Having grown up with nicads, I'm reluctant to charge faster than 0.1C, but
it appears that 0.3C and even 0.5C is acceptable with good-quality NiMH
cells.

That does seem to be the case. It appears that there are a couple of
ways to charge NiMH cells fairly safely:

[1] At a nice, slow rate - .1C or even a bit lower. At this low rate,
the cells don't heat up very much at all, and thus don't exhibit the
zero-delta-V or negative-delta-V full-charge signature. For that
reason, neither a voltage- or temperature-sensing approach can be
used reliably, and a timed charge shutoff is the only alternative.

[3] Use a relatively fast charge rate - no less than .3C, with .5C or
even 1C being common. The cells do warm up significantly during
the fast-charging, and then their temperature starts to rise
sharply (and the voltage reaches zero-delta-V and starts down into
negative-delta-V) at full charge. Today's fast chargers seem to
use either T or delta-T as the primary shutoff indicator, with
delta-V as a secondary, and a timed shutoff as the final failsafe.

Early versions of the Maha/Powerex MH-9000 had a reputation for
occasional shutoff failure - they would not detect full charge
reliably, and the batteries would become quite hot as they cooked
away. Powerex revised the charger - I think they tweaked the firmware
logic having to do with the shutoff detection - and I understand that
they *raised* the minimum recommended fast-charge rate from 0.3C to
0.5C. Presumably this results in a more rapid temperature rise at
full-charge, and makes it easier to detect.

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