That is a fault with your charger more than a fault of the batteries. I can
charge LSD NiMH batteries in a few hours in my Maha charger.

Sure, I can charge then in a few hours with a better charger too. But the
cheaper fixed current ones won't. The one that takes 24 hours is the
one that came with them. :-(

There could be a number of things going in here.

You mention the higher resistance of the low-self-discharge cells.
Frankly I'm sceptical that this, by itself, could be great enough or
have enough effect on the battery to reduce the charging efficiency by
as much as you have observed. The low-self-discharge cells seem to be
able to discharge at rates of C/2 or even C/1 without their terminal
voltage dropping by very much, which means that the internal losses
aren't very high.

It's possible that the cheap charger you got with the batteries,
simply isn't very good - it may not be delivering as much current into
the batteries as it was supposed to. Possibly its internal current
regulator is poorly designed... if it consists only of a series
resistor hooked to a poorly-regulated DC voltage, then modest
variations in the DC supply voltage (from its internal transformer) or
in the battery's terminal voltage during charging could make a big
difference in the amount of current that the charger actually delivers
to the battery. You might be charging at a C/20 rate rather than a
C/10 rate. If the charger design was originally created for
lower-capacity cells, and wasn't revised when the capacities were
increased, it wouldn't be surprising if it's slow.

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. There seems to be a significant
"overhead cost" to charging... a fairly high fraction of the first
50-100 mA or so that you push into an AA cell just turns into heat,
rather than recharging the electrochemistry. Your charger may be
using such a low current rate that it's not actually making much
headway against this issue.

My impression is that a lot of the cheap/inexpensive "overnight"
chargers were originally designed for NiCd cells, and have been
re-branded as dual-chemistry (NiCd/NiMH) chargers without significant
change. Since NiMH cells typically have around twice as much capacity
as NiCd cells of the same size, it's not surprising that the charging
takes a long time.

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