> > Interestingly, rechargeable cell allowable trickle currents as a % of
> > 1 hour capacity tend to rise with decreasing physical size until
> > allowable trickle is far larger than nominated fast charge rate.

> With nickel cells, as long as the heat can get out trickle charge
> current shouldnt be a problem correct? =A0So as you increase the ratio
> of surface area to =A0volume (decreasing overall size) it should handle
> a higher 'C' trickle charge.

The argument sounds OK enough BUT at the top end with eg NimH AA
something has changed. One factor in being able to maintain a trickle
charge is the ability to recombine electrolysis products formed at the
electrodes. This requires catalytic material. It may be that as more
and more capacity is shoehorned into a fix package size the ability to
provide enough material that is not related to energy storage becomes
too difficult.

AA cells of over 2000 or so mAh from reputable manufacturers recommend
NO trickle charge or  low AND extremely short duration trickle charge.
This does  not appear to be a thermal issue. Gas formation and water
loss will both kill cells and both are exacerbated if recombining
catalysts are inadequate. The consequent venting will also expel
corrosive electrolyte.

The cube square law certainly seems to work at the bottom end with the
massive allowed trickle charge currents relative say to battery 1 hour
current capability - but this doesn't explain the immunity to
electrolysis. Presumably at low energy densities their is ample
volumetric capacity for catalytic material.


            Russell McMahon

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