From: Mike Nickerson via EV
I think the problem happens when the constant current charger
doesn't stop when the cells are full. The cell can't store the
energy, but it has to go somewhere. They are sealed, so it can't
just bubble off water like lead-acid cells do. The energy breaks
down the electrolyte and builds pressure until the cell finally
blows.
Once full, I don't think it matters how large or small the current
is. It will cause damage and it is additive.
Jan Steinman via EV wrote:
I'm not saying you're wrong. But as an electrical engineer, no one
has explained it to me in a way I can understand yet.
No working cell should see more than a few hundred milliwatts in the
scenario cited. This is roughly the energy dissipated by your left
ear. (Well, maybe you left hand, unless you're exerting yourself.)
The energy dissipation alone is not enough to cause the damage
shown.
Heat merely accelerates the whole process. If the power level, or heat
dissipating ability of the cell keep the temperature down, these
destructive side effects still occur -- they just take longer.
Overcharging can destroy a cell in an hour at high temperatures. Or in a
few days at room temperature.
As Cor mentioned, are these cells actually sealed? With no safety
vent? I guess I can imagine an explosion in such a case, but still,
basic thermodynamics don't support that. Pressure from disassociation
should lead to re-combination elsewhere, due to the limits of the
power supplied. Cells that size should be able to dissipate low tens
of watts safely. Energy out must not exceed energy in.
They have a safety vent; but it if opens, the cell is ruined. Gas (which
means water) is lost. The vents do not reseal, so they continue to lose
water every time it is charged. The remaining electrolyte gets stronger
and stronger as water is lost, and starts corroding things inside.
Before long, the cell fails completely.
There are recombination reactions, but they don't work very well in nimh
cells. They *can't* work well, because the hydrogen is in fact one of
the active elements. Nimh cells are really nickel-hydrogen cells, with a
spongy metal plate to adsorb the hydrogen so the pressure inside doesn't
get too high.
If you keep charging a fully-charged cell, you keep disassociating water
to produce hydrogen and oxygen. There is no place for it to go, except
to increase the pressure inside.
Does that help?
--
Sometimes when you innovate, you make mistakes. It is best to admit
them quickly and get on with improving your other innovations.
(Steve Jobs)
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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