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G8MNY > TECH 09.09.19 20:07l 66 Lines 3182 Bytes #999 (0) @ WW
BID : 20880_GB7CIP
Read: GUEST
Subj: Recovering Old NICAD/NIMH Cells
Path: IW8PGT<HB9ON<IW2OHX<IQ2LB<F1OYP<AB0AF<GB7YEW<GB7CIP
Sent: 190909/0905Z @:GB7CIP.#32.GBR.EURO #:20880 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
(Updated Feb 04)
Old Nicads batteries & sometimes NMHI batteries usually end up with several
cells being short circuit, as they suffer from crystal growth in the
electrolyte. This is often called the memory effect as a battery with 1 cell
shorted seems to have very short life as the battery voltage is unusable soon
after charging. The chemistry of NMHI batteries are supposed to be more immune
to this problem but often they are not!
To reclaim the cell before charging, (as normal charging will not remove
the short) a very high current pulse must be used.
With a sealed battery pack this can blow any internal safety fuse! So it is
best done when U have opened the pack. The good cells will withstand the high
current OK, but having them in series can reduce the current pulse depending on
how you are applying it, if you have a really bad cell.
Current up to 10 x C (e.g. 5A on a 500mAH AA cell, 40A on a 4AH D cell) can be
safely applied until the pack warms up, (this is how fast chargers detect 70%
charge & drop back to trickle).
For more difficult cells, current over 10 x C may be required, this can destroy
the internal cell wires, but you have nothing to loose! Charge up a large
capacitor e.g. 10,000uF to say 40V & connect to cell (computer must not be
nearby! & mind the sparks) with thick wires, the few 100A of pulse current
usually clears the short after a few pulses. This method is less likely to fuse
the internal connections than putting the cell across a car battery!
Once all the cells are over 1V & stay there for a while, trickle charge (Cx0.1)
them until warm or 14 hours. A few cycles of discharge & charge will normally
bring back old cells to 100%.
If the capacity is less than 100% after a few cycles or there is excessive volt
age (>2V) across a cell when charging, it is O/C or dry & will need to be
replaced.
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Tony, G8TBF@GB7NND says cells going short due to crystallisation is not due to
the electrolyte but the cadmium electrode; cadmium has a property of 'cold
crystallisation' (like Tin & Zinc) & can form 'needle' type spikes that bridge
the cell.
The way to prevent this is to occasionally give the cells a long charge at a
low current (<= 10 hour rate). This dissolves any 'needles' & replates the
surfaces.
The other effect of the cadmium crystallisation is the 'memory effect' - this
happens when cells are not fully discharged for long periods, & is caused by
the 'deeper' parts of the cadmium plate crystallising.
The effect is that once the amorphous metal on the surface of the plate has
been dissolved during discharge & the crystalline part is exposed, the cadmium
is much less reactive - a crystalline structure simply does not break down as
easily as amorphous metal.
The cure for 'memory effect' is to deep-discharge the cell, by connecting
(e.g.) a 10 ohm resistor across it & leave it until absolutely dead.
Normal recharging will then give full capacity & no 'memory'.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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