>From Nickel-Cadmium Battery Application Handbook, Gates Energy
Products, 3rd edition (1986), section 4.5.3:
The effects of elevated charge temperature on the immediate cycle
capacity of the cell have been discussed in Section 3.2.1 and 4.3.3.1.
Cells exposed to overcharge for very extented periods of time,
particularly at elevated cell temperatures, may develop an additional
shortcoming called voltage depression. This phenomenon is one in which
the cell voltage is depressed approximately 150 mV below the normally
expected values which were calculated on Figure 4.19. This depression
affects Eo and is independent of discharge rate.
This depression effect initially appears on the discharge voltage curve
near the end of discharge. With extension of the overcharge time (non-
discharge) of the cell, this depression progresses slowly toward the
mid-point and beyond. Accompanying this effect of depression in the
voltage dimension of the curve is an actual slight increase in the
capacity dimension as illustrated in Figure 4.21. This depressed
voltage effect is an electrically reversible condition and disappears
when the cell is completely discharged and charged (sometimes called
conditioning). It thus appears only on the discharge following a very
extended overcharge. It will reappear if the extended overcharge is
repeated.
The phenomenon which causes this depressed voltage is continuous
overcharging of the active material of the electrode. The effect is
erased by discharging and recharging that portion of the active
material which has experienced the extensive overcharge. For this
reason the depressed voltage effect in the discharged portion of the
curve is erased by the very act of observing it, when the discharge is
carried beyond the first knee of the depressed curve. Complete
discharge, and subsequent full charge, essentially restores the curve
to its normal form.
The reversibility of this effect is probably the very characteristic
that gives rise to the misnomer memory. When cells are subjected to
continuous charge/overcharge, with only modest discharges (repetitive
or otherwise), the revesibility of the effect actually prevents the
voltage depression from occuring in that portion of the electrode
active material which is cycled. The voltage depression phenomenon is,
however, not erased from that portion of the electrode material which
has been subjected to continuous overcharge but NOT discharged. In this
situation, whenever the cell is discharged deeper than recent previous
discharges and reaches the beginning of the previously uncycle
material, the voltage may decrease 150 mV per cell. This misleads the
observer into believing that the discharge is at the knee of the normal
discharge curve and erroneously concluding that the cell remembers and,
thus, delivers only the amount of capacity previously repetitively
used. Instead, the phenomenon is actually related only to extended
overcharging and incomplete discharging, not repetitive shallow
cycling. This is because that portion of the electrode material which
has experienced overcharge and not been discharged for an extended
period of time slowly shifts to a more inaccessible form.
The depressed voltage effect can of course cause loss of useful
capacity in those application cases where a high cutoff voltage
prevents complete discharge of the minimum capacity cell in the
battery. If voltage depression has occurred, complete discharge
requires continuation down through the depressed knee to that voltage
level which keeps all the electrode material active ...
------------------
From: Tom Trottier, President, ACT Productions Inc.
infoanim@xxxxxx http://www.act.ca
+1 613 594-4829 fax +1 613 594-8944
199 Holmwood Ave, Ottawa, Ontario, Canada K1S 2P3
"Make it as simple as possible, but no simpler" - Einstein
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|