Message 5545.26
From: Pat McCarthy (EVERSEEKER)
Nicad "memory" is an urban legend.
NOT exactly true.....
To: Jonathan Ridder (JRIDDER)
Mar-4 5:13 pm
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 an Engineering Handbook published by Sanyo, makers of `Cadnica' NiCads:
Nickel-cadmium batteries have a `memory effect' in which the voltage drops by 2 levels during discharge after shallow charge/discharge cycles. In applications, when discharge end voltage is highly established, apparent decreases in capacity and operating voltage are shown. In the following, `C' is the amp-hour capacity of the battery. For example, if you have a 5 A-hr battery, a charging rate of `0.1C' is 0.5A. A graph shows a series of discharge curves (voltage versus time) for a battery under the following conditions: (1) Fresh battery, (2) First discharge after 100 cycles, (3) Second discharge after 100 cycles, (4) Third discharge after 100 cycles. The `cycles' consist of a 0.1 C charge for 10 hours followed by a 1C discharge for 10 minutes. The `discharge curve' is measured after a final 0.1C charge for 16 hours followed by a discharge rate of 1C. The four curves are almost identical. As near as I can read the graph, The first discharge after 100 memory cycles drops to 1V after about 63 minutes, compared with about 66 minutes for the fresh battery. The number of cycles Cadnica batteries can withstand depends on the depth of discharge. When the cell is discharged to a greater depth, the number of cycles decreases. Some folks have recommended that occasionally you intentionally completely discharge your NiCads to eliminate the memory effect. The above implies that that's a bad idea. The bottom line - yes there is a memory effect, but it is very small and not worth worrying about. (also from them:) As long as a Cadnica battery is charged at an input rate below the specified value [typically 0.1C] internal gas pressure remains at a low level. However, heat generated by gas recombination causes a rise in cell temperature. When overcharging is repeated often, heat deteriorates the cell and shortens its service life. So don't overcharge excessively. It's OK to leave your battery on the charger overnight (up to 18 hours or so if the battery was fully-discharged) but don't hook it up to the charger and go away for a week.
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