paulb@zds-ux.UUCP (Paul E. Berg) (09/27/90)
In article <1236@limbo.Intuitive.Com>, taylor@limbo.Intuitive.Com (Dave Taylor) writes: > Michael Portuesi writes: > > > This is nothing you weren't warned about in the manual. Have you > > considered leaving the unit plugged in when you are not using it? > > Yeah, but that'll "teach the batteries bad tricks" -- my understanding > of how rechargable batteries work is that you're not supposed to leave > them plugged in for great lengths of time, as they will hold shorter > and shorter charges. Questions about charging NiCad battery packs and comments about reduced battery operation time after extended charge periods seem to appear at regular intervals in this group. This information is extracted from G.E. and Sanyo Nickel-Cadmium battery application handbook sections describing ``Discharge Conditions'' and ``Voltage Depression''. ... Cells exposed to overcharge for very extended periods of time, particularly at elevated cell temperatures, may develop a shortcoming called ``voltage depression''. This depression effect initially appears on the discharge voltage curve near the end of discharge. With extension of the overcharge time of the cell, this depression progresses slowly toward the mid-point and beyond. G.E. gives a value of 150mV for this depression. |x discharge | xxxxxxxxxxxxx*xxxxxxxxxxxxxx x="normal" discharge voltage | y x curve | yyyyyyyyyyyyyy* | xy y="depressed" curve | xy +-------- discharged capacity ---------- Note the actual slight increase in the capacity dimension that occurs with the depressed voltage effect. This depressed voltage effect is an electrically reversible condition and disappears with one or two complete discharge/charge cycles. It will reappear if the extended overcharge is repeated. The continuous charge/overcharge moves the first knee of the depressed curve toward time zero. Shallow discharge does not erase the voltage depression effect which occurs with overcharge -- this is the ``memory effect''. You have a problem with voltage depression if the depressed voltage is below minimum for your application. Some devices detect the end of useful battery life by detecting the voltage drop over time indicating the knee has passed rather than by looking at actual voltage levels -- this sensing technique can be fooled by voltage depression. If your laptop reports low battery and then reports battery good, you may have just passed the knee. Note that you must continue to discharge the battery in order to erase the depressed voltage effect even if your laptop refuses to run at this lower voltage -- that could be a problem... If a reduced capacity is observed, and it is not cleared by complete discharge cycles, you have probably reached the end of your battery's service life. A number of factors affect battery life -- you have control over very few of them. Wear-out failure is typically either: - due to an internal short caused by defective active materials and deterioration of organic materials (such as a separator) - due to the drying out of the electrolyte. Nickel-Cadmium batteries are sealed, but leakage will occur from the safety vent if: + the battery is charged at too high a rate + the battery is over-discharged until the reversal of polarity + the battery is used at extremely high or low temperature Current NiCad batteries are reported by the manufacturers to sustain over 1000 charge/discharge cycles with ``no noticeable drop'' in capacity. So leaving you laptop ``plugged in'' and giving your battery a continuous overcharge is not a problem if you have a well designed machine [it should run when the battery's on its voltage-depressed plateau :-)] -- if the battery's service life is short, the cause is not continuous overcharging... --cheers --____ ___ ____ /| ) /) / ' /| ) Paul E. Berg / |-'__ // /-- / |-< _ __ _ Zenith Data Systems (_/ (_(_(_(_(/_ (___,o (_/___)_(/_/ (_(_)_ ZDS Technology Center voice (616) 982-5124 /| 150 Hilltop Road paulb@zds-ux.ZDS.COM fax (616) 982-5690 (/ St Joseph, MI 49085
jbrindle@polyslo.CalPoly.EDU (Joseph S. Brindley) (09/27/90)
I understand that the drop in voltage can be reversed with a complete discharge but how can I achieve this discharge? I don't care for Traveling Software's idea of running the disks until they crash so is there some other alternative such as a fixture I could construct to achieve discharge? thanks -- <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> < jbrindle@polyslo.Calpoly.EDU * "Try not...Do. Do or Do not, > < Joe Brindley * there is not Try." > < "North to Alaska" * Yoda. >