Path: utzoo!attcan!uunet!husc6!cmcl2!rutgers!apple!voder!pyramid!prls!philabs!micomvax!ray From: ray@micomvax.UUCP (Ray Dunn) Newsgroups: comp.sys.ibm.pc Subject: Re: Overcharging NiCad batteries? (long) Summary: some answers Keywords: NiCad rechargeable batteries, laptops, foreign power Message-ID: <1301@micomvax.UUCP> Date: 28 Sep 88 18:32:35 GMT References: <3008@dalcs.UUCP> Reply-To: ray@micomvax.UUCP (Ray Dunn) Organization: Philips Electronics Ltd. (TDS - Montreal) St. Laurent QC, Canada Lines: 129 In article <3008@dalcs.UUCP> lane@dalcs.UUCP (John Wright/Dr. Pat Lane) writes: >... >Anyway, they say that there is a considerable danger in overcharging the NiCad >batteries and that repeated overcharging will reduce the length of time the >batteries will hold a charge to virtually nil. They recommend charging for >8 to 10 hours and *no* more... >... Further, they say that damage can be done by >charging the batteries before they are fully exhausted. Ah. NiCads. There should probably be an entry in the net.announce.newusers "commonly asked questions" to cover nicads! It's usually in consumers.house or models.rc or whatever but it *does* come up once or twice a year, and is a subject full of myths and old-wives-tales! Hope this is of use and worth the volume. >Specific questions: >1. The 8 to 10 hours recharging time...is that affected by whether the machine > is on? Would that mean that you should not use the machine on and plugged > in for more than 10 hours...after 10 hours, unplug and use the batteries > till they're gone, then go back to external power for another 10 hours and > so on...till you give up and go get some sleep :-) 8-10 hours is *NOT* a trickle charge, so the batteries *will* overcharge if left longer at this rate, or if left for a full 8 hours when they were only partially discharged. Overcharging causes permanent chemical changes which reduce the life and capacity of the cells. Use an outlet timer when using this charger. Normally, a trickle charge is the 14-16 hour rate, which is about 1/12th to 1/10th of the current of the one hour capacity of the pack - thus a 1.1 amp-hour nicad is charged at about 100ma for 16 hours. At trickle charge rate, nicads will *not* suffer significant damage if left charging for longer than 16 hours, but see below. Most equipment comes with a trickle charger. This makes life easier but charging times longer. Problem number 1: If *kept* fully charged permanently, nicads will loose capacity. If routinely discharged by only a fraction of their capacity, the available capacity will tend towards this fraction. This "memory" (actually a chemical change in the cells) can be avoided (and to a certain extent reversed) by cycling the cells through *complete* charge/discharge cycles. So, for *best* performance, don't plug them in during equipment use, or routinely charge between uses unless the cells have fully discharged. Although it *is* a good idea to fully discharge between charges, it is often a pain in the neck. Going through a full discharge cycle a couple of times a month, say, is often a good compromise. Balance the cost of possibly replacing the nicads prematurely, or getting less capacity, against the inconvenience of handling them "properly". A good solution to the problem is to have two or more battery packs, to match your use pattern. One in use, the other(s) on trickle charge. This may not be convenient if the pack is built-in to the equipment. Problem number 2: A nicad pack is a battery made up of several cells. Individual cells discharge before others. If current continues to be drawn from the pack after one or more cells have discharged, then current is forced through these cells and they become *deep discharged*, an irreversible chemical change takes place. Nicads have a very flat discharge curve with a sharp "knee" near the discharged point, so as soon as a voltage dip is seen (e.g. screen dimming, shaver motor slowing down etc) the pack should be considered dead and recharged. Continuing to use a pack until it is completely dead will always deep discharge one or more cells to some extent. Damage is cumulative. Odd tips: A bad pack often only has one or more bad cells, and can be recovered by replacing only the bad cells, not the whole pack. *Occasionally* a cell will "die" because a crystal of chemical grows internally and shorts the electrodes inside the cell. Such a pack can often be recovered by "zapping" - discharging a high value electrolitic capacitor through the cell to burn away the crystal. Deep discharged cells cannot be recovered in this way. Radio Shack now cells, sorry, sells, a few odd-sizes of nicads in acceptable capacities - e.g. sub-C. I only say this because they are convenient, and are often cheaper than original-equipment too. The manufacture and disposal of NiCads represent an environmental problem. Cadmium is an important heavy metal pollutant. >Side issue: >I have a car cigarette-lighter adapter as well... >... They also say that the battery cannot be charged while the machine >is on when running from a car battery because there is not enough power... Surmise: If the nicad pack gives 12 volts (10 cells - actually 12.5-13volts at full charge), then the car battery's 12 volts (actually 14.2 when fully charged) is approximately the same. Potential difference to charge the nicads is now marginal at best, and the current taken to run the machine *could* create enough loss in the cable from/to the lighter to tip the scales. > I was >curious about this because (I could be wrong here) I thought the regular ext. >power adapter supplies 12V-DC and if the voltage is the same, would the laptop >not try to draw enough amps to run the machine and charge the batteries both? If the external supply gives 12 volts, it will probably do so when the specified current is being drawn - if you put a voltmeter on the output when it is not connected, you will probably see 18 volts or so (I assume a typical simple non-regulated external supply). Regulation (to 5 and 12volt?) will probably be provided in the machine. >... [can we use a cheap 220/110 voltage converter] Yes. Disclaimer: These are my own musings, not necessarily representing any connection with the real world, and certainly not with my employer. -- Ray Dunn. | UUCP: ..!philabs!micomvax!ray Philips Electronics Ltd. | TEL : (514) 744-8200 Ext: 2347 600 Dr Frederik Philips Blvd | FAX : (514) 744-6455 St Laurent. Quebec. H4M 2S9 | TLX : 05-824090