Path: utzoo!utgpu!watserv1!ria!uwovax.uwo.ca!telecom-request From: ntg!dplatt@apple.com (Dave Platt) Newsgroups: comp.dcom.telecom Subject: Re: Help Needed With NiCads on my Panasonic Cellular Message-ID: Date: 15 Mar 91 18:38:36 GMT Sender: Telecom@eecs.nwu.edu Organization: New Technologies Group, Inc. Palo Alto CA Lines: 62 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 207, Message 6 of 9 In article Scott Meyers writes: > I have a Panasonic Transportable phone. I know about the problems > associated with NiCads when you try to recharge them before they've > been totally depleted. How can I discharge the battery beyond the > handset just cutting off. I figured something like a light bulb and > some leads to connect to the battery terminals but that seems crude > and slow. Any suggestions. Thanks in advance... What you've suggested is crude and slow ... and, if you're talking about a typical NiCd battery pack, it can damage the cells. The _only_ safe ways to discharge NiCd cells are either [a] one at a time, or [b] with the cells connected in parallel. In either case, a light bulb or a resistor does a fine job. This is easy to do if you have single 1.2-volt cells (e.g. AA or C or D size); you can home-brew up a discharging fixture with a battery holder, light, and resistor from Radio Shack. Discharging NiCd cells that are connected in series (e.g. when they are wired up in a battery pack) is a BAD idea. The reason is that it's quite common for one cell in the pack to have slightly less capacity than another (manufacturing differences, etc.). If you try to discharge the battery all the way to zero, the weakest cell will reach zero volts while the other cells are still "live". As the remaining cells continue to discharge, the current will be flowing through the dead cell ... in effect, trying to "charge" it in reverse (as if you'd put the cell into a charger backwards). This will damage the cell ... it may lose some of its ability to store electricity, and/or develop an internal short. Because the cell has been weakened, it will run down even sooner the next time you use it ... and if you then attempt to drain the whole battery, you'll damage the weakened cell even sooner. Eventually the cell may short itself completely, and your battery pack's output will be 1.2 volts below what it should be ... your phone will probably reject it. It's true that some NiCd cells have been shown to exhibit a "memory" effect, if you repeatedly discharge them to an identical point and then recharge them. I've heard that modern NiCd batteries have pretty much eliminated the memory effect. In real-world use, the memory effect is likely to do much less damage to the cells in your battery than over-discharging them will. NiCd cells have a very flat voltage output ... they deliver very close to 1.2 volts until they've almost completely discharged, at which point the voltage drops off very rapidly. This is unlike the behavior of lead-acid, carbon-zinc, and alkaline batteries, whose output voltages drop off more gradually and smoothly as they discharge. Well-designed NiCd-powered equipment will detect the voltage dropoff, and will shut down immediately to avoid over-discharging the cells and damaging the battery pack. So, your best bet is probably to keep a spare well-charged NiCd battery with your phone. When the phone reports "Low battery" and shuts down, it means that the battery you've been using has only about 5% of its capacity left and should _not_ be discharged further. You can then swap in the new battery, and recharge the old one without worrying about the memory effect. Dave Platt VOICE: (415) 813-8917 UUCP: ...apple!ntg!dplatt USNAIL: New Technologies Group Inc. 2468 Embarcardero Way, Palo Alto CA 94303