> Subject: Batteries: why do they die and how to prevent it? > In-Reply-To: <[EMAIL PROTECTED]> > > Subject says it all. >
This short question could lead to an infinitely long answer. To keep it brief, I'll limit the discussion to just rechargeable batteries. Rechargeable batteries work because their underlying chemical reactions are nominally reversible. You might think, then, that they should have a nearly infinite lifetime. Unfortunately, the reversibility applies only to the chemical reactions, and not necessarily to the physical/mechanical changes that accompany charge-discharge cycles. As you discharge a cell, physical transport of material generally takes place. Recharging generally moves the stuff back, but not necessarily to their exact, original locations. So, you might build up peaks and valleys over time. Eventually, some of those peaks can puncture insulation and cause shorts, or the valleys can effectively create regions of high resistance (or even open circuits). A constant challenge of battery designers is to minimize these sorts of consequences. The famous (and misnamed) "memory" effect in nickel-based cells is an example of a problem caused by physical changes. In this case, overcharging (most commonly caused by continuing to charge -- even at a low level -- a NiCd or NiMH (yes, really) battery even after it's been topped up) can cause the growth of surprisingly large crystals within the battery (with an ordinary microscope, you can readily see the difference in crystal size for a virgin battery, compared with one that has been consistently overcharged -- but be aware that nickel-based compounds are toxic, so don't cut open batteries unless you know what you're doing). As they grow, the cell resistance increases, reducing the cell's ability to deliver energy to a load. Eventually, the crystals will grow large enough to puncture insulation and cause a short. Sometimes, you can fix this by applying a huge current for a short time (e.g., from a big fat capacitor). To prevent "memory" is easy: Don't recharge NiCd cells before you've really drained them. Using them just a bit, and then popping them back into the charger turns out to be a terrible thing for NiCd/NiMH batteries. If memory problems do arise, a couple of thorough discharge-charge cycles will help break down the crystals. Many battery chargers/reconditioners attempt to perform this function. However, they generally operate on an entire battery pack. If there's a shorted cell inside, reconditioning will generally fail. Even worse, discharging such a pack can result in the application of reverse voltages across one or more cells, and this will destroy them fairly quickly. So, if you do decide to recondition cells, you really need to do it on an individual cell basis. That generally means popping open battery packs, or drilling/melting access holes, etc. You have to be pretty determined to want to go through that trouble, but that's the best way to keep those cells going for a long time. Lead-acid batteries also suffer degradation from physical changes, but they manifest themselves differently. Here, you must *avoid* deep discharges, because the lead-acid system produces a chemical, lead sulfate, which is an insoluble insulator. Deep discharges create a layer of this insulator that is too thick, and the cell resistance increases more or less permanently. That's why you will often seriously wound or even kill a car battery if you leave your headlights on overnight. For these batteries, you want to treat them more or less the opposite way you would treat NiCd cells. Keep lead-acid batteries fully charged as much as possible, don't discharge them deeply, and never store them in a discharged state. Hope that this is what you were looking for. -- Prof. Thomas H. Lee Center for Integrated Systems, CIS-205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 http://www-smirc.stanford.edu 650-725-3709 ph, -3383 fax -- PowerBooks is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Enter To Win A | -- Canon PowerShot Digital Cameras start at $299 | Free iBook! | Support Low End Mac <http://lowendmac.com/lists/support.html> PowerBooks list info: <http://lowendmac.com/lists/powerbooks.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:powerbooks@;mail.maclaunch.com> To unsubscribe, email: <mailto:powerbooks-off@;mail.maclaunch.com> For digest mode, email: <mailto:powerbooks-digest@;mail.maclaunch.com> Subscription questions: <mailto:listmom@;lowendmac.com> Archive: <http://www.mail-archive.com/powerbooks%40mail.maclaunch.com/> --------------------------------------------------------------- >The Think Different Store http://www.ThinkDifferentStore.com ---------------------------------------------------------------
