Refractory |
memory effect mythI have, upon occasion, heard people explain that the nickel metal hydride battery of cordless drill X is superior to the nickel cadmium battery of cordless drill Y, because NiMH has no "memory effect", but NiCd does. People have all kinds of strange religious beliefs about the rituals that must be performed to exorcise the Memory Monster. If they were just dancing around their laptop, MP3 player or cordless drill waving incense sticks then I wouldn't care. But they're usually doing unnecessary charge/discharge cycles. Which is bad. If you flatten a battery before you recharge it - some people flatten their batteries manually, and some chargers do it automatically - you greatly reduce the life of the pack. A given battery pack may last for 500 full cycles, or 2000 partial ones. If you're fully flattening the battery by actually using it, then fair enough. But flattening it as part of the recharging process is goofy. "Memory effect" is now used as a general term for anything that makes a battery not deliver its full capacity. What the term originally referred to, though, is a phenomenon that's probably never actually been observed in consumer hardware. True memory only happens in sintered plate NiCd cells (which aren't necessarily the sort of NiCd you're using, and are of course completely different from any kind of NiMH cell), and it only happens when you precisely discharge a cell to exactly the same level over and over again, and recharge it without any overcharge. True memory effect happens in satellite power systems, electronics test labs, and practically nowhere else. Cheap trickle chargers always overcharge if you leave them long enough, and quality consumer NiCd chargers also slightly overcharge, because the slight voltage drop that happens when you do that is what they use to pick the end of the charge cycle. So even if you're using sintered plate NiCds, which you're probably not, your charger will cure memory effect anyway. What people nowadays call memory effect is a combination of two things. One - cell aging. Batteries don't last forever. The older they get, the less capacity they have. Live with it. Two - "voltage depression". Voltage depression is a problem with NiCd batteries and, according to some sources, also with NiMH, and it doesn't affect the battery capacity much at all. Rather, the battery voltage drops unusually quickly as it discharges. Gadgets that monitor their battery voltage therefore think the battery's flat earlier than they should. There may be lots of capacity left at the slightly depressed voltage, but the gadget doesn't know that and flashes its "low battery" warning. Fully discharging cells cures voltage depression, but if you fully discharge a whole battery then the stronger cells in the battery will "reverse" the weaker ones. The weaker ones go flat first, and then get charged backwards by the others. This is very bad for the weaker cells, and will kill a battery pack quick smart. So don't do it. Fancier discharging hardware lets you set a voltage to discharge to - say, 0.9 volts per cell. A NiCd or NiMH cell that's down to 0.9 volts under moderate load has practically no charge left; it's very nearly dead flat. But stopping the discharge at that point, rather than letting the pack slump down to zero volts, should save weak cells in the pack from any significant reversal. Well, unless they're so weak that the pack's toast anyway. There's still not a whole lot of point to doing this, though, unless you've got a scientific battery care regimen and want to start every charge from a precisely known state. |