cliff@cs.arizona.edu (Cliff Hathaway) (08/10/90)
With the plethora of cordless, rechargable appliances and tools in our
machine room tool cabinet, the question of the which charging schedule
will prolong the life of the batteries and provide a reasonable useful
charge has come up. The best information we have is that the batteries
should not be recharged until they have been drained to avoid giving
them a memory (e.g., if a fully-charged flashlight is consistently used
for 5 minutes, and then recharged, it will soon develop a memory, and will
only shine for 5 minutes before it is discharged).
1) Will this really happen?
2) Does it only happen to a certain type of rechargable batteries?
3) Is it possible to overly drain rechargable batteries so that they will
no longer take or hold a charge?
4) What is the lifespan of a typical rechargable battery?
My apologies if this has been covered recently; I just started following
these newsgroups.
Cliff Hathaway
Dept. of Computer Science (602)621-4291
University of Arizona cliff@cs.arizona.edu (internet)
Tucson, Ariz. 85721 {cmcl2,noao,uunet}!arizona!cliff (uucp)ftpam1@acad3.fai.alaska.edu (MUNTS PHILLIP A) (08/10/90)
In article <105@javelina.cs.arizona.edu>, cliff@cs.arizona.edu (Cliff Hathaway) writes... >With the plethora of cordless, rechargable appliances and tools in our >machine room tool cabinet, the question of the which charging schedule >will prolong the life of the batteries and provide a reasonable useful >charge has come up. The best information we have is that the batteries >should not be recharged until they have been drained to avoid giving >them a memory (e.g., if a fully-charged flashlight is consistently used >for 5 minutes, and then recharged, it will soon develop a memory, and will >only shine for 5 minutes before it is discharged). There was an article in "Power Electronics" magazine a couple of years ago about this very topic. The consensus of the manufacturers was that there ain't no such thing, or least they can't duplicate it under lab conditions. The problem is with overcharging: Nickel Cadmium batteries (the kind used for virtually all small rechargeable battery packs) are just like car batteries in that the electrolyte is mostly water. If you overcharge, the water evaporates, just like in your car. But NiCads are sealed so you can't replace the water. (I have seen wet-cell NiCads from the phone company.) When you discharge a battery for 5 minutes and then run it through a full charge cycle, you inevitably overcharge it. If you always run your batteries down before charge, they don't overcharge. This is what I read and it fits my own small experience. Incidentally, I hear rumors about a "millenium" cell from Gates that comes with a lifetime guarantee: if it ever fails to hold a charge, they will replace it. Perhaps they have found a way to lick the electolyte loss problem. Philip Munts N7AHL NRA Extremist, etc. University of Alaska, Fairbanks
adam@media-lab.MEDIA.MIT.EDU (Adam Glass) (08/10/90)
cliff@cs.arizona.edu (Cliff Hathaway) writes: >3) Is it possible to overly drain rechargable batteries so that they will > no longer take or hold a charge? Yes. When batteries go into this state, they're called "deep discharged." It has not happened to me, but my RC battery charger has lots of caveats about over- and undercharging your NiCds. >4) What is the lifespan of a typical rechargable battery? The batteries I have are rated for about 500 charge cycles. Keep in mind, however, that they are built to handle rapid charging and discharging (i.e., being charged at 6A for 15 minutes, or discharged at 10A.) I believe that you're supposed to slow-charge the batteries every once in a while to help cancel the adverse effects of the rapid-charging. This rejuvination I have personally witnessed; every Thursday night, before the Friday races, I set the battery packs on "trickle" charge... I have heard some talk about zapping the "whiskers" that develop in NiCds after a while. Come to think of it, I may have seen these so-called "whiskers" -- whenever I go out on a remote shoot, I have to take about 5 camera batteries because they're so old, and each one holds it charge for about 25 minutes, whereas two years ago, the same batteries would last an hour or more. On a related topic: NiCds will drop a few tenths of a volt when they're "peak" charged. In the RC business, we use "delta peak chargers" to stop charging the batteries when the voltage drops by the characteristic .1 or .2 volts. How would one go about making one of these peak detector chargers? I've seen one which only uses one 8 pin chip (presumably some sort of comparator) and various secondary components: resistors, capacitors, maybe a transistor or two, and a relay to cut off power. Adam -- "I'll post more when I know more." -- George William Herbert
dbell@cup.portal.com (David J Bell) (08/10/90)
>>With the plethora of cordless, rechargable appliances and tools in our >>machine room tool cabinet, the question of the which charging schedule >>will prolong the life of the batteries and provide a reasonable useful >>charge has come up. The best information we have is that the batteries >>should not be recharged until they have been drained to avoid giving >>them a memory (e.g., if a fully-charged flashlight is consistently used >>for 5 minutes, and then recharged, it will soon develop a memory, and will >>only shine for 5 minutes before it is discharged). > There was an article in "Power Electronics" magazine a couple of years ag o >about this very topic. The consensus of the manufacturers was that there ain' t >no such thing, or least they can't duplicate it under lab conditions. BZZZZTT! Wrong.... I don't know where they got that idea, that NiCd cells don;t develop "charge memory". It doesn't occur to the extent suggested in the first posting (5 minute life, eventually) unless a cell has been ruined, but NiCds, and most other secondary cells need to be reconditioned once in a while. I am an engineer in a spacecraft power systems division of a large aerospace firm; there is a marked improvement in "old" cells after they have been fully discharged, then shorted down with ~1 ohm resistor for 12-24 hours. The available capacity increases 50-100% in some cases... There is a more serious problem, when discharging exhausted cells: there is a possibility of one (weaker) cell being reversed, that is - discharged through zero, by the rest of the cells discharging through the load. This is usually what ruins a rechargeable pack... Dave dbell@cup.portal.com
hsd@raybed2.msd.ray.com (HERBERT DASILVA) (08/14/90)
In article <105@javelina.cs.arizona.edu> cliff@cs.arizona.edu (Cliff Hathaway) writes: >With the plethora of cordless, rechargable appliances and tools in our >machine room tool cabinet, the question of the which charging schedule >will prolong the life of the batteries and provide a reasonable useful >charge has come up. The best information we have is that the batteries >should not be recharged until they have been drained to avoid giving >them a memory (e.g., if a fully-charged flashlight is consistently used >for 5 minutes, and then recharged, it will soon develop a memory, and will >only shine for 5 minutes before it is discharged). > >1) Will this really happen? Yup. >2) Does it only happen to a certain type of rechargable batteries? Ni-Cads only, gel cells and lead-acid batteries do not exhibit these symptoms unless damaged. >3) Is it possible to overly drain rechargable batteries so that they will > no longer take or hold a charge? Yup. This is called 'cell reversal'. More on that later... :-) >4) What is the lifespan of a typical rechargable battery? Very variable... Depends on the battery manufacturer and how they are used... Here's the poop (this is probably gonna be long, and kinda technical, but you might get some neat facts to know and tell from it :-)... When cells are discharged just a little, and then recharged to full, they begin to 'forget' how much juice they can actually hold. The peak voltage remains the same (or close), but the amp/hour rating drops, so less "run-time" is available. If this continues for long periods of time, the power loss can become permanent. Rechargeable batteries of the same size and manufacture are not all exactly the same. Some have more capacity than others, so unless the batteries are "matched" so that all the batteries in the pack are the same (or REALLY close), your pack is only as strong as its weakest cell. This means if you drain the pack all the way down to the dregs each time, the weak cell(s) can empty out before the strong cells. At this point, lights start to dim and motors slow down ALL OF A SUDDEN. What has happened is that the weak cell(s) are dead, and the powerful cells are pushing juice through the dead one(s). This is most common in power drills and screwdrivers where the amp load is pretty high, and damage can be done the fastest. If this happens a lot, the weak cells will be damaged, and can actually CHANGE POLARITY. This is a 'cell reversal'. The pack now seems weaker, because the damaged cell(s) never take a full charge again (and remember - these were the weak cells to begin with). A damaged battery will have a lower peak voltage AND lower amp/hour rating. This is often confused with a 'memory', because the symptoms appear the same. So, you don't want to drain them to DEAD, just to WEAK. This is the 'best' way to prolong useful battery life for household use. Another enemy to Ni-Cads is heat. During normal charging and discharging Ni-Cads become warm. This is normal. Slowly, over time, this warming and cooling will age the battery, and its capacity drops. If a rechargeable battery is overcharged or shorted out, it will overheat and expand (as heat tends to do to things :-). Ni-Cads have built in 'vents' to keep the batteries from exploding, but when a battery 'vents' the battery suffers a loss of material, and again, a lower peak voltage and amp/hour rating occurs. Overheating is rapid aging. Discharged Ni-Cads should not be recharged until cool to the touch. This generally applies only to high-amp tools like cordless drills and screwdrivers. The WORST thing you can do to a cordless tool is drain it dead, and then immediately put it back on charge to see if you can use it again right away, "just for a few minutes more", but then again, if you need the tool... The 'best' thing to do is get a tool with a detachable battery pack, and then have an extra pack or two charged , so that you can cycle them. No, putting them in the refrigerator does not help :-). If your batteries have developed a memory, but have not been damaged, you can usually recover a large portion of the 'forgotten' storage capacity by repeatedly charging to full, and then discharging to 'weak' a couple of times as best as the charger allows. If this method does not improve the performance of the batteries, they may be damaged, or just plain 'old'. Also, if a pack has been 'damaged', some of the strong cells may still be perfectly good! Anyone who is into serious RC Car racing would have the equipment 'reflex' charge a pack or test the cells... A good set of batteries will take 500 charges even when "abused". Worst offenders are items which are always on charge, but are seldom used. The problem occurs because Ni-Cads slowly discharge even when not in use, and then the charger "tops them off" to be full again, so this is like the batteries being constantly discharged A TINY BIT and then recharged, which is not the best. But then again, you can't take a fully charged item off charge and let it sit, because it will slowly discharge, and might not be ready for use when you need it. It takes about 30 days for a full battery to discharge completely. The ideal would be to set an item on charge overnight the day before you need it. Obviously this doesn't work well for lights, phones, vacuums, and other use-on-an-impulse items, but does work well for drills, screwdrivers, and other "project" type tools. When used "correctly", the life of Ni-Cads can be 1000 or more charges! For example: My cordless phone would sit on the charger all day, and only get discharged when I was on the phone. The first set of batteries lasted about a year. My dad bought the same phone at the same time, but he would take it off the charge stand and carry it around the job site all day, and only return it to the charging base at the end of the day. His first set of batteries lasted over 3 years! (This was 5 or 6 years ago when Ni-Cad technology was young). I've had good luck with Radio Shack AA batteries and Sanyo C batteries. These are what I use for RC Car racing. Our batteries go through hell, and we have gotten to know them inside and out, as a cell which has lost even just 5% of its capacity can be the difference between finishing a race and "running-out- of-juice". Other guys I race with have used GE AA batteries, and they seem to work well too. Of course, if you have a custom pack for your favorite rechargeable tool, you're stuck using that pack... I hope this takes away some of the mystery of Ni-Cads. More technical data available on request :-) Herb "RC Race Rascal" DaSilva > > Cliff Hathaway > Dept. of Computer Science (602)621-4291 > University of Arizona cliff@cs.arizona.edu (internet) > Tucson, Ariz. 85721 {cmcl2,noao,uunet}!arizona!cliff (uucp)
harry@neuron1.Jpl.Nasa.Gov (Harry Langenbacher) (08/16/90)
I asked a battery-man here about that a couple of years ago, but I am confused by his answers. Yes batteries do have "memory", but he said that letting a battery sit around in a discharged state, or to use them in a nearly discharged state was the worst thing, so he recommended faithfully keeping them charged up. I would appreciate hearing a less confused answer on this than my memory provides - perhaps I've been running discharged too long. - Harry Langenbacher 818-354-9513 harry%neuron1@jpl-mil.jpl.nasa.gov harry@neuron1.jpl.nasa.gov 128.149.12.7
adam@media-lab.MEDIA.MIT.EDU (Adam Glass) (08/17/90)
harry@neuron1.UUCP (Harry Langenbacher) writes: > have "memory", but he said that letting a battery sit around in a > discharged state, or to use them in a nearly discharged state was the > worst thing, so he recommended faithfully keeping them charged up. This is exactly what battery manufacturers say to do. All of my NiCd cells are supposed to be left in a fully charged state between uses. People with SCE cells have an even rougher time, as the cells have to be pulse-charged and meticulously maintained. Such is the price of 1700 mAH sub-C batteries. Adam