darford Posted March 30, 2002 Share Posted March 30, 2002 I probably will invest in a NiMH charger and batteries for all the AA devices the kids and I have, but I have a shelf life question. According to the Rayovac web site, they recommend that if the cell is unused for a week, you should recharge it. Does "unused" mean sitting on a shelf, or in a unit but powered off? In other words, what shelf life can I expect if the cells sit in an FRS radio, camera, or handheld scanner that may be used infrequently? If they'll be dead in a week, maybe I'd be better off sticking with alkalines. TIA. Quote Link to comment
+rdw Posted March 30, 2002 Share Posted March 30, 2002 They will lose some charge within a week, but they will definitely NOT be dead in that time. I can usually go a month on the same batteries in my digital camera with no problems. The best thing about NiMH is that you can top them off whenever you want (when it is convenient for you) since they don't suffer the memory effect of NiCd batteries. "Unused" means any time it is not powering something. rdw Quote Link to comment
MajBach Posted March 30, 2002 Share Posted March 30, 2002 NiMh can self discharge as much as 4% per day. You may want to consider a charger that keeps it 'topped' up when left in the charger. I bought the Rayovac PS3 charger. It doesn't have this function but it charges batteries independantly and can charge both NiCD, NiMh AND alkalines. I use alkalines in tool box flashlights and other applications where shelf life is important. It is also a relatively cheap charger. Be sure to be NiMH batteries with higher mA/h ratings. This measures the total capacity of the battery. Some are now as high as 1800, which is comparable to a disposable alkaline. MajBach You can't have everything. where would you put it? Quote Link to comment
+Renegade Knight Posted March 31, 2002 Share Posted March 31, 2002 NiMH batteries do suffer from memory effects. They are not as bad as NiCad but you should still discharge them before charging them again. You have to move to the lithiom ion world to get out of memory effects. However all batteries have maximum number of recharge cycles they can undergo before they go bad. Quote Link to comment
+Cricketman Posted March 31, 2002 Share Posted March 31, 2002 Where can I find rechareable Li ion AA batteries? Does anyone know what the mAh ratings are for these batteries? Thanks in advance Wil Quote Link to comment
+Rich in NEPA Posted March 31, 2002 Share Posted March 31, 2002 There's no such thing as NiCad battery Memory Effect. ~Rich in NEPA~ === For every credibility gap, there is a gullibility fill. === [This message was edited by Rich in NEPA on March 31, 2002 at 12:59 PM.] Quote Link to comment
+ApK Posted March 31, 2002 Share Posted March 31, 2002 Voltage Depression Effect. Quote Link to comment
+Rich in NEPA Posted March 31, 2002 Share Posted March 31, 2002 The term "memory effect" refers to a specific situation that occured in certain (very early) satellite applications, which used a NiCad cell made with sintered plates. No commercial consumer-grade NiCad cells have ever been made with this process. So, all you are doing is "redefining" the term, which is a simple trick of semantics ... like calling a dog a cat. "Voltage depression" (which is NOT "memory") is something that can occur with any of the current rechargeable battery technologies. If you read the article I quoted (or dozens of other "independant" test results), you'll see that voltage depression is indeed a real issue and a problem. Memory effect is NOT. Let's face it ... NiMH and Li-Ion battery manufacturers like to play on consumer ignorance (and dread!) of NiCad memory when they claim that THEIR products have no "memory." Of course that's true, because they know that memory doesn't exist, and are cleverly implying that it does in competing products. Their own advertising counts on the fact that, as everyone knows!!!, NiCads have memory. See what I mean about gullibility? [Edit] Ooops! ... I forgot there was something else I wanted to mention. When someone is aware of a misconception of this kind, isn't it advisable to make an effort to correct it rather than encouraging it? Cheers ... ~Rich in NEPA~ === A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. === [This message was edited by Rich in NEPA on March 31, 2002 at 06:43 PM.] Quote Link to comment
+Rich in NEPA Posted March 31, 2002 Share Posted March 31, 2002 The term "memory effect" refers to a specific situation that occured in certain (very early) satellite applications, which used a NiCad cell made with sintered plates. No commercial consumer-grade NiCad cells have ever been made with this process. So, all you are doing is "redefining" the term, which is a simple trick of semantics ... like calling a dog a cat. "Voltage depression" (which is NOT "memory") is something that can occur with any of the current rechargeable battery technologies. If you read the article I quoted (or dozens of other "independant" test results), you'll see that voltage depression is indeed a real issue and a problem. Memory effect is NOT. Let's face it ... NiMH and Li-Ion battery manufacturers like to play on consumer ignorance (and dread!) of NiCad memory when they claim that THEIR products have no "memory." Of course that's true, because they know that memory doesn't exist, and are cleverly implying that it does in competing products. Their own advertising counts on the fact that, as everyone knows!!!, NiCads have memory. See what I mean about gullibility? [Edit] Ooops! ... I forgot there was something else I wanted to mention. When someone is aware of a misconception of this kind, isn't it advisable to make an effort to correct it rather than encouraging it? Cheers ... ~Rich in NEPA~ === A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. === [This message was edited by Rich in NEPA on March 31, 2002 at 06:43 PM.] Quote Link to comment
peter Posted April 1, 2002 Share Posted April 1, 2002 I agree with Rich on the "memory effect." As to the original question of self-discharge, Duracell has a chart that shows the dependence on storage temperature at: http://www.duracell.com/oem/Rechargeable/Nickel/selfdis.asp . Note that at room temperature the cells will discharge about 20% in a month. The rate of self discharge can go up markedly as the cell ages. Self-discharge has become less of an issue for me since I got the Ray-O-Vac '1-hour' PS-4 charger. Now I can just recharge before using the cells if they are significantly discharged. Rechargeable Lithium ion cells were brought up above. These have a higher voltage (3.7) than other common cell chemistries so they're not interchangable with AA cells. Quote Link to comment
+ApK Posted April 19, 2002 Share Posted April 19, 2002 quote:Originally posted by Rich in NEPA:[Edit] Ooops! ... I forgot there was something else I wanted to mention. When someone is aware of a misconception of this kind, isn't it _advisable_ to make an effort to correct it rather than encouraging it? Certainly. In the interest of proactive education then, perhaps rather then telling people "NiCd Memory is a myth", it would be more usefull to say "The effect you're seeing is not really 'memory', it is something else." ApK Quote Link to comment
+ApK Posted April 19, 2002 Share Posted April 19, 2002 quote:Originally posted by Rich in NEPA:[Edit] Ooops! ... I forgot there was something else I wanted to mention. When someone is aware of a misconception of this kind, isn't it _advisable_ to make an effort to correct it rather than encouraging it? Certainly. In the interest of proactive education then, perhaps rather then telling people "NiCd Memory is a myth", it would be more usefull to say "The effect you're seeing is not really 'memory', it is something else." ApK Quote Link to comment
+Renegade Knight Posted June 23, 2003 Share Posted June 23, 2003 It would be best to explain what the heck it really is. My NiCAD's do have a memory effect due to poor usage habits. Call a rose by any other name and the result is the same. You bleed when you use it wrong. ===================== Wherever you go there you are. Quote Link to comment
+Prime Suspect Posted June 23, 2003 Share Posted June 23, 2003 Back to the subject... NiMH batteries will lose about 1% of their power every day. Doesn't matter if they're on a shelf or in your GPS. Despite what was posted here, NEVER purposely run down NiMH batteries all the way down. This can permanently damage them. "Don't mess with a geocacher. We know all the best places to hide a body." Quote Link to comment
+Rich in NEPA Posted June 23, 2003 Share Posted June 23, 2003 quote:Originally posted by Renegade Knight:It would be best to explain what the heck it really is. My NiCAD's do have a memory effect due to poor usage habits. Call a rose by any other name and the result is the same. You bleed when you use it wrong. The explanation is right there in front of your nose! All you have to do is read. Once again let me state: you are NOT experiencing "memory effect" no matter what the manufacturers' advertising lackies try to convince you of. Overcharging, and to a degree overheating and overdischarging (when one or more cells in a "battery" is depeleted before the others, and the voltage across it reverses), causes voltage depression—i.e., loss of capacity due to damage to the cell which is for the most part irreversible. This is not the same thing as the so called memory effect. It affects all rechargeable battery technologies. To be fair, NiCad cells are actually a little more tolerant of abuse than NiMH cells. Cheers ... ~Rich in NEPA~ --- A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. --- Quote Link to comment
+Rich in NEPA Posted June 23, 2003 Share Posted June 23, 2003 quote:Originally posted by Prime Suspect:Despite what was posted here, NEVER purposely run down NiMH batteries all the way down. This can permanently damage them. Howdy, PS! I'm not trying to be argumentative, but actually a single cell can be discharged completely without harming it. The problem comes when you are dealing with a "battery." (It really is a matter of semantics here.) By definition a battery is typically two or more cells connected in series and/or parallel. For example, if your GPSr, flashlight, radio, etc., takes 4 cells, those 4 cells taken together comprise the battery. In addition, it is very difficult and/or too expensive when building a battery to match the cells so that they all have exactly the same capacity. Therefore, when two or more cells are used in series in a battery and you run the battery down completely, at some point one of those cells will reach zero volts before the others. When this happens, that cell experiences a reverse voltage across it and, depending on the degree, it damages the cell reducing it's capacity and causing even more of a mismatch. Continued deep discharging will eventually kill the cell completely, and subsequently the entire battery itself (or in the case of your GPSr, the "set" of cells you are using in it) will become useless. The general recommendation is to discharge a battery only to a level of roughly 0.9 to 1.0 volt per cell. Most recently designed electronic devices are designed to shut down automatically when this battery voltage level is reached. This prevents any damage due to inadvertent deep-discharging. Hope this helps. Cheers ... ~Rich in NEPA~ --- A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. --- [This message was edited by Rich in NEPA on June 23, 2003 at 04:21 PM.] Quote Link to comment
+Renegade Knight Posted June 24, 2003 Share Posted June 24, 2003 Ok I looked this up and found a good article on the memory effect myth. Lets compair. The Myth. Don't charge your NiCAD's before they are discharged or you will develop a memory effect and you will eventually get a situation where your battry gives out before it's really discharged. The Truth. If you overcharge the battery you will cause a situation where the voltage will drop off before you discharge your battery completly. The working difference. For all intents and purposes to Joe Battery user. None. The working solution for both myth and reality. Use the battery until it discharges then charge it; but don't overcharge it. This can repair some of the damage done. The difference between the solutions for myth and reality. Nada. The behavior that causes the problem is the same the solution is the same. Why is different but since the behavior is the same it's rather a moot point and so the "Myth" continues. ===================== Wherever you go there you are. Quote Link to comment
+Renegade Knight Posted June 24, 2003 Share Posted June 24, 2003 Two competing links on this. Yes there is a memory effect. http://www.batteryreviews.com/memory.html No there isn't a memory effect. http://www.dansdata.com/gz011.htm ===================== Wherever you go there you are. Quote Link to comment
+Rich in NEPA Posted June 24, 2003 Share Posted June 24, 2003 ~Rich in NEPA~ --- I'm not really a electrical engineer, but I did stay at a Holiday Inn Express last night. --- Quote Link to comment
+Searching_ut Posted June 24, 2003 Share Posted June 24, 2003 Regarding the term “memory effect”, I’m not so sure it’s incorrect to say it doesn’t apply to the results of short cycling. Most engineering papers I’ve seen in the last decade or so, define the resulting chemical/crystal changes that result as “memory” It’s one of those language items that has evolved over time, like the definition of light, or even the terms used to define GPS. What exactly is the thing we hold in our hands? As for the Rayovac PS4 quick charger initially discussed, I highly recommend it. You can buy it in a package with a cigarette lighter adapter, which makes the unit handy for travel. It will charge individual cells, and gets the job done in a hour or less depending on how far down the battery had been discharged. It also reverts to a trickle charge once the batteries are charged, which can be handy, although I doubt you’d want to leave them in there for months at a time. As for the Batteries themselves, Rayovac claims a discharge rate of 15 percent per month, but don’t mention the storage temperature the rate applies to. Seems about right to me for normal room temp, but I’ve never cap checked um to see how much capacity they really loose over time. I wouldn’t recommend them for use in your smoke detector, or the emergency flashlights you keep lying around, but I’ve found them to be great for my assorted radios, routinely used flashlights, GPS units etc. Quote Link to comment
+Renegade Knight Posted June 24, 2003 Share Posted June 24, 2003 quote:Originally posted by Rich in NEPA:_ _ http://img.Groundspeak.com/cache/64361_900.gif _~Rich in NEPA~_ __--- I'm not really a electrical engineer, but I did stay at a Holiday Inn Express last night. ---__ Now that was good. That's worth a beer if we ever meet. ===================== Wherever you go there you are. Quote Link to comment
+Bloencustoms Posted June 25, 2003 Share Posted June 25, 2003 The way I understood it, NiCd's would develop crystals in their electrolyte after repeated charge/discharge cycles. These crystals limit the cell's capacity. There was a company (Tekin) that used to make a "Reflex peak detector charger". This charger would monitor voltage in hundredth of a volt increments. As soon as the voltage "peaked" the charger would revert to trickle charge (a hundred milliamps or so). There was a mode that you could select to recondition the cells (reflex mode) that would reverse the polarity of the charge current every fifth pulse. This was said to break up the crystals and restore much of the original capacity of the cells. However, I got out of the R/C car racing, and have heard little about NiCd's since. There was also a method to prevent cell reversal that should be effective (albiet costly) even today. You purchase many, many cells, then charge them all fully. Measure the time it takes for each cell to discharge under an equal load. You then make batteries (more than one cell) out of the most closely matched cells in the lot. For instance, if your GPSr requires two cells, and you have two cells that each discharge fully in 2052 seconds, and 2048 seconds, they are fairly closely matched. Obviously, the time differential is dependanton the load (in amperes). The greater the load, the quicker the cells discharge, and the more difference a second makes. Two closely matched cells can be discharged more completely without fear of reversal. The trouble with cordless drills, cell phones and other rechargeable devices is they don't take the time to match their packs. It would be prohibitively expensive. A regular R/C car pack that cost $20 for a 7.2V sub c NiCd pack can cost $60 for a "1 second" pack. Meaning that the greatest difference in discharge time under load is 1 second from the highest capacity cell to the lowest. These packs are incredible to see in operation. The car is going full speed, then suddenly it just dies. No slowly winding down. "The fertilizer has hit the ventilator" [This message was edited by BloenCustoms on June 25, 2003 at 06:51 PM.] Quote Link to comment
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