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Just bought my first cordless power tool (a B&D 14.4 v drill). What are th rule of thumb for maximizing battery life/performance? (run the batteries way, way down the first couple times? Do not ever recharge until they are fairly dead? Charge the batteries up after every use even if not run down much?). No such guidelines included in the users manual. Thanks
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Tom,
You need to look into the back issues of FHB. A few years ago there was a review of cordless drills with a side bar on batteries. In brief - 1) It's best not to drain the battery dead before recharge, 2) heat buildup is the enemy of battery working time and overall life. (Which explains why the old 9.6V Makitas last forever: They have a huge surface-to-volume ratio that dissipates heat)
*Is it just me, or are Dewalt batts crap? 12V specifically. And is that the real story on those old Makita 9.6 sticks? They do seem to be better than any others, wish they'd fit some of my other tools. Joe H
*I spent the last 8 years designing Ni-Cd's, both cells and batteries, for the aerospace industry. Gerard's advice is quite sound. #1 killer - heat #2 killer - overcharge #3 killer - overdischarge.If they get significantly warm on charge, take them off as soon as they are charged. This is more important as the pack ages for a number of complicated reasons I won't bore you with. I actually charge mine in the fridge when my wife will let me. I also store them in the fridge.Don't let your tool bake in the sun or (this is probably the #1 cause of cell phone failures) leave it in a hot car or truck.Recharge as often as practical.
*Tim, I always thought (maybe incorrectly) that you are not supposed to charge a battery before it's dead because it causes it to have a shorter "memory". When do you charge them?
*Tim,Please fill us in on the "complicated, boring" reasons. Have you seen the price of the things lately?
*OK, you asked for it!Recharge Frequency: Battery life is a direct function of how often the active materials in the cell is cycled between charge and discharge. Shallow discharges (charging often) are less detrimental than deep discharges. The reasons are many and complicated. In short, the positive (nickel) electrodes expand and contract during cycling. While slight, this expansion and contraction leads to physical breakdown of the electrode. In the early stages, the electrical paths of conduction become broken on a microscopic scale rendering pockets of active material electrically isolated. Eventually, it can even lead to bits of active material coming loose from the substrate. The negative electrodes also suffer from deep discharges. Cadmium from the negatives can go into solution. WHen it recrystalizes on the electrodes, it often form cadmium dendrites (stalagtites) that can grow and poke through the separator material. When this happens, the cell goes completely dead. These mechainsms contribute to early failure when a cell or battery is discharged properly, meaning that it is not over-discharged. I'll get to that in a minute. To give you an idea of how significant this is, here's an example.If you completely discharge a pack everytime, it might last 1,000 cycles under perfect conditions. If you were to cycle that same pack to 25% (1/4 discharged), you might get 6,000 cycles. If you look at the total energy delivered, the lower depth of discharges deliver much longer life. This appears to also be an exponential function, so as you go lower on DOD, the life increases exponentially. Memory is a myth in modern cells. Back in the early space days, there were some examples of memory, but it is really a misunderstood problem. Memeory is observed when a battery is cycled many many times to exactly the same depth of discharge every time. If, after this cycling, you attempt to discharge to some lower level, you may see that the battery will not provide the energy. All that is usually required to fix the problem is to cycle the battery through one or two deep discarges. In practical use, we all occasionally " deep discharge" our batteries accidentally. That fact coupled with the fact that most modern cells are virtually immune to the problem makes intentional deep discharges unnecessary. Deep discharges are bad news for portable power tools. Multi-cell batteries are never perfectly balanced. One or more cells always reach the fully discharged state before the rest. When this occurs, the rest of the cells drive these low cells into voltage reversal. If this happens at high currents (like you would see when operating the tool)the positive electrodes start to emit hydrogen gas. Hydrogen gas does not readily recombine inside a Ni-Cd cell, so internal pressure just builds. If this happens repeatedly, the pressure will build to the point that the safety vent operates. If this repeats, the cell eventually fails due to electrolyte loss and a thing called loss of precharge. No sense digging into those subjects, I think it should suffice to say that you should recharge well before the tool quits operating. Heat kills Ni-Cds. I don't have a clear understanding of the mechanism because it is electrochemical in nature and I'm a mechanincal engineer, but I do know that it is usually not reversible. One episode of leaving your pack in a hot vehicle can permanently remove 10-20% of your capacity depending on the temperature and duration. I'm tired of typing, and I'm certain you're tired of reading. Any specific questions, let me know.Tim
*Wow. Thanks for the info Tim. I have been "deep discharging" my stuff (phone, computer, camcorder, etc.) because of concerns about "memories." Now I know better. You would think the manufacturers of this stuff would do a better job keeping buyers informed.It sounds like the general rule of thumb regarding charging is 'charge often.' Are there any classes of batteries for which this would be a bad rule?
*Tim, Thanks for all that info, I guess we just have to listen to the tool to try to get a feel for when its starting to go.
*Tom,I'm only really familliar with Ni-Cd, NiMH, and lead acid. All three benefit from similar treatment.Allen,It's tough sometimes to tell when your battery is close to dead. One of the main selling points of Ni-Cds is that they provide close to full power all the way to the end of the discharge and then they conk out all at once. I also fool with electric powered model airplanes, and you can sometimes get yourself in a tight spot when go from full power to no power in 30 seconds.
*Tim...I used to work in TV...we shot on Sony Betacam. We RELIGIOUSLY drained our batteries, for cameras and lights, every night. We even had a discharging set up, so we could kill them dead before charging. The batteries for the cameras were Sony NP1s (commonly called chocolate bars, in the biz). Were we wrong...or were these the "old style" cells you referred to?Great info, Tim. You actually raised my eyebrows!
*Tim: Submit an article to FHB.
*I also have been totally discharging my batteries and when my cell phone battery stopped holding a charge the guy at the repair place said that I had not been letting it run down enough before charging it. It is a nimh battery. I guess he was wrong too. Good information to know, thanks.
*Andy...Hey Andy? Like lonecat says, get this guy to write an article on how we are all destroying our batteries. Andy?
*Tim, What about charging too early or before needed? does this cause damage? Thanks again!
*Tim, what about leaving them in the charger? I think my Makitas shut off, not sure about the Dewalt. My old Milwaukee stays on I think, or the light does and so do (did, I threw both drills in the trash)Bosch. Am I hurting the batts by leaving one in all the time, or do they care? My Dewalts go dead by themselves, nothing else does. Joe H
*I've also heard some chargers will switch to "trickle charging" once the battery is fully charged. Is this harmful to the cells?
*Joe,I've read some chargers shut off when the battery is fully charged; some don't. My owners manual says mine does not.
*Tim Kulin has my vote for the "Most Thorough" letter, ever written on this website! Wow! Just think of all the conversations going on at jobsites across America. Thanks for taking so much time set the record straight. You The Man.
*All,Thanks for all the kind words! I'm glad to see all those years paying off for someone. I was out of town this week, so I'll try to answer all the questions at once.This question about deep discharges is more complicated than I indicated (I was hoping to stay below 10,000 words). The other part of the story goes like this:Memory is only one form of capacity loss. It almost never occurs in anything except space use. There are other capacity loss mechanisms that can be addressed with a complete discharge. Some of the causes are so complicated, I don't have a clear understanding of them. However, I have seen many many examples of a cell or battery that performed poorly before a complete discharge, then performed well after one. There are probably two general classes of problems 1) active material becoming not-so-active, and 2)cell balance issues. The first is beyond the scope of my understanding and is, in any event, more difficult to fix. The second is extremely common, and slightly easier to fix. Cells get out of balance in a battery for a number of reasons, none of which are within your control as the end user. Some cells will be at higher or lower states of charge than others. This causes all kinds of problems like reduced capacity and reduced life. If you want to know how those things work, lemme know. Anyhow, the answer is to try to keep your battery balanced. There are two basic methods. The first involves trickle or "overcharging". When the pack is fully charged, some cells lag behind. The charger will sometimes switch to trickle mode, and this allows the lagging cells to become fully charged, while the charged cells just generate a little heat. Sounds good, right? Problem is, it's not very effective. Lots of good engineers have worked on the right kind of trickle charge to get this to work, but so far, nothing I have seen has been very effective. The second way is much more effective, but much more difficult to do. It involves discharging the pack completely, all the way down to zero volts. This is ususally accomplished by shorting the whole pack with a load of some sort. The problem with this is that some cells can be driven into reversal (see the other long post) and suffer damage. A better way to do it is to short the cells individually. This works great on an aircraft battery, but is not so easy with the type of batteries we deal with. The solution I use is to short the entire battery with a high value resistor, like 200-300 ohms. With this high value, the discharge rate is very low and can take days. A low discharge rate means that the cells that are driven into reversal will be driven into reverse gently. Oddly, a cell must show a negative voltage greater than about -.15V before hydrogen begins to form and the cell is damaged. I only use this treatment when a pack seems bad to me. I don't do it regularly. If you would like to try it, go to radio shack and get enough power resistors to give you about 250 ohms. Run your pack down on the tool, then rig up some way to connect the resistors to the battery. Check periodically with a volt meter the battery voltage. Start the clock when the pack reaches 0 - 0.05 V and let it sit with the resistors for another 12 to 24 hours. Charge normally and away you go.Allen,I don't think you can charge too often. You can certainly charge too long, but probably not too often.Joe and Mike and Tom,Trickle charging is a not bad thing up to about 24 or 48 hours. Beyond that, I would say it's a bad thing. The easiest way to tell if your charger is trickle charging is to feel the pack. If it's warm, it's charging. This subject is debated at the highest levels in the industry, so you will probably get multiple answers from multiple experts. This is my opinion base on my experience. Personally, I watch until the light on my DeWalt charged says it's charged, then take the battery off. Once in a great while, I will leave a battery on overnight. Specific to DeWalt, I noticed a contractor with his battery charging. Out of curiosity, I touched it. It was REALLY hot. The poor guy thought this was normal and wondered why his batteries went bad so quickly. That's just one instance, but I'd be wary if I were a DeWalt owner.I've often wondered if this kind of information might be useful to a magazine like FHB or FWW. I have no clue how to go about submitting, but if someone here knows, please let me know.
*Andy-I'm with Chris on this one. If you can't get Tim to write an article, this definitely rates inclusion in Best of Breaktime.Thanks again Tim. Great info! Mike
*Hey Fellas,DeWalt recalled most of their chargers because they were faulty. As Tim noticed about his frined's, they were overheating, and in some cases caused a few fires. This overcharging/overheating problem caused a few fatal fires before they decided to recall them. The overcharging/overheating also ruined quite a few batteries. If you have one of the older DeWalt chargers, and notice that your batteries are not lasting very long (before they become garbage), then you may very well have one of the recalled models.And why doesn't the manufacturer give more detailed info regarding the charging cycles and methods for their batteries? Two reasons. First, most owner manuals are written overseas. They are translated into English, and sometimes they don't get it all correct. Other times they simply try to keep the in fo as simple as possible. Second reason, the manufacturer also sells batteries. Batteries are big business. If they tell you how to keep the batteries lasting a long, long time, then they couldn't sell as many batteries to you. One example of how manufacturers are trying to stay ahead of the aftermarket battery companies - they are starting to change the way the battery fits their products. Sometimes it is a simple, small change that makes aftermarket batteries not fit as well, or not fit at all. Other ways are to completely change the battery numbering system, confuse the consumer, and then tell ya that the only CORRECT battery to use is their battery. Some have started to include in their warranty info that you have to use THEIR brand of battery to keep the warranty intact.Great info Tim. James DuHamel
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Just bought my first cordless power tool (a B&D 14.4 v drill). What are th rule of thumb for maximizing battery life/performance? (run the batteries way, way down the first couple times? Do not ever recharge until they are fairly dead? Charge the batteries up after every use even if not run down much?). No such guidelines included in the users manual. Thanks