I was just looking through the Mar/Apr issue of Wood magazine, where they did an article on myths of rechargable batteries. One thing they said really confused me - they said that rechargable batteries should NOT be discharged completely, and that you should recharge them as soon as you feel your tool slowing down, because if a cell is totally discharged it can switch polarity and ruin the whole battery (or something like that). I'd always heard that with NiCad batteries, you DO want to discharge them completely, so they don't develop a 'memory' and accept less of a charge each cycle. Does anyone have a good explanation for why one or the other is true, or firsthand experience trying it both ways? Also, they said that NiMH batteries are really not superior, because even though they can have larger amp-hour ratings, they don't last for as many charge/discharge cycles. I'd also read elsewhere that NiMH batteries don't develop a memory, which seems to me like it would make them last longer. Has anyone used both types side-by-side through the whole life of a battery? TIA, Andy
Well, it's not about incomplete discharge, it's about frequent discharge to the same level of discharge.
Well, it depends.
It depends on what you're using it for. A cellphone where you throw it on the charger when it beeps, you probably want the nickel metal hydride, because a NiCd will (if memory is real - cue debate on that now) eventually consider anything below that point not to be there.
It's complicated. Basically, given a choice, I'll go with the Nickel Metal Hydride, but often it's a case of "Here's what battery works with this" and that's your choice.
NiCad battery memory problem can only occur under special circumstances, which never occur in how any one uses NiCads, so don't worry about it.
You do want to discharge a cell, but not reverse the polarity. A battery has many cells. Take the battery voltage and divide by
1.3 to get the approximate number of cells. If you have a 14V battery pack, you do not want the voltage to drop more then 1.3 Volts. If more than that, you have a good chance of reversing the polarity on one of the approximately 10 cells.
Nicads have a fairly constance Voltage until they reach the discharge level, then the Voltage drops quickly. If you start the recharge just as the Voltage drops, i.e. when the tool starts to slow, then you recharge when the battery has mostly discharged.
People have done side-by-side testing. Let's hope they reply to your posting with the results or references to the results.
NiMH batteries are good for *fewer* charge/discharge cycles than NiCad. you get more watt-hours in less space, but pay for it with reduced cycles. I'm don't know just where it works out in terms of 'lifetime" watt-hours. I suspect that NiMH are a tad more expensive per lifetime watt-hour.
*early* NiCad batteries did have a 'memory effect'. If you're dealing with batteries manufactured in the last 15 years (at least), it is a _non-issue_, due to changes in battery design.
Running NiCads MULTI-CELL BATTERIES _all_the_way_dead_ is a bad idea, for the reasons mentioned in your posting. When dealing with SINGLE CELLS (1.2V units) running them all the way down is not harmful, and can extend lifetime _somewhat_.
NiCads have a fairly sharp 'knee' in the output curve -- when output starts to fall off, they are "quite close" to being totally run-down.
This is also what I have heard. I have no real proof or reference, but my understanding is the whole memory thing was a real problem back in the '80's or when ever NiCad's first came out. The manufactures kind of quietly fixed the problem years ago, but a most of us that can remember when Nissan was Datsun just can't get it out of our heads, so the problem now lies in the memory of the user, not the battery. As others have said, recharge both NiMH and NiCad as soon as they start to slow down. - Or so I have been told.
Everything I've read in the last couple years said that the memory effect was not a factor in any nicad batteries anymore. What is bad is heat, and letting them discharge completey before recharging makes them get hotter during recharging, thereby lowering their longevity.
I don't buy that. They STILL have a memory effect, but perhaps over many more cycles than once before.
The memory is due to crystals forming between the electrodes each time the battery is discharges, recharged. They close the gap between the electrods, thus reducing it's capacity.
I've renewed run down (and newer NiCad) batts for my cordless phone by "shocking" them with 12V (they're 3.6v packs) for a second or two. The crystals burn off.
Larry, what is the full technique? Does one apply the shocker to the positive, the negative, or both? Should I use the car battery or one of those little AC transformers?
Really. Then what does the NiCd battery rejuvinator, sold by Physio-Control, actually do? Because before rejuvinating a NiCd defibrillator battery, it gives you the capacity of it, then it does it's cycles protocol, and measures the capacity after the rejuve, and the capacity has improved.
I don't think it's snake oil, what with being certified by the FDA as a piece of test equipment for calibration of medical devices, and all. Maybe they know something about NiCd batteries that you do not?
I think that articles such as this serve to feed the raging debate over batteries. I'll express an opinion based on personal opinion. I am not an expert by any means. I am an electrical engineer who has had to work with batteries at various points of my career. Battery chemistry and design is a complex matter. Its not as simple as classifying batteries into Nicad and Nimh and LiIon. The spectrum of design characteristics supports a broad swath of appplication requirements. Though Nimh and LiIon have become prominent in consumer electronics, you still find a lot of Nicad in heavy industrial applications because it works better.
I used to do all kinds cycles and incantations and records on my batteries based on my "expert" knowledge. Today I don't do any of that. I throw it in the charger and swap when something gets low (cordless drill, hand held tape recorder or whatever). I don't pay any attention to rules or times or myths. People basically get really ill when they have to replace the first batteries on their cordless drill because the price is a real shock. When you buy a cordless drill with two batteries, about 60-75% of the cost is the batteries. I'm waiting until the pair of batteries on my Dewalt drill are completely useless. Than I will buy a whole new drill. In the meantime, I just swap batteries everytime I start a new project in the shop - sometimes daily. It works fine for me.
FYI: Cell reversal is caused when a cell in a pack has a lower capacity than the other cells and that cell reaches 0 volts before the others. The other cells in the pack then pump current backwards through the depleted cell causing the reversed voltage.
The NICAD memory effect, according to the history/legend that I heard, dates to the late 50s and the Vangard space program. the US lauched the Vanguard satelites into orbits that maintained precisely the same amount of time time (to the second) in the sun, charging the batteries, and in the dark, discharging the batteries. Thus, the batteries were charged/discharged to exactly, and I mean exactly, the same point for their entire life time. They developed a memory.
So there are 2 conditions for memory.
You must be using ancient NICADs (I think someone else pointed this out)
You must discharge precisely to the same point each time.
Using todays hand tools in normal everyday use, you are unlikely to achieve these conditions.
Now, there is an effect called charge depression, or capacity depression where the discharge curve is lowered, not shortened. The end effect is to make it look like there is a memory. However, it is caused by overcharging and not by repeated discharging to the same point. This effect is what most people refer to when they say they fixed a capacity problem by deep cycling a battery.
Also, some cells develop large crystals on their plates that can grow large enough to puncture the insulating separator between the two plates of the cell. This shorts out the cell and unless you can blow out the short the battery pack will exhibit reduced voltage and will appear to have reduced capacity. Pulse chargers, where high current for brief periods of time (pulses) are used to charge the battery, are sometimes good at blowing out the short but you have to realize, once that insulator is punctured, the cell is essentially destroyed. you may be able to bring it back to life by zapping it but the cell essentially self discharges and the crystals grow back. You might as well buy a new pack rather than fool around with zapping cells. Or at least start saving your pennies because you're going to have to buy a new pack sooner rather than later.
BTW, in order to effectively zap cells, you need a really high current source. the best way that I found is to connect another fully charged nicad cell across the shorted cell. positive to positive, negative to negative. I'm talking cells here, not the entire battery. The charged cell will discharge into the bad cell and each cell should end up half charged. You should monitor the voltage of the pair to see that the short really is cleared or else you're going to be dumping an extreme amount of current into a low resistance short which can only lead to bad things happening.
Obligatory warning time: zapping cells can result in dangerous buildup of hydrogen gas in the depleted cell. Be careful and monitor cell voltage to ensure the short is cleared.
On the subject of these crystals, they really only short out a cell if the cell is discharged. On a fully charged cell, they tend to blow themselves out with a current surge when thy puncture the separator. Thus you should always try to keep your packs charged, especially over the winter when they're sitting in drawers, and the incidence of shorted cells should be reduced. They will still tend to self discharge, but they will be at full voltage.
Since there are opinions being slung left and right, I'll give you mine.
First take a look at this web site
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can read the bio and there is a bunch of information there, so I won't repeat it here.
Second, my opinions:
I've been using NiCad's for 15 years flying R/C airplanes. In that time, I have never had a battery develop a memory nor have I ever had a battery fail in flight. I have made, literally, hundreds of flights and put my batteries through many charge-discharge cycles (but not to complete discharge). I cycle my batteries once a year on a modestly priced battery cycler. It discharges the batteries down to 1.1 volts per cell and then recharges them. The ONLY reason I do this is to determine the capacity of the battery pack. If my pack drops below 80% of stated capacity, it flies a bench from then on.
I believe the secret is do not leave a battery pack on constant trickle or ever let it stay completely discharged for any length of time. I wrecked a number of packs before I figured that out (using expensive battery maintenance equipment I might add). I do, however, keep them charged by charging them at least every 2 or 3 months. That is not a problem since they usually get this through normal use.
I maintain my Dewalt cordless drill in a similar manner. I don't check for capacity, but I do note how long the packs last in use (no crash possibility here). When I graduated from a 9.6 volt drill to a 12 volt drill, I passed the older Drill to my BIL. I had it 4 years and he is still using it 6 years later with the original packs. The 12 volt drill is, of course, 6 years old and still on the original packs.
this is good adice while not using my home made bike light I charge the cells once a month to keep them from getting too low. the advantage to NiMH cells is that they are lighter for the power they contain. on those bigger packs that can make a difference. so higher amp hours per ounce is a big deal when you get big packs. another advantage to NiMH cells is they die fast so you don't ge a slow rundown on your tool. Knight-Toolworks
Older and cheap NiCads will suffer the memory effect (the correct term is memory effect as they don't actually develop a memory they just show symptoms of it). As NiCads were improved the memory effect was worked on and is pretty much non existant. For example the pair of batteries that came with my 1995 makita drill I almost never fully discharged and they lasted until 2003. NiMH and Lithium ION do last fewer complete discharge cycles, however it takes many partial cycles to make a complete cycle therefore you can top them off many many times, in fact its better to charge them each time you use them rather than waiting for the drill/saw to slow. I still see people fully discharging and recharging their laptop batteries or pulling them out of the laptop and storing them then wonder why they don't last more than a year or two while my always in and topped off last 4-5 years.
Cell phones have pretty much all gone to lithium ion which gives an even better power to weight ratio than NIMH.
For all practical purposes the memory effect doesn't exist in quality ($) batteries like you would get for a cordless drill (i.e. it may show after several thousand charge/discharge cycles but the drill would be worn out by then too), the cheap nicads you get in a $19.99 cordless phone usually due from being overcharged due to having a very simple charging circuit and show the memory effect after a couple years.
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