NiMH charging question

[JohnnyDo]

Originally Posted by W.K.Shuridys

Alright, there is a lot of bad information in this thread that needs to be corrected.

First of all, for all practical purposes there is no such thing as memory. What everyone likes to call memory is actually voltage depression and is NOT from failing to discharge your batteries fully before recharging. If anything, discharging your battery is a needless task that in fact harms your cells by reducing their remaining cycle count. Furthermore, discharging your batteries past 0.9V/cell will cause them to go into reversal which REALLY destroys them, and is probably the cause of most nickel-based battery failure.

You are quite wrong on the point about discharging bellow 0.9v/cell. Cell reversal happens when a cell in a pack is at 0v but other cells are at a higher voltage thus shorting out and causing voltage reversal on that perticular cell. The 0.9v/cell theory is to average out the fact that at this point the cells int eh pack are approx 99% discharged and on average each cell in the pack is ready to be charged up again. Eg. in a 7 cell pack, the individual cell voltages *may* be at : 1.0, 0.9, 0.9, 0.7, 0.8, 1.0, 1.0. This total value = a pack voltage of 6.3v with a "average" cell voltage of 0.9v. Given the above example of cell voltages, if you were to discharge the pack so that cell #4 (0.7v) went down to 0v the voltage of the rest of the cells would be: 0.3, 0.2, 0.2, 0.0, 0.1, 0.3, 0.3. now if you continued to discharge the pack the cell which is currently at 0.0v will go into voltage reversale and be shorted out to the other cells in series with it. It is at this point and ONLY at this point that cell reversal happens. Bottom line is that each cell is unique and during it's life will give slightly different perofrmance results then the other cells in the pack, this is why there is a slight difference in voltage levels during discharging and why the 0.9v/cell theory is used as it protects the average Joe Consumer from discharging the pack past a point where one or more cells in the pack go into voltage reversale.

I won't comment on the "memory" issue as it's very much an open debate to alot of people including myself. Any long term R/C racer has experianced it and why they do their best to look after their $200 matched battery pack. I will leave the subject at that.

Originally Posted by W.K.Shuridys

Also bad, contrary to Illusion's post, leaving your pack connected after it's charged IS harmful to your cells and you definitely SHOULD worry about unplugging your charger at the proper time. A wall wart is simply a transformer that supplies a fixed amount of current and voltage and has no peak detection circuitry to tell it when the battery is full. What happens is, as explained by the following link, the chemistry of the nickelic hydroxide changes from the beta to the gamma form on the positive plate which has a potential of some 40-50 mV less than the beta form. That means in an 8.4V pack, you are losing some 300-400 mV total from your pack. Charging with a trickle charger encourages this.

This is true only *IF* your charger does not have a trickle charge built in that will put through a very small ammount of current. If your charger does not have this feature and continues to charge the pack at say 300ma, then yes over charge will happen and dammage will occure. Trickle charging is useually C/100 which is 1/100th the capasity of the cells used. Charging at this low rate will keep the cells in the pack in a fully charged state without damage as it offsets the cells inhearant discharge rate. Even some battery manufacture spec sheets I have seen has specificied trickle charge currents (usually around the 20 - 30ma area). Of course this is dependant on your battery's and is in no way LAW.

Now would I keep my battery packs on a trickle charge 24/7. Generally no. It's a pain becuase I'd have to have a charger to each of my packs. It's much much easyer to discharge my packs down to 0.9v and store them in that state then charge them before I need them. This will guarentee I don't experiance the memory effect (see web link in previous post), and this will keep my pack fresh and fully charged when I need it. It is also good for NiCad cells be cycled every once in awhile for best performance. Cycling a NiCad will degrade it's long term life cycle slightly, but overall it improves it's performance. To me this is a worthwile trade off.

[Gryphon]

Good reply, although to be fair I was oversimplifying the explanation of the cell reversal phenomenon for the average reader. As long as they know enough to keep their cells above 0.9-1.0V they should be fine.

As for memory effect, General Electric (whom I trust to be something of an authority on matters electrical) has stated they are unable to reproduce ACTUAL memory effect outside of strictly controlled aerospace applications where NiCds were charged and discharged to EXACTLY the same points time and again. Consumer batteries indeed suffer various types of failure but thus far none have been attributed to memory.

Battery chemistry is incredibly complicated, and from my limited knowledge it seems that another common mode of failure attributed to memory is dendrite formation on the positive plate. If a dendrite grows to sufficient length and penetrates the adjacent separator it will short out the cell, obviously rendering it ineffective. Zapping the cell with high voltage/current will destroy the dendrite although if the separator is penetrated, it does little good as the capacity will be severely reduced, and another dendrite will likely be forming soon anyhow. Even if the separator isn't penetrated obviously a large dendrite won't have as high a surface area as small crystals so the cell is rendered less efficient.

I know that the new modern wonder-NiMHs have a recommended charge rate of some 5 to 6 amps, and the Intellect series I'm told actually PREFER heat while charging otherwise they will not gain full capacity. This goes against all conventional wisdom regarding charging as far as I know. I find this to be a fascinating subject but sadly there is a lot of misinformation or unspecific data available on the net. Sites like Battery University are a good start but are geared more towards the average user than someone who would like to study the subject in detail.

I wonder if there are any good textbooks devoted to the subject?

[ILLusion]

Originally Posted by W.K.Shuridys

I took that to mean you were saying it's alright to leave it plugged in and not have to wory about disconnecting it on time. I apologize if I was mistaken but that seems to be how it's worded.

Ah, no, I meant it was okay for him to unplug it before it was fully charged.

Because his original question was along the lines of "is it okay to unplug it before it's done charging."

My answer was to "not worry about unplugging."

[GMTII]

1

[JohnnyDo]

Originally Posted by W.K.Shuridys

Battery chemistry is incredibly complicated, and from my limited knowledge it seems that another common mode of failure attributed to memory is dendrite formation on the positive plate. If a dendrite grows to sufficient length and penetrates the adjacent separator it will short out the cell, obviously rendering it ineffective. Zapping the cell with high voltage/current will destroy the dendrite although if the separator is penetrated, it does little good as the capacity will be severely reduced, and another dendrite will likely be forming soon anyhow. Even if the separator isn't penetrated obviously a large dendrite won't have as high a surface area as small crystals so the cell is rendered less efficient.

another trick is to bang the battery on it's terminals on a hard surface such that you don't dent or damage the battery but to give it a jar to loosen up these crystals.

I think you are right in that more info in the form of a book would be helpful, but such a book would probalby be out of the scope of education and understanding for most if not all of us. I know chemistry is way outta my league. Not only that but new battery types and the chemicals manufactures use to make batteries are constantly changing and thus the rules we follow or try to follow offten get skewed from this. I personally don't see a real 100% accurate way to get out of this situation other then to do what we all currently do and make the best of what we know. In the end though I'm pretty sure the mesures we take one way or the other will not sevearly influence the performance or life of our packs past the point where the battery pack just dies becuase of use. Nicads typically only have a cycle life of what 1000 charges give or take at best.

[Gryphon]

Yeah a thousand at best, probably half that under most consumer circumstances unless they know enough to care for them to 1000 cycles.

I found this text: http://www.knovel.com/knovel2/Toc.jsp?BookID=627 Looks to be what I'm searching for although it was published in 2002, and has precious little information regarding the developments as of late in LiPos, high capacity NiMHs, etc. Sounds like a Fourth Edition is warranted.