Batteries
Q. How is the voltage of a pack determined?
A battery pack consists of a number of cells, wired in series. Therefore, the voltage for the pack is equal to the number of cells times 1.2 volts (Nicad cells provide 1.2 volts of electricity). However, because of a cell's internal resistance, the actual voltage you are getting is slightly lower; more like 1.1 volts per cell or even down to 1 volt per cell in the higher current installations.
Q. What are milli-amp hours?
The milli-amp hour is the standard unit of storage capacity for a cell.
It is analogous to "gallons of fuel" for an internal combustion engine. The milli-amp hour rating of a cell tells how many constant milli-amps of current can be supplied by the pack for one hour. This rating can be used to find the duration that a battery pack can provide given a certain draw.
Because cells are wired in series, the milli-amp hour rating of a pack is the same as the milli-amp hour rating of a single cell.
Q. How fast can I charge my batteries?
Different cells can withstand different charging rates. Check with the manufacturer to make certain you don't damage your pack.
For fast charging, most packs can be safely charged in 15 minutes, which requires a charging current of 4 times the capacity of the pack.
Slow charging is usually done at a rate of 1/10th the capacity, or C/10. Cell manufacturers list this as the charge rate in which the cells will not vent (release gases that build up from overcharging). However, even this charge rate can reduce the life expectancy of a cell if left on after the cell is fully charged.
Q. What is Nicad memory?
Besides a hot topic of debate, you mean?
Nicad memory is commonly explained as a loss of cell or pack capacity after repeated charges and discharges to the same level. It is actually a voltage depression in the cell that causes it to appear as if the cell isn't charged. The cause of memory is beyond my capacity to understand or explain, but suffice to say that repeated charges and discharges to the same level have been reproducibly shown by battery makers to cause voltage depression. Therefore, Nicad memory is a very real phenomenon. How often it occurs is a subject of much debate. Don't bring it up unless you are prepared.
To avoid Nicad memory, simply fast charge your batteries regularly and provide an occasional deep discharge to 1.0 volts/cell under a light load. Also, avoid trickle charging batteries for long periods. Fast charging negates the effect of Nicad memory.
GE did a study on Nicad memory and concluded that memory cannot occur if any one of the following conditions are met:
1. Batteries achieve full overcharge (peak charge)
2. Discharge is not exactly the same each cycle- plus or minus 2-3%
3. Discharge is to less than 1.0 volt per cell.
Much of this information comes from the Nicad faq referenced in the Internet Resources section of this faq.
Q. What is Cell reversal?
From the nicad faq:
"In a battery, not all cells are created equal. One will be weaker than the others. So, as the battery is discharged, the weakest cell will use up all its active material. Now, as discharge continues, the current through the dead cell becomes a charging current, except that it is reversed. So, now reduction is occurring at the positive terminal. As there is no more nickelic hydroxide, it reduces the water, and produces hydrogen. Cell pressure builds, and it vents. The cell has lost water and the life of the cell has been shortened
"This is the big danger of battery cycling to prevent memory. Invariably, unless one is very careful, one ends up reversing a cell. It does much more harm than the cycling does good. Also, keep in mind that cells do have a finite life. Each cycle is a bit of life."
Q. Should I cycle my packs?
Weigh the dangers of cell reversal versus the dangers of Nicad memory and decide for yourself.
Some people discharge their packs to 0 volts per cell and say they have never had a problem. Others say that cycling below 1.0 volt is damaging. I have never witnessed Nicad memory, but I have never witnessed cell reversal either. Use your best judgement.
Q. Can I deep discharge an individual cell safely?
Individual cells (i.e. NOT IN PACKS) can be discharged to 0 volts per cell safely. Cell reversal can't occur with individual cells. In fact, cycling an individual cell is a good way to determine its exact capacity. This is how packs are "matched".
Q. What is the discharge of a Nicad like?
Well, look at the following graph and you'll get an idea.
Time
The graph tries to show that a Nicad provides an initial surge of power (at around 1.2 volts or higher), then provides a pretty much constant number of volts until its capacity is almost entirely depleted. This means that the voltage level of a cell is NOT proportional to remaining charge. By the time a cell reaches 1.0 volt, it is almost entirely discharged.
Q. What is "blackwire" on the negative leads of nicad battery packs?
The Black Wire Syndrome
The black wire syndrome is an occurance in battery packs (Ni-Cds) where the negative wire becomes corroded (turns from shinny copper to blue-black). This is the result of either a shorted cell in the pack, the normal wearout failure mode of Ni-Cds, or cell reversal when a pack is left under load for an extended period. The sealing mechanism of a Ni-Cd cell depends to some degree on maintaining a potential across the seal interface. Once this potential goes to zero the cell undergoes what is called creep leakage. With other cells in a pack at some potential above zero, the leakage (electrolyte) is "driven" along the negative lead. It can travel for some distance making the wire impossible to solder and at the same time greatly reducing its ability to carry current and even worse, makes the wire somewhat brittle.
A switch left on in a plane or transmitter for several months can cause this creepage to go all the way to the switch itself, destroying the battery lead as well as the switch harness. There is no cure. The effected lead, connector, switch harness must be replaced. This leakage creep takes time so periodic inspection of the packs, making sure that there are no shorted cells insures against the problem.
The cells should also be inspected for any evidence of white powder (electrolyte mixed with carbondioxide in the air to form potassium carbonate). In humid conditions this can revert back to mobile electrolyte free to creep along the negative lead. Some "salting" as this white powder is referred to, does not necessarily mean that the cell has leaked. There may have been some slight amount of residual electrolyte left on the cell during the manufacturing process. This can be removed with simple household vinegar and then washed with water after which it is dried by applying a little warmth from your heat gun..
Q. How do I match cells without spending a fortune on expensive equipment?
A. Since few of us can afford a turbo matcher and it really isn't that important for sport flying here is what I do to make semi- matched packs from inexpensive cells.
The lower tech way requires only a DVM and your regular charging gear plus a notepad.
1) Check the individual cell voltage with a DVM. If the cell is at Zero volts then it will probably never be any good for any use.
2) Discharge the individual cells to <0.5 volts. I use a 0.5 ohm power resistor for this but you can use around 30 ohms overnight.
3) build a pack so that cells can be easily changed out. I like 7 cells as a max for this as too many cells makes it hard to do some of the testing.
4) Charge the pack at c/10 for 24 hours. The cells should be warm at the end of this period.
5) Discharge the pack into a load. A motor with a light load works pretty well. 5-10 amps is a good enough load. Watch the voltage of each cell. You are looking for cells that have lower voltage than the others. Keep an eye on it and when it starts to slow down figure out which cells died first. Mark those some way. Stop discharging when one cell drops below 0.5 volts. I usually put a little number on the cell to tell me which ones died in which order.
6) Charge the pack at c/10 for 16 hours. repeat 5 and 6 a few times. note any change in cell order. Sometimes after exercising the cells a few times the worst ones become good. Change out any weak cells and start over. After a few iterations you will have a pack that with cells that will dump close to each other.
7) Fast charge the pack at a 3c or even 4c rate. Touch the cells often during the charge to see if any cells get hot during charge. If all cells get hot then reduce the charge rate. If only one or two you might want to swap out those cells and start over. The cells you pull out are probably fine for the most part. You can generally make a pack out of the culls that works fine but has slightly reduced capacity. I like to take these cells and charge them up and let them sit a couple of weeks. If they still have nearly a full charge I make 4 and 5 cell receiver packs out of them and sell the packs to the 1/4 scale gas guys. They love them cause I only charge $15 for such a pack and they last for years. If you have a charger like the 110d or the 112d that will tell you the energy put into a cell you can do this a different way. This is a slightly higher tech way but requires a smarter charger. You can substitute the charge time for the amp hours figure if your charger displays that.
Another method:
1) Discharge the cell to zero volts. I use a 0.5 ohm power resistor. a 25 to 30 ohm resistor overnight will do this as well.
2) Charge the cell in the peak detecting charger at a 3c rate. Use the same rate for all cells of a given type. Watch the temp of the cell and if it gets hot during the charge cull that one out. When the cell peaks you need to record the peak voltage and the ah that went into the cell.
3) Repeat 1 and 2 a couple of times. The values you get will become fairly consistant unless the temperature changes a lot.
4) Select cells with similar capacities and peak voltages in that order. The cells with the lowest peak voltage are the better cells. They will have the higher voltage under load. You learn a lot about batteries when you play with them like this. I hope this helps!
Q. What does "C" mean?
A.
"C" is specified as the discharge rate which will discharge a fully charged battery in one hour. It is the same number as the cell capacity but in milli-amps (mA) i.e. for a 500 maH cell, the C rate is 500 mA, for a 1700 mAH cell C is 1700 mA (or 1.7A). It is also convenient to give charging rates in terms of C. For example, the standard slow charge rate for all Ni-Cds is C/10 (one tenth of C). For an 800mAH battery that's 80mA. Fast charge rates are around 3C or 4C.
Note that charging a battery is not completely efficient. You have to put around 30-50% more energy into a battery than you will get out. So the charge time at C/10 is 14-16 hours not the 10 hours you might think from the basic arithmetic.
Q. What are NiMH batteries like?
A.
NiMH (Nickel Metal Hydride) cells are similar to the more familiar Nicads in many ways. A cell has a voltage of 1.2V and they are made in the same range of sizes. Compared with Nicads, NiMH cells of the same size and weight generally have a higher capacity (mAH) but cannot be charged or discharged quite so fast. They have a higher internal resistance so they will not deliver as much current as Nicads. They can be very useful for sport flying where good duration is preferred to the ultimate in power and they are particularly useful for indoor and slow flight when their relatively light weight for capacity is prized.
Like all battery technologies NiMH cells are improving all the time. At the time of writing the best of the bunch are the new 3000mAH cells in Sub-C size, capable of currents over 30A.
Q. Can I use Lithium batteries for electric flight?
A.
First a warning. There are a number of different types of batteries with Lithium in the name. Of these, rechargeable Lithium Metal cells are the most practical for use in electric flight at the time of writing (Aug 2000). The most other types of lithium battery technology are either too fragile, unable to deliver high currents or not easily rechargeable though some people are apparently successfully using rechargeable Lithium Ion cells despite the fact that they need very special charging conditions. Be careful what you are buying. You should check that you are getting RECHARGEABLE Lithium Metal cells, some Lithium Metal cells are primary cells, i.e. non-rechargeable.
Rechargeable Lithium Metal cells, made by Tadiran, have a very high energy density i.e. high WattHours for very low weight. When reading the mAH rating for these cells remember that they deliver 3V per cell rather than the 1.2V of Nicad/NiMH. However they can only deliver low currents (up to about 3A) and they require specialised chargers. They cannot be charged with standard Nicad chargers and are very easily destroyed if charged incorrectly. They also have a shorter life, typically 100 charge/discharge cycles rather than 500/1000 for Nicads. Their main use is for very lightweight indoor models typically using the small coreless motors sold for that purpose.
Some people have indicated that rechargeable Lithium Ion cells can also be used with care. They deliver around 3.6V per cell. Since they will not deliver very high currents they are sometimes used in series/parallel configuration e.g. 2 series sets of 3 parallel cells will give 7.2V and current draw up to about 10A, enough for very long durations with a Speed 400 size motor. Like Lithium Metal cells these must not be charged using a standard Nicad charger. They have an extremely poisonous organic electrolyte and can explode if charged incorrectly. They require a constant voltage charge upto about 80% capacity followed by a tapering current charge for the remaining 20%. Because they are so difficult to handle they are never sold as individual cells only in ready made packs for laptops, camcorders etc with the necessary safety devices built into the pack. You should not pull these packs apart unless you are confident that you understand the likely problems and are willing to take responsibility for any accidents.
Q. What do the battery codes like SCR, AE mean?
a. Steve Lewin
The following coding is specific to Sanyo Nicad cells though most of the major manufacturers use very similar codes.
The first few characters in the cell name, eg "KR" or "N", represent different types of internal construction. Cells coded "RC" are actually "N" type cells optimised for RC use, eg RC2000, RC2400.
"N" and "RC" cells have a lower internal resistance than an equivalent capacity "KR" cell. This makes them more suitable for our fast charge/high discharge-rate type of application. The low internal resistance means less heating of the battery and therefore less wasted power.
The numbers give the cell capacity in mAH e.g. 1000, 1700.
The letters immediately following describe the case diameter.
(Increased capacity is obtained by making the cell longer.) Case Code Diameter Std Length
AAA 10mm 45mm
AA 14mm 50mm
A/AF 17mm 50mm
SC 23mm (sub-C) 43mm
C 26mm 46mm
D 34mm 58mm
The final character/s R, E, K, etc represent the cell performance or capability. Of these, it is "R" which is normally of most interest to us as this means the cell is suited to rapid charge. To achieve this the cells generally have a lower internal resistance and are also suited to high discharge rates. Some "E" cells are also used, e.g. 600AE, "E" standing for extended capacity.
Thus 1700SCR = 1700mAH in a sub-C Case and is an "R" type (high rate) cell.
1500AE= 1500mAH in an "A" case and is a E-high capacity cell.
Generally a specific RCxxxx cell is roughly equivalent to NxxxxSCR i.e. RC2400 would be effectively the same as an N2400SCR (if that cell existed).
A listing of most of the currently available Sanyo NiCd cells can be found on Sanyo's own website at Battery Data.
Panasonic NiCd codes are almost identical though they have an additional performance code of "P" which is the one most useful to us. P-type cells are specifically intended for both rapid charge and high discharge (they are designed for rapid recharge power tool use). The complete specifications for all Panasonic batteries is available on-line at the Panasonic OEM Battery site
Q. Is it safe to solder directly to batteries?
It depends who you listen to. In the safety instructions of every major manufacturer of Nicad and NiMH cells that I have seen there is a very definite instruction that you should never solder directly to the cells. The reason for this is that the cell has a number of components which are made of plastics and which can be damaged by excessive heat. In particular there is a vent designed to allow excess hydrogen to be safely removed from the cell. If you damage the vent, oxygen from the air can enter the cell and the internal chemistry will be severely degraded, dramatically reducing the life of the cell.
However these same manufacturers will tell you that you must not charge a cell in less than an hour and you must not discharge at higher than 15C (7.5A absolute maximum for 500AR cells !). By their standards we are already abusing their cells horribly.
Most of us do not have access to the spot welding machines recommended by the manufacurers. Most cells available to us with spot welded tags have only a few welds and thin tags, usually of the wrong material. They will not take anywhere near the currents we require. If you have access to battery packs made with thick pure nickel tags and at least 6, preferably 8 welds per connection these are probably the best of all possible worlds. For most of us the only method available for constructing packs is soldering.
To make soldering as safe as possible you must use a HOT iron with a large heavy tip so that it keeps its temperature when you put it in contact with the cool cell. 40Watts is the practical minimum. The objective is to have the iron contacting the cell for as short a time as possible. NOTE : soldering guns are bad for this use because they have very small tips and cool down too much meaning that you hold them in contact for far too long giving the heat time to spread to all the plastic parts. Each soldering operation should ideally take less than 2 seconds. 5 seconds is about the limit. Clean everything well and degrease carefully. First tin the contact and the wire or bar separately. Then let them cool. Then reheat the wire, then the cell and put them together. Hold them in contact until the solder has solidified, which only takes a few seconds. Practise with some scrap cells until you are comfortable that you can do each operation as quickly as possible. |
