Q. How does a NiCad cell work?
A. A typical nicad consists of plates wound into a compact roll, isolated from each other by a porous plastic separator and immersed in a caustic electrolyte solution (potassium hydroxide), Nickel hydroxide (NiOOH) is the active material in the positive plate (anode) which, during discharge, reduces to Ni(OH)2 by accepting electrons from the external circuit. When the cell becomes fully charged, the charging current is converted to heat and the cell becomes warm. A Nicad has a negative temperature voltage co-efficient so the warming of the cell results in a depression of the voltage (the “delta peak”). Delta peak chargers rely on this peak to determine when to stop the charge.

Q. What happens if a cell is overcharged?
A. When both plates are completely charged and charging is continued, hydrogen is generated by the cathode and oxygen by the anode. Gas pressure builds up in the cell. A safety vent will release gas pressure if it gets too high, but the pressure is kept partly under control by making the cathode larger than the anode. The anode becomes fully charged first, which inhibits production of hydrogen at the cathode. Thus the cell can be overcharged at a slow rate safely for long periods of time without a noticeable loss of performance. Overcharging at fast rates, however, will cause both heat and pressure to build very quickly, possibly more than the cell can handle, so it may self-destruct.

Q What do the letters on a cell (eg, “KR1800SCR”) mean?
A. The following are the designations used on Sanyo, the most popular brand of nicad cells:
"KR" means a standard cell. "N" means a cell for general use. The number which follows refers, of course, to the capacity of the cell in milliamp-hours (mah). An 1800 mah cell means it can (theoretically) be discharged for 1 hour at a current of 1800 milliamps (ma).
We can apply multipliers and dividers to that, therefore 1800 ma can be discharged for 1/2 hour at 3600 ma (3.6 A), or 2 hours at 900 ma, or 4 hours at 450 ma - and so on. Later when we get to talking about charge rates, the capacity of a cell will be referred to in amp-hours (C).The letters following the capacity refer to the physical size of the cell - or more specifically to its diameter. All AAA cells have the same diameter but not necessarily the same length. Similarly all AA cells are the same diameter as are all A cells, etc.
"SC" refers to sub-C size cells. There are also C and D size cells, but these are not much used in radio control except for lighting glow plugs.
The final letters refer to the type (or construction) of the cell. "R" represents rapid charge/discharge type, "E" means an extended capacity cell, "C" represents ultra polypropylene (even further capacity).
Therefore a cell marked KR1800SCR is a standard cell of 1800 milliamp capacity, sub-C size, rapid charge type (as commonly used in 6/7 cell electric flight packs).
There are two other letters (eg, “EH”) stamped on Sanyo cells and these refer to their year of manufacture. However I have yet to ascertain the code they use.

Q. What is “memory” effect?
A. Memory effect refers to a loss of capacity caused by shallow discharging cells to approximately the same level time after time then re-charging them.This can occur when a model is given the same number of flights week after week. There is a lot of  controversy over memory effect. Some say it is a myth and that most cases of a battery losing its capacity are due to a single cell in the pack weakening or failing causing a drop in voltage of the pack as a whole. Others insist it does exist and offer complicated technical explanations to support their beliefs. I believe it’s better to be on the safe side and suggest you deep discharge your nicad packs using a purpose made cycler or simply by leaving your aircraft’s switch on for 5 or 6hours every couple of months, then charge the pack for 14-16 hours.

Q. What is the effect of heat and cold on cell capacity?
A. Both hot and cold environments will reduce the capacity of a nicad pack. At 40 degrees celsius a pack can lose 40% of its capacity - and it doesn’t take long for a nicad pack wrapped in foam in the interior of an aircraft to reach 40 degrees on a hot summer’s day. Furthermore a hot battery does not exhibit much of a delta peak, so if you try to top up a hot battery with a peak charger, the charger may not sense the peak and may over-charge. Electric flight batteries which heat up in flight due to the high rate of discharge should be allowed to cool before re-charging. The top electric flyers use Eskys for this purpose.

Q. What charge rates should I use for nicad packs?
A. For battery packs with standard (KR) or extended capacity (E) the safest charge rate is the C/10 or “overnight” rate (eg, 60 ma charge rate for a 600 ma battery). A full charge from a discharged state will take 14-16 hours. Even if left on charge for considerably longer, the pack will not heat up enough to be damaged. Also, if the cells of a pack start out at different states of charge (often the case if stored for a time) this charge rate will bring all cells up to a common state of full charge without damaging the cells which became charged first.
Delta peak and “burp”  chargers have become very popular. Standard nicad cells can be safely charged at rates up to  2.5C, i.e., 1.5A for a 600mah pack. At high charge rates a cell which has started out at a higher state of charge will charge first and overheat as the other cells approach full charge, and this can damage the pack. If you are using fast charge rates it is a good practice to use an overnight C/10 rate about every third or fourth charge to ensure the cells don’t become mismatched. The first charges after long-term storage and the first charge of a brand-new pack should always be at the slow rate.
Rapid charge (R-type) cells have lower internal resistances and can handle much higher charge rates. Electric flyers charge them at rates of 5C or more without noticable deterioration (at 5C a pack can be fully charged in 15 minutes). Even with R-type cells it is a good idea to charge them at the overnight (C/10) rate when new and the first time after long storage.

Q. How do I know what state of charge my batteries are in?
A. There is no real way of measuring the state of charge of a pack without completely discharging it. Voltage measurements can provide a rough indication, but can also result in big errors and lead to a false sense of security. For example you could measure the voltage of a pack which was originally flat but been has been on charge at the C/10 charge rate for - say - 1/2 hour. It would probably show 5.4 volts and lead you to believe it is fully charged, But it’s certainly not - it wouldn’t hold that voltage for more than a minute or so - especially under load. If you are really keen you could determine how much flying time you get from fully-charged packs. Then keep track of how long you fly at a flying session (approximately) and estimate the amount of full charge you have used up. Then charge the proportionate amount before the next flying session. If in doubt fully cycle (discharge then charge) the packs.

Q. What about trickle charging?
A. Yes, you can keep your transmitter and flight packs charged between flying sessions by trickle charging. The best method is by pulse charging, which helps to prevent the formation of calcium bridges across the plates of the cells. You can set up a pulse charger at very low cost by using the plug-pack charger that came with your radio. Simply buy a cheap appliance timer and plug the charger into it. Set the timer so that it is on for 1 hour per day. If you have had a full day of flying, then when you get home set the timer so that the on/off triggers come up in about 14 hours time and hook up the batteries so that the timer is on. In about 14 hours the timer will shut off the charger and therafter it will switch it on for 1 hour per day. Cells on trickle charge are always ready for immediate use.

Q. What causes a nicad to fail?
A Nicad cells can be damaged by heat, physical damage, and physical and chemical aging. There are three main failure modes. The first is open circuit failure. This may be a mechanical break in the internal connections, or a loss of electrolyte through the vent in the cell evidenced by a white powder which accumulates around the positive terminal. The next is short circuiting, caused by either a failure of the separator to prevent the plates from touching one another, or more commonly cadmium migration forming a crystal or dentrite across the separator. Finally a cell can just become worn out. It becomes leaky, won’t fully charge, loses capacity, self discharges rapidly, often develops a grayish powder around its terminal.

Q. Assuming there’s no visible damage or corrosion, how can I tell if my packs are still good?
A. The best way is to invest in a good cycler and keep records of your packs’ performance. Capacity measurements should be made after running the battery through several cycles. Keep records of each pack’s measured capacity when it was new, and after long-term storage, and watch out for any degradation in performance.
A degradation of more than 20% of rated capacity should be suspect. I find checking my batteries at the field during the day with a digital voltmeter which shows voltage under a typical load is a good idea. As a result I have not had a crash due to battery failure in 20 years of flying.

Q What is black wire corrosion?
A. If you examine an old TX or RX pack which has lost its punch you may find the wire coming from the negative terminal is corroded, possibly black in colour, brittle and impossible to solder. It is usually corroded for its whole length, from the negative terminal right up to the switch or plug it is connected to. The negative pin on the plug may also have a green verdigris-like deposit. It is only the negative wire which suffers from this problem. The cause of black wire corrosion has been the subject of much conjecture over the years, but the most plausable explanation is the escape of material from the inside of the cell, probably the potassium hydroxide electrolyte, through the bottom of the cell casing. However the cause of black wire corrosion isn’t important - just examine your batteries occasionally to ensure they are not suffering from it. If you find black wire corrosion in the negative lead from a pack, the pack has “had it” and should be discarded. It is useless trying to press it back into service by soldering on a new lead and plug.

* Adapted from articles by Roy Bourke in “SAM Speaks”, the journal of The Society of Antique Modellers. References: “The Quiet Revolution” and “Electric Motor Handbook” by Robert J Boucher plus other publications.