There are several sizes of NiCads -- the smaller, lower-capacity cells such as the 1000 mAhr cells provide lighter weight (1.4 oz/cell) at the expense of short flights. Larger, higher-capacity cells such as the 1400, 1700, and now 2000 mAhr cells yield longer flights but do exact a weight penalty (1.9 oz/cell). The model type and pilot preference dictate which is most appropriate for the situation. In general, I recommend using the 1700 SCRC's (and the new 2000 SCRC's if you can find them) if your plane can support the weight. If not, the 1000 SCR's are good choices. For motors with very low current draw, like the Speed 400 motors (< 10 amps), you may wish to consider the 500-600 mAH cells (either rapid-charge, or high-capacity).
|Cell name||description||weight||internal impedance|
|KR600 AE||600 mAH, slow [dis]charge||18 g||8.5 milliOhms|
|N500 AR||500 mAH, rapid charge||19 g||9 milliOhms|
|N1000 SCR||1000 mAH, rapid charge||41 g||4.5 milliOhms|
|N1400 SCR||1400 mAH, rapid charge||53 g||4 milliOhms|
|N1700 SCRC||rapid charge||56 g||3.6 milliOhms|
|N2000 SCRC||2000 mAH, NEW rapid charge||58? g||?? milliOhms|
With gas planes, this question can be answered by knowing (1) how quickly your engine consumes fuel, and (2) the capacity of your fuel tank. With electrics, this is identical, although the exact wording is different. For electrics, the motor duration depends on (1) the amount of current your motor typically draws, and (2) the capacity of your nicad batteries.
Let's take the standard sub-C size 1400 mAh battery. The label 1400 mAhr reads "1400 milliamp-hours". That means you can draw 1400 milliamps (=1.4 amps) of current for 1 hour before the battery is depleted. Similarly... you can draw twice the current (2.8 amps) for half the time (30 minutes), four times the current (5.6 amps) for 1/4 the time (15 minutes), etc etc. Get it?
Important point! The NUMBER of nicad cells for a motor dictates the output power; the cell TYPE dictates the capacity (and the subsequent flight time).
So... knowing your batteries' capacity is like knowing the size of your fuel tank! The question now is how fast you're draining that tank! As a rule-of-thumb, most cheap tin-can motors draw 10-20 amps or so, sport cobalt motors draw 25-35 amps, and the hot FAI cobalt motors draw 40-70 amps at full throttle. To measure the current draw EXACTLY, you need to borrow or buy an ammeter.
We can now see why a throttle control is so useful -- motor run times are typically pretty short. From our example above, a hot cobalt motor drawing 30 amps will run a typical 1400 mAhr battery pack dry in about 3 minutes! But throttled back, you can get long flight times!
Nicads are not only important to the electric RC flier, we all depend quite heavily on those little 1.3 volt wonders to keep our receivers running strong. However nicad batteries remain one of the most neglected and misunderstood elements of our models.
Most transmitter and receiver packs consist of 'standard' charge cells (SC-class). Such cells are designed for an overnight (10-14 hours) charge cycle at a current rate of 1/10th their capacity. That is, a 500 mAh receiver pack may be best charged for 10-14 hours at a 50 mA current rate. A more rapid 1 hour charge may be occasionally done at a current equal to the capacity rating (for our example of the 500 mAh receiver pack, this is ~500 mA = 0.5 A).
Electric propulsion nicads must withstand torturous conditions by comparison with radio nicads. A competition-wind cobalt motor can draw as much as 70 amps from a battery pack, and we fliers demand that we be able to recharge at the car in 15 minutes or so! Since normal nicads aren't built to withstand this sort of abuse, electric fliers use rapid-charge cells (SCR-class). These cells are designed for high current charges and loads, and feature correspondingly lower internal impedances for higher power throughput. For such cells, one can readily charge the cells at a rate of 4 times the capacity in 15 minutes. For most electric nicads, which fall in the 1000 - 1700 mAh capacity range, a 4-5 amp charge rate is appropriate.
A third class of cells, called 'SCE', 'AE' or 'KR' cells, sport high capacities for suprisingly light weight. Unfortunately, these cells are usually not optimum for electrics since they have relatively high impedances and therefore cannot be rapid-charged nor discharged. They do make interesting choices for the low-current Speed 400 motors, however. They, along with the Hydrimax series of metal-hydride cells, need to be improved further to allow for reliable propulsion use in high-current electric R/C planes.
Many modelers believe that they should 'deep discharge' their nicad packs so that a nicad 'memory' doesn't develop. Although this point has some relevance (although most nicads today are fairly resistant to the 'memory' effect), one is more likely to severely damage the pack through cell-reversal by doing this. That is, if the pack isn't completely balanced, one cell will be 'lower' in charge than the others and will reverse polarity by the time the others reach zero charge. Reversal of polarity, while reversible with a a quick burst of high (5-10 amps) charge current, is very damaging to the cell and will dramatically reduce it's reliability and capacity.
For most purposes, one should discharge the pack gently to 1.1 volts/cell and then charge at an overnight rate to balance the pack. This is generally sufficient for most use, even for large electric flight packs. However, I have found that a periodic individual cell deep discharge is beneficial to really balancing a pack and diagnosing its condition. To do this, discharge to 1.1 volts/cell as before and then wire up a tiny motor or flashlight light bulb to each cell individually. Discharge each cell completely. You may find some dispersion in the charge state of some of the cells. This discrepancy will probably be 'fixed' by the deep discharge (but if it doesn't, you should probably toss out the bad cell). When you are done, resume charging as before.
I usually individually deep-discharge my flight pack cells once every 4-8 weeks and my radio packs every 6 months.
I store my nicads in a charged state, although there is no clear evidence that storing them in an idle (disconnected) discharged state is at all harmful.
Most fliers desire a little more control over their airplanes, however. The next simplest mechanism is to have a mechanical or electrical switch in the plane that may be activated remotely. This allows the pilot to turn the motor off and on at will. For sailplanes and small models, this is usually sufficient.
The most efficent and popular way to control the motor is through the use of an electronic speed control (ESC). This device switches the motor on and off rapidly, with a switching duration dependent on the throttle setting. Thus, the pilot has a proportional throttle control analagous to gas-powered planes. This is the most suitable solution for sport planes since the pilot can throttle back after aerobatic maneuvers and takeoffs and landings to conserve power for longer flights. There are two types of controllers sold for planes: