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by Bob Aberle
Charging: With respect to charging, you will hear the following terms: overnight charging, field fast charging, and trickle charging. The overnight charge rate is the one with which you should become most familiar.
All battery charging, regardless of the type, is done as a function of the battery's rated capacity. You want to know what a battery's capacity is before you put it on a charger because too small a battery charged at too high a charge rate will be "cooked." On the other end of the scale, too big a battery charged at too low a rate will never get to full charge.
The overnight charging rate has been established for many years as the capacity of the battery divided by 10 (usually written C/10). This is the rate most RC-system dual chargers use.
When employing a C/10 overnight rate, you are instructed to leave the batteries on charge for at least 10 hours. You could let them go for 16 or even 24 hours, and you would never hurt the batteries. This rule applies to Ni-Cd and NiMH cells.
As a beginner in the hobby, you shouldn't experience any problems if you use the unit that came with your new RC system to overnight-charge (the night before you intend to fly) the transmitter and airborne battery. As you progress in the hobby, you will learn the rest of what is necessary to maintain your battery packs.
I need to repeat several items I have already mentioned in this series because they are so important. First, when plugging your charger into a household 115VAC outlet, make sure that the outlet remains "live" during the overnight-charge period. If you plug your charger into an outlet that is connected to your shop lighting, you'll turn off your charger when you turn out the lights, and nothing will be charged.
Second, don't be tempted to restore only enough charge to cover what you took out at the flying field. When you return home, don't put the batteries on charge for only two hours if you flew for two hours that day. Ni-Cd and NiMH battery chemistry is such that it needs the C/10 charge current (rate) and a minimum of 10 hours of charging. Anything less than 10 hours can cause problems.
Third, never charge a relatively high-capacity battery at a lower rate, then leave it on for a much longer period of time to compensate. In the past couple of years, new NiMH cells of the same pen cell (or AA size) have come on the market with higher and higher capacity ratings. The AA Ni-Cd cells were usually rated at 500-700 mAh capacity. Now these same-size cells have capacity ratings upward of 1600 mAh.
Let's say you replace your original RC-system batteries with 1600 mAh cells. Apply the logic that the charge rate must be C/10, or 1600/10, which would require a 160 mA charge current, but your RC-system charger produces only 50-60 mA charge current. Being smart, you use that RC-system charger and compensate by leaving the batteries on charge for 24 or 48 hours.
Don't do this! That battery will never achieve full capacity unless you get the current up to C/10. When using the higher-capacity cells, you must purchase a charger with increased output.
One of the best for this type of charging is the ACE R/C Digital Dual Variable Charger (DDVC). It has two outputs that can be adjusted continuously from 0 up to approximately 300 mA current and is capable of handling as many as 10-12 battery cells. In the preceding example you would attach your 1600 mAh eight-cell battery to the DDVC, dial up 160 mA current, and let it run overnight for at least 10 hours.
Don't worry about how many times you charge your batteries overnight. At that rate, you could practically leave them on charge all the time. I've never gone quite that far, but on many occasions I've left batteries on the C/10 rate for 24 or 48 hours with no degradation in performance.
I generally put my battery packs on C/10 charge (such as the RC-system charger) the night before I plan on flying. If it is raining the next morning then several days go by before I have the opportunity to fly, I charge the batteries overnight again. There may have been only a few days between charging, but at this safe charge rate more is always better!
Trickle charging is done at a rate considerably lower than C/10. We generally refer to a trickle charge as C/50, so that a typical 500 mAh battery would be charged at only 10 mA (500/50 = 10). At that rate the battery can be left on trickle for an indefinite period.
All of my RC transmitters are left on constant trickle charge following overnight charging. I have four transmitters that have been on trickle charge for more than five years and still have close to their full rated capacity.
The advantage of trickle charging is that the batteries are available the moment you want to fly. However, you can only maintain a charge level at trickle; you can't recharge a battery that has been used at the trickle charge rate.
Photos show my shelf-mounted ACE R/C Digipulse Multi-Charger. It has six individual outputs, each of which can be adjusted for charge currents of roughly 5- to 200 mA. A charge current can be set up and will remain until you change it. After a timed 16-hour period, each output reverts back to a pulsed trickle charge that will keep the batteries at peak performance, ready when you are.
There is even a backup battery that protects the charger's memory in case of a power failure. With the variable-charge-current feature, you can set each output to produce exactly the C/10 rate for each battery capacity employed (up to approximately 2000 mAh).
As I stated, all of my RC transmitters stay on trickle charge. On the other hand, almost all of my airborne RC equipment is powered from the same cells that run my electric motors. For those packs, of which I must have more than 50 at any given time, I don't employ trickle charging because it could be a logistical nightmare of chargers and wiring.
Field fast charging is a high charge rate that allows Ni-Cd cells to be completely recharged in approximately 20 minutes and NiMH cells to be completely recharged in 30 minutes.
The specific rates are 3C for Ni-Cd and 2C for NiMH. If your Ni-Cd battery pack is rated at 300 mAh and you want to field fast charge it, you would select a 900 mA current (300 x 3 = 900). At that current, a totally depleted battery could be fully charged in roughly 20 minutes.
All field chargers operate from 12-volt DC expressly so that they can be conveniently connected to your car battery. The question is, Why would you want to field charge if you had charged your batteries overnight at home before you came to the flying field?
A person might resort to field fast charging to extend his/her flying time at the field. I get approximately two solid hours of operating time from a single RC transmitter; that is a lot of flying. Because of that, I have never resorted to field fast charging any of my RC transmitters.
There is a more compelling reason not to fast charge a transmitter. Most RC transmitters have a protective fuse in the primary power lead, which is sometimes rated at only 0.5 amp (500 mA). If you hit that circuit with a fast charge current of 1.5 amps (1500 mA) you will likely blow the fuse. In many cases that will require the unit to be returned to the factory for maintenance.
The only time I ever have the need for "extra power" at the flying field is when I compete in the AMA Nationals at Muncie, Indiana. Because of the possibility of a flyoff (in the event of a tie score) at the end of the day, I bring a second freshly charged battery pack and simply swap packs. My general rule is not to fast charge a transmitter battery pack at the field.
Four- and five-cell receiver (or airborne) battery packs tend to run out of power faster than the transmitter, so you might want to fast charge this type of battery pack. I prefer to substitute a freshly charged second pack rather than resort to fast charging. Many RC-system batteries were not intended for ever being field fast charged.
If you are into electric-powered flight, as I am, you will have to resort to field fast charging for the second and all subsequent flights of the day. Most of my electric-powered models employ battery packs with 10 cells or less. This enables me to use an electronic (motor) speed controller that includes a Battery Eliminator Circuit (BEC).
The BEC allows me to share the single large airborne battery pack between the motor operation and the RC system. After each flight, that battery is field fast recharged in 20-30 minutes. After that, it is ready to run the motor and the RC system. Because of this "sharing" advantage, I rarely use the regular four- or five-cell RC-system airborne battery packs.
I've painted the picture of charging. Field fast charging, under most normal conditions, is unnecessary for the average fueled-powered-model enthusiast, but that capability is a must for electric-power modelers.
The only "fast" charger of choice is called a "peak detect" charger. The peak-detect circuit will permit fast charging up to the moment of full charge, at which point the battery will also be at peak voltage potential. Within a few seconds thereafter, the peak-detect circuit operates and turns off the charger. This feature provides for full charging (up to full capacity) and for automatic cutoff after full charge is reached. This is the only way to go!
Some inexpensive chargers simply employ a 15- to 20-minute timer. The problem with this is that it cannot sense the state of the battery's charge, so it is easy to undercharge or overcharge the battery—a bad idea!
There are several "automatic" peak-detect chargers on the market; two that come to mind are the FMA Direct Super Nova and the Dymond Modelsports Super Turbo. In automatic mode, these chargers sense the battery's cell count and capacity rating, and they automatically set the proper charging parameters. The concept involves averaging many of the parameters and applying conservative charging numbers, but it does work and is worth your extra attention if you become serious about electric-powered flight.
With regard to field fast charging, only peak-detect charge Ni-Cd or NiMH battery cells. Make sure your particular cells are rated for fast or elevated-level charging. Ask your battery dealer or distributor if you're in doubt.
Only fast charge your battery packs after they have had a chance to cool down; never charge a hot battery pack! (Warm to the touch is okay.) Never use a peak-detect charger to charge any Lithium-type battery, be it metal, ion, or polymer.
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