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REMOTE CONTROL BY BOB YOUNG Servicing batteries & chargers This month is the last of the series on servicing R/C equipment & we will finish off with nicad battery packs & chargers. Nicads can cause all sorts of problems, far beyond what most people expect from such a widely used component. As noted in previous columns, nicads are statistically the number one cause of trouble (after finger trouble, that is) in the modern R/C system. The key to successful modelling is preventa­tive maintenance and I cannot stress this strongly enough. The foregoing is no reflection on modern nicads for they are a vast improvement on the old button cells that I cut my teeth on. It merely indicates that the improvement in quality of modern elec­tronic equipment has completely outstripped that of the nicad. The problem with any battery, nicads included, is they contain corrosive substances which will eventually eat their way through the battery casing given enough time. Thus, if your R/C set has batteries in it, then you will eventually have battery trouble. It is as simple as that. To compound the problem, rechargeable batteries must have venting as a safety measure. Thus, at some point, corrosive gases will be vented into the transmitter (or model). This venting is usually caused by overcharging or overheating the battery. Fast charging is hard on batteries and cell life is reduced accord­ingly. However, cells designed for fast charging will not be as badly effected here as, say, cells designed for low discharge This photo shows a standard plugpack charger (left), of the type sold with all modern R/C systems. Also featured is a cycling charger made by Silvertone. This unit features 240VAC and 12VDC charging. The 12VDC feature is very useful for field charging. In operation, the start button initiates discharge & when the voltage endpoint is reached (1V per cell), the unit automatically commences the charge cycle. 72  Silicon Chip currents and trickle charging. Calculator batteries fall into this category. Fast overcharging is deadly, as is drawing too much current from cells designed for low current operation. One very common mistake made by modellers is to buy cheap nicads which are usual­ly designed for calculator use and then use them in the receiver battery pack. The instantaneous start-up current on a servo motor can run as high as 1A. Thus, a model with six servos could easily require up to 6A at some point in its operation. Calculator batteries will turn up their toes very quickly indeed under these conditions. However, calculator batteries are usually OK in the transmitter battery pack, as the current drain is in the order of a steady 100-150mA. Receiver packs Receiver packs should be made up from good quality cells designed for high current operation. The other problem introduced by low rate cells in receiver operation is the internal resist­ance of the cell. Low rate cells have a higher internal impedance and this will cause spikes on the receiver’s supply rails. These spikes can cause serious problems, particularly at extremes of range when all the servos start to jitter. This raises the aver­age servo current and causes a voltage drop from the battery pack, thereby reducing the range. This reduced range further increases the servo jitter and suddenly the receiver/servo/batteries are locked into a death spiral. The result is one broken model, one modeller scratching his head over why his receiver ran out of range, and a receiver and battery that works perfectly once the abnormal load is taken off the system. If you suspect this sort of problem, do a range or sensitiv­ity check with only one servo in the system. Repeat this test with all servos included and note the difference in range. If there is a significant difference in range, suspect the batteries or noisy servos. Remove one servo at a time, replacing the previous­ly removed servo and again note the range. If one servo shows up as a problem, then check the noise suppression filters in the servo. If the servos all check out OK, suspect the battery. Replace the battery with one made up from high rate cells. This should cure the problem. Fast charging raises the temperature of the cell, as does excessive current. Accidental short circuits are quite damaging to nicads, not necessarily immediately but later on in their life. Worse still, the vented gases given off during the trauma will start to corrode any surrounding metal and this includes electronic components anywhere near the batteries. With this in mind, start your battery servicing with a good look around the area in which the battery is housed. Check to ensure that PC boards are clean and not showing signs of corro­ sion. Corrosion on PC boards with solder resist masks often shows up as a dark green stain. Hot, vented battery gases will rise and lay a corrosive and conductive coating on any surface directly above the battery. More often than not, this happens to be the encoder PC board in the transmitter. The conductive coating can have a devastating effect on timing circuits and in time eat its way right through copper tracks. Black wire syndrome I have spoken often about the “black wire syndrome” in the past and I will mention it again here. Black wire is a corrosive process in which all of the copper in the negative lead (usually) is gradually replaced by some sort of black garbage. This wire has increased resistance and becomes very black and brittle. It will eventually fall off the terminal it is soldered to but not before it transfers the corrosion to anything to which it is connected. Thus, the battery connectors, switch harness, and the switch contacts themselves must be subjected to close scruti­ny. Nor is this effect confined to the negative lead. I have seen all types and colours of wire effected. To check for black wire, simply pull back the insulation as near to the solder joint as possible. If the copper or nickel plating is bright and shiny, just put the insulation back where you found it. If the conductor is showing signs of staining or corrosion, then replace the entire length with nickel plated multi-strand wire. Do not use untinned copper conductors on battery leads. If the battery lead in the transmitter has a connector in it, then pull the connector apart and check for corrosion on the pins. Once black wire has set in, this connector will not pull apart as the two halves become welded into a corrosion riddled whole. Do not bother to try to clean this connector if it has mild staining, for it will just keep staining. Just replace it or bypass it directly with a hard wired installation. I might add here that CRC-226 helps to minimise the forma­tion of this type of corrosion, particularly on connectors and switches, so keep the ends of the batteries, connectors and switches moist with a coating of CRC-226. Do not forget to in­spect the charging leads and socket as these are all in the battery path and often corrode badly. The charging socket is also open to dust, moisture, overspray from painting models and worst of all, burnt castor oil fumes. Check these items carefully and keep up the CRC. This business of overspray is a point I have never raised before but I have lost count of the number of servo leads I have had to replace because someone has forgot to mask their servo connectors before spraying the model. Painted servos and connec­tors may make a great fashion statement but they do not enhance the electrical performance of the system. If you must spray the model with the servos, VCR ALIGNMENT TOOL KIT • 7 Assorted head & guide aligners • Hex key set • Retaining ring remover • 3 Reversible screwdrivers – SML – Flat – Philips • Spring hook • Fitted vinyl • Micro screwdriver • Zippered • VCR head puller Our Low Price $99.95 WOMBAT COMPONENTS WOMBAT COMMUNICATIONS 83 - 85 Railway Ave Werribee, Vic 3030 Phone: (03) 742 7330 Fax: (03) 741 6834 battery packs and charge receptacles in it, then please make sure they are well sealed against paint. Once you are satisfied that all of the external areas are free of corrosion, then it is time to go into the battery pack itself. Take off the heatshrink enclosing the bundle of cells and examine the ends of the cells. If they are badly salted, the pack should be dumped. Mild salting can be cleaned off with a toothbrush and CRC-226. Mild salting does not mean the pack CALLING ALL HOBBYISTS We provide the challenge and money for you to design and build as many simple, useful, economical and original kit sets as possible. We will only consider kits using lots of ICs and transistors. If you need assistance in getting samples and technical specifications while building your kits, let us know. YUGA ENTERPRISE 705 SIMS DRIVE #03-09 SHUN LI INDUSTRIAL COMPLEX SINGAPORE 1438 TEL: 65 741 0300    Fax: 65 749 1048 June 1994  73 REMOTE CONTROL – Servicing Batteries is at its end of life but does indicate that the periodic inspections must be increased in frequency. Once this salting starts, black wire is not far behind. Sadly, having made a statement like that I must now qualify it. First, black wire is not confined to the negative lead so check all leads. It can occur with no evi­ dence of salting on the batteries so stay alert. Black wire is an elusive foe so always exercise extreme caution when inspecting the batteries and associated components. One other point – do not use your best soldering iron tip when soldering badly corroded battery terminals. This corrosion will contaminate the tip and make soldering very difficult for some time. Finally, be sure to wash your hands after handling old batteries. They are often coated with chemicals which have spilled over the entire surface of the battery. These chemicals are very corrosive. If the cells are moist from internal chemi­cals, dump the pack. Castor oil often migrates down the battery lead in the model and coats the cells under the heatshrink and it is sometimes difficult to tell the source of the moisture. deliv­er a trace that is quite distinct with a high start point, gently sloping centre portion and the sharp drop at the end. The big difference between the two, however, is that the Yuasa trace is on average 0.1V higher over the flat portion, whilst the Saft curve is at least 10-20% longer. All nicads will exhibit these distinct differences as they reflect the internal construc­tion, chemical composition and condition of the cell. As batteries age, they change the shape of the graph and again it is possible to tell an ageing battery from its graph. And yes, you can see evidence of memory, even in modern cells. Some traces exhibit a very distinct kink in the voltage curve which in some cases goes completely with cycling. I have always recommended cycling before every charge for two very good rea­ sons: (1) it gives you a known starting point for your charge time, thus helping to prevent overcharging; and (2) it moves the chemicals around inside the cell and helps to minimise the formation of crusty deposits on the plates. The modern battery is supposedly “Be sure to wash your hands after handling old batteries. They are often coated with chemicals which have spilled over the entire surface of the battery”. It is now time to check the cells electrically. First, charge them fully and if you have the facilities, then cycle the batteries once or twice and note the capacity. At Silvertone, we do a routine cycle and battery graph on all sets we service. This tells us the full story: cell voltage, state of the cells and capac­ity. In time, I have learned to interpret these graphs and can almost tell the brand of cell from the shape of the trace. For example, Yuasa 500mAh cells deliver the almost classi­ cal nicad graph – ­ high start point, flat middle section and sharp drop off at the end. Saft 500mAh cells, on the other hand, 74  Silicon Chip immune from overcharging and Saft state that at the C/10 rate, their cells will stand 20,000 cycles. In practice, I have found that Saft cells give excellent service over very extended periods of time. I still get sets in that I built in 1974 with original battery packs in good condition. Thus, I am very reluctant to change brands for this reason. The problem with some batteries, particularly those offering high capacity in the AA size, is that this extra capaci­ty is obtained by leaving out some the safety chemicals. Thus, cell life is often reduced as a result. Finally, inspect the external casing of the cells for crash damage or other physical distortions. Cells often get pushed in during a crash and in time will rupture in use. Check the termi­nal voltage on each individual cell after a full charge and a partial discharge. All cells should be within 0.1V of each other. If one cell is outside this limit, be careful. I do not recommend replacing one cell in an old pack but sometimes it is a matter of convenience and cost. Once you are satisfied that the cells are OK, replace the heatshrink sleeving and remount the battery. A good source of small quantities of the larger diameter heat shrinks suitable for battery packs is your local model shop. They use these sizes for covering helicopter rotor blades. Chargers With regard to the charger, there is not much to go wrong with these units. Check for paint overspray on the connectors as these things kick around the workshop and are rarely if ever looked after. A good scrub with the toothbrush and CRC-226 is all that is usually needed. Most chargers supplied with modern R/C systems are simply a plugpack with two diodes, two LEDs and a dropping resistor for each battery. The occasional LED goes out and sometimes the transformer goes open circuit but these are rare failures. A quick check of the voltage on the output pins is in order, making sure that the polarity is correct. Often, modellers replace these connectors to suit the set they are using. Likewise check the soldering on these joins if they pres­ent. Some modellers have never learned how to solder and twisted leads are the order of the day. Finish off with a quick check of the current from each charge lead. This will normally be the C/10 rate or 50mA for a 500mAh cell. To my mind, a better way to handle nicads is to discharge them immediately after use so that they are stored in the dis­charged state and the internal chemicals are in their least active mode. Thus, they are least likely to get up to mischief between sessions. The cycling charger is ideal for this approach. When the discharge is complete, the discharge relay drops out and the unit does not even need to be unplugged. you then simply switch on the charge power the night before your next modelling session and 14 hours SC later you are ready to howl.