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RE.MOTE CONTROL · By BOB YOUNG The care and feeding of battery packs Sooner or later, everyone involved with remote control realises that without good batteries, a fancy model is a dead duck. It is particularly unfortunate if the battery dies when your model is in mid-flight. The heart of the modern R/C system is the battery pack which, nowadays, usually consists of rechargeable nickel-cadmium cells. Statistically, the battery is now the number one killer of R/C systems and it is the very first item that I check on systems in for repair. That is not to say that nicads are unreliable far from it. But modern R/C designs and the components in them have become so reliable that the nicad is now the weak link in the system, primarily because of its inherent corrosive nature. However, there is a more obvious :reason why nicads have become the number one problem: they are prone to operator error and I mean operator error in a big way. Because any discussion on the care and feediR-g-of nicads is so vast, I intend to cover only the more obvious problems that present themselves to the R/C modeller. Operator error I define operator error as the inappropriate choice of cell type and the actual handling of those cells once installed. To begin, they must be recharged correctly and let me tell you it is unfortunate that the most popular fly82 SILICON CHIP ing time is Sunday morning. This means that the batteries must be recharged on Saturday night. Now strange things happen to human beings on Saturday nights, amongst which they stay out late and drink too much of that frothy brown liquid. If recharging is remembered at all, it is often in the early hours of the morning, resulting in a charging period well short of the required 1 O to 14 hours .. More seriously, the model · is usually banned from the nice warm house by a long suffering better half, and recharging often takes place in a cold garage. This further reduces charging efficiency and cell life. Sunday morning thus sees the R/C junkie, desperate for his weekly fix of fun in the sky and sun, drag himself out of the cot, thoroughly unsure of his position in the world and the state of charge of his batteries. In trying to relate to his position in the world he probably falls back on that good old Australian question: "Did I have a good time last night or what?" As he most likely cannot remember he p:roba bly consoles himself with that equally famous Australian reply, "Gawd I feel crook, so I must have". Unfortunately there is no such simple way to gauge how the nicads fared, short of doing a timed discharge and recharge, which will not help get our R/C desperado to the flying field in time for that contest. So, reaching once more for another Australianism, he sets off with a "she'll be right mate". A trifle facetious perhaps but this is a scenerio that I have encountered many times in my career in the field of R/C modelling. Of course, few customers have the courage to admit it but some have. I have done similar things myself and although I have never partaken of · the amber fluid, my desperation to fly has certainly overridden commonsense on occasions. And yes, I have forgotten to recharge my batterie·s once or twice. Access to a good power supply allows a rapid charge but the average modeller has no such recourse and will often in desperation fall back to the "she'll be right" panacea. The lessons learned are usually bitter ones and age and experience soon teaches one to put batteries on charge before one goes out on Saturday night. However, there are always new chums arriving in the hobby and the same mistakes keep re-appearing. Different batteries The modern nicad battery, along with all modern electronic components, has suffered from the process of proliferation. The result is a bewildering array of components with very subtle differences in the local hobby stores. 1.6 - - - -- ..-------r- - - r - - - - r- - -, w "'et ~ 11---+--+-- -+- --+--+-t+--+~ > -' -' ~ o.8 1-- ......L---'----+-- -tt,8C,---,";;4c-t~,c:.--:o:-l.2:lc 0.81--- CNARGE : 0.1C x 16Hr DISCHARGE : 0.2C, 1C, 4C, BC TEMPERATURE : 2o·c 0·6 L _ _2_0 _ _4_0_ __.. &o- - ~ 80~ - ~ 10-=o-~ 120 0 -1-- -+ - - - + - - - + -----+------1 0. 2 DISCHARGE CAPACITY (¾) _ 6===:::l::=N!;,; i·Cd~IN::::TE::RN=:Al:::,R:r:ESl:S:TA:NC::: E±== = ;= 0 60 ==---! 60 Fig.2: the effects of discharge current on cell efficiency. Note that a discharge current of 4C results in the efficiency dropping to about 90%. The output voltage is also reduced along the entire discharge curve. DISCHARGE TIME (HOURS) Fig.1: one of the dangers with nicads is the sudden voltage drop at the end of the discharge curve. There are fast charge, low discharge rate batteries; slow charge, high discharge rate batteries; calculator batteries; torch batteries; portable radio batteries and dozens or perhaps hundreds more. What does it all mean and more importantly, which one do I use in my R/C set? To answer this, we must have a very clear idea of what the batteries will be called upon to cope with in the locations in which you intend to use them. In this month's column, we will confine the discussion to the transmitter and the receiver battery packs. A later column will discuss the more demanding ultra high discharge rates encountered in electric powered aircraft and cars. This discussion will cover the construction of different cell types as well. In the R/C transmitter (Tx), the current consumption is usually a steady 150mA or thereabouts, depending primarily upon the Tx's power amplifier stage. Therefore, the demands on the construction of the battery are low and most lowcost 500mA.h cells will do the job nicely. In my opinion, based on 2 7 years of dealing with nicads, the overriding factor is the quality of the cell construction and the safety chemicals included to provide overcharge protection. A good quality cell will provide around 10 years of trouble-free service in the transmitter. But "black wire" syndrome is the big problem and periodic inspection is a must, even with high quality cells. For a detailed discussion on the "black wire" syndrome, see the February 1990 issue of SILICON CHIP. The manufacturers' instructions often point out that battery boxes are not recommended and if used they must have nickel or nickel plated steel terminals. Copper, zinc, aluminium and chrome will readily corrode. Even nickel terminals will still tend to oxidize and must be wiped clean regularly. A spray of CRC-226 helps minimise this effect. This effect is one of the great mysteries in using nicads and the major cause of failures. Manufacturers go to great pains to point out that the cell is sealed and that it can withstand the normal 50mA charge for extended periods and yet the cell still promotes corrosion both on contacts and on circuit boards in the near vicinity. Modellers rarely clean the contacts or check for "black wire" or other signs of corrosion until some catastrophic failure occurs. I feel that Tx batteries should be charged out of the Tx case if possible. I still have yet to see a satisfac- tory explanation for the "Black Wire Syndrome", yet this problem can eat the negative wiring loom right out of electronic equipment and is a major source of device failure. The position for the receiver battery is vastly different. A servo at start up will draw an instantaneous current which may be as high as one amp. More usually this figure runs at 600mA. Thus, four servos leaping into life simultaneously will draw 2 to 3 amps which is quite a load for a 500mA.h battery. Helicopters The position in a helicopter is by far the most demanding, for several reasons. First, modern helicopters require around six servos plus a gyro. Second, the helicopter has almost no natural stability and therefore must be flown constantly. Thus, the servos rarely rest and as a result, current consumption is very high. The situation in a model car is not as critical for two reasons: first , most cars run only two servos and second, the car can be stopped im- 1.4 1.3 > w "'et 1.2 = C > -' -' w '-' 1.1 CHARGE : 150mA (C/3.3) x 5Hrs. TEMPERATURE : zo•c ~ ~- i--.....- \ " 1A (2C) 20 - "'I \ 250mA\ (C/2) 500mA' (1C) 40 60 80 DISCHARGE TIME (MINUTES) 100 120 Fig.3: voltage curves for a consumer-type cell at C/2, C & 2C. Note the rapid voltage drop at the endpoint in each case. SEPTEMBER1990 83 Now the important point is that the voltage drop across the batteries at these high currents can be considerable in cells not intended for high discharge rates. This results in supply rail noise finding its way into the Rx and decoder circuits, reducing range and causing excessive servo jitter in weak signal areas. In fact, the situation can very quickly deteriorate into a closed loop with the supply rail spike generated by the servos starting causing a decoder fault which will in turn cause the servos to start again, thereby re-injecting another spike and bringing about complete loss of control. The Rx battery, designed for low current operation, virtually collapses under a constant 2-3 amp load and the model is by now irretrievably out of control. I shudder when I open some sets to find cheap calculator nicads, usually designed for 50mA constant current load. Cells designed for high current usage have end welded plates and other features to reduce internal resistance and thus internal heating. High rates of discharge will reduce cell life even in cells designed for this usage. This is one very good reason for using the largest capacity battery weight will allow. I refuse to guarantee the repair unless those cells are replaced, for I know from past experience the set will be back soon. Unfortunately, this time I will be blamed because I was supposed to have fixed it. In ..:;- 3 0 0 1 - - + - ' - . . + - ~ - - - - + - - - - - - - t ~ g 250 l---+--+-.......-1----+----,f----l ~ ~ l!; 2001---+--f->'---'lr----+-----,f-----t ~ 1501---1---1---i.:::o..,---'",d-----,f-----t w d ► "' 1001---i--+--l---+-~i-=--... 0 10 20 60 40 100 80 DEPTH OF DISCHARGE, DOD(%) Fig.4: depth of discharge (D.O.D) vs. cell life. The graph shows that the deeper you discharge the cells, the lower their cycle life. mediately the first signs of trouble show up. It takes time to land an aircraft and often that time is just not available because of the very sharp "knee" on the voltage curve. A 500mA.h pack will give about 2.5 hours in a 4-servo aircraft but only about 45 minutes in a 4-servo helicopter. The usual pack size for helicopters is 1.2Ah. There is an important point to note here. Modellers tend to learn from experience that a 5-hour charge is enough for, say, four flights; the industry rule of thumb being one hour of charging per flight. What can happen is that a windy day calls for more control inputs and thus higher current consumption and the reduced charging time resulting from our late night out (referred to above) is just not sufficient. The result may be a crash on the last flight. 100 80 ' -r--- ---- the R/C world, once you have repaired a set, it seems that you are held eternally responsible for that set. I used to joke that even if the wings fell off the aircraft, I would be blamed, until one day the wings did fall of a model and I was blamed. That joke lost its appeal thereafter. The moral of this story is do not send a set in for repair with inappropriate nicads installed, without expecting to renew them. The only thing worse is to send a set in for repair without the batteries used on the fatal occasion, for the most probable cause of the problem was those batteries. In that situation, you will only end up with a "defect not confirmed" tag and a bill for checking the set. After re-installing the defective batteries, a second crash is the certain result. Yet over and over again, sets arrive for repair with inappropriate nicads or without battery packs. As stated previously, the battery is the heart of the R/C system and the well being of that system is in the hands of the operator. Furthermore, cutting corners on the cost of Rx nicads is -being very foolish indeed - a model travelling at lO0km/h can make a siza ble hole in somebody's head. Choosing batteries How then do we arrive at the choice of an appropriate battery pack? Most commercial R/C equipment comes complete with nicads and charger and thus presents no problem, as the manufacturer ensures 40 80 i---.. ~ _/ .,,,...-- ► ~ ... / 40 CAPACITY MEASURING CONDITIONS : CHARGE: 0.1C x 16Hrs. DISChARGE : 0.2C EV : 1V !f:! 0.5 ..__T5!P~~E. 0 0 ~ ~ ~ ~ 1~ No. OF CYCLES Fig.5: discharge capacity vs. number of cycles for a typical nicad cell. Under normal conditions, nicads are good for over 500 charge/discharge cycles. 84 SILICON CHIP 35 E w 50 ---_,,r./ / --- C 30 ~ ~ ... ~ V/2ftESSURE / 20 \ -~-- .- = .;-- =... CYCLE CONDITIONS : CHARGE: 0.lC x 11Hrs. DISCHARGE: 0.7C x 1Hr. w 5 VOLTAGE 60 C "' ~ - ~ w 25 .., 20 100 150 CHARGE INPUT (% OF CAPACITY) Fig.6: cell voltage, internal pressure and cell temperature as a function of charge input. Note how the cell voltage drops if charging continues after it is fully charged. Note also the increases in temperature and pressure as charging proceeds. Fig.7: the effects of temperature on cell voltage during charging. Nicad capacity is specified at 20°C and must be derated for higher temperatures. CHARGE : 150mA (C/J_j) x 5Hrs. - 1.6 / 1.3 v --l---- ~ / 1o•c 2o·c V 4s•c 1.2 0 2 CHARGE TIME (HOURS) that the correct cell type is fitted. The problem arises with sets sold for dry battery operation which have nicads fitted by the operator, and in sets in which the original cells have been replaced. In addition to this, the performance of the set on dry batteries or inappropriate nicads is very much influenced by the design of the Rx circuit. Such matters as decoupling, voltage stabilisation and low voltage operation all play an important part in this situation. Fig.1 illustrates the basic pros and cons of dry batteries versus nicads, when used in R/C systems. The very flat voltage curve and extremely low internal resistance of the nicad puts it clearly in front of the dry cell. In fact, it amazes me that dry cells give as satisfactory a result as they do and it speaks volumes for the quality of modern circuit design. However, they can cause excessive servo jitter as the cell ages. Fig.1 also shows one of the basic dangers in using nicads and that is the rapid voltage drop [beyond the "knee") at the end of the usable portion of the curve. Nicads pushed to their limit can collapse in the space of a 15-minute flight with very little warning. The moral here: land at the first sign of trouble and check range and battery voltage as well as for mechanical defects in the airframe. Modern nicads fall into broad catagories regarding design and construction and the Panasonic catalog lists the following types: Standard, Rapid Charge, High Temperature, High Capacity, High Rate Discharge and Rapid Charge, Super High Capacity and Rapid Charge, Memory Backup and Consumer Type. From this bewildering array, which cell do we choose? To begin, we must establish how . long we wish to operate between charges. A 500mA.h cell will quite safely provide 2 to 2.5 hours of operation on a 4-servo model aircraft. The same size cell will provide approx 4 hours operation on a standard Tx. This is usually considered adequate for most modelling applications. Next, we must establish what type of load 2.5 amps represents in relation to a 500mA.h cell. Obviously 2.5 amps drawn from a lead acid car battery is not a heavy load but does it constitute a rapid discharge from a 500mA.h cell'? This is not so easily settled and there is no definition of what constitutes a rapid discharge rate in any catalog that I could find. Fig.2 does give some clue in that 4C (4 times the cell capacity in milliamps, 4 x 500 = 2000mA or 2 amps) is beginning to stress the cell and efficiency has dropped to 90% of normal. Note also that the voltage available has fallen, despite the low internal resistance. Fig.3 shows the voltage curve for a consumer type cell at C/2, C and 2C. Thus, in the interests of efficiency, cell life and voltage available, it pays to use the largest capacity cell that the weight penalty will allow. Note that 2.5 amps from a 500mA.h cell is 5C while the same current from a 2.5A.h. cell is only lC. The airborne battery cells should be a high discharge type if the capacity is kept to a minimum. The AA cell is a borderline case and may be a high quality standard cell. One final word here on the effects of genuine interference: if you are using inappropriate, aged or otherwise defective cells, all of the servos will begin to chatter when interference is encountered. This gives rise to the condition described earlier, thus ensuring a crash, whereas cells in good condition may ride out the crisis. Finally, a brief word on charging: nicads are very simple to charge and under ordinary conditions will give well over 500 cycles in their lifetime. Fig.4 shows the effect of depth of discharge (D.O.D) on cell life. In this regard, there is an ever raging argument in R/C circles concerning the use of cycling chargers and whether to discharge every time before charging or not. My opinion is that it is worth doing. Why? Fig.5 shows the cycle life of nicads based upon the 100% D.O.D cycle. As can be seen, the minimum life is 500 cycles. Now this represents 10 years of charging every Saturday night with a full discharge before every charge and that is the minimum figure. Don't overcharge Overcharging can also damage nicad cells. One big problem faced by the model aircraft people in particular is the situation where a set is charged on Saturday night but the model is not flown the following day. Next Saturday, what to do? The set is still charged although self discharge will have reduced that charge by an amount unknown to the modeller because that rate depends on all sorts of things including cell age, internal condition, temperature and so on. Also, if the kids have access to the garage, they often show Dad's pride and joy to their mates and use half the charge in the process. Rarely is this communicated to Dad. The moral? Do yourself a favour. Discharge the cells immediately to their endpoint voltage (1. 1V per cell) and then charge them for the full 14 hours. Charge as close to the flying session as possible. Also, replace the cells every 5 years and check them every 6 months for corrosion. Replace any airborne cells involved in a heavy crash, particularly if physically damaged. High "G" forces can internally weaken a cell which can result in a later failure in flight. ~ SEPTEMBER1990 85