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REMOTE CONTROL BY BOB YOUNG Nicad cells for high rate discharge and the jetpipe temperature shoots up into the dangerous area. As a result, a complex battery analyser is used in order to determine the state of charge and performance of each individual cell in the battery pack and great care is exercised in order to keep those batteries in top condition. As it is, there is just enough capacity in a good battery pack to effect a start on one engine. The second engine cannot be started until the battery has picked up sufficient charge from the operating engine. In fact, the instructions state that a start must not be attempted on the second engine until the charging current has fallen below 100 amps. As stated previously, these batteries need to be good. The aircraft itself is of Brazilian design and manufacture and is intended for use in South America. The middle of the Amazon Basin is no place to have a flat battery. All in all, it is a very demanding situation. Radio control modellers do not place such stringent demands on their battery packs with regard to life threating situations. However, the poor long suffering R/C serviceman who fails to get his customer's R/C into tip-top shape for that competition very defi- High discharge rates can cause real problems for nicad batteries. This month, we look at those problems and discuss the solutions. In September we dealt with nicad cells for transmitter and receiver use in aircraft and discussed the difficulty in determining what constituted a correct rate of discharge for a 500 mAh cell. This month we will not face this difficulty for we will be dealing with awesome capacity/discharge ratios that may result in run times of as short as 15 seconds to 5 minutes. No confusion here. This is high rate discharge with a capital "H''. I would like to open the discussion with a little tale of an aviation application of nicads in a light twin-engine aircraft called a Bandierrante. This is a 20-seat commuter machine powered by Pt-6 turbo props. Now starting one of these little turbines is quite a trick as they must be spooled up to 12-15% of normal rpm with the starter motor before in.troducing fuel. The higher the rpm on the starter, the lower the jetpipe temperature and the less the thermal stress on the hot parts of the engine, - -100 ae 0 -~ 80 1.6 r---__ - ?: 60 ·c::; "'a. 0"' 40 20 particularly the power turbine. The problem is that a sufficient mass of cooling air is required before the fuel is introduced. Any pilot foolish enough to disregard the instruction prohibiting the application of fuel before the 12% rpm figure is on the clock is liable to find his power turbine dripping onto the tarmac. The problem is that this calls for starter engagements of quite long durations, typically 10-12 seconds. This demands the best batteries available and nicads are the order of the day. The Bandierrante battery consists of 20 cells of 36Ah capacity (Ah stands for ampere-hours) which gives some inkling of the discharge current involved. The start up surge is 1500 amps, falling as starter RPM increases. This whole situation is extremely complex for batteries are heavy and must be carried at the expense of payload. Thus, the smallest battery possible is carried. However, if these batteries sag in voltage or the motor is slow to start, the start-up rpm falls - - - --- 1.5 1-------,1------,---;------t-1 .4 ,_ 2 .0 3 .0 4 .0 Discharge rate (C mA ) Fig.1: capacity vs discharge rate. High discharge rates increase cell stress and give shorter cell life. 104 SILICON CHIP Q) 1.2 ~ 1.1 > 1.0 0 Charge . C / 1 OX 15hrs. 20°C(68 °F) Discharge : Cut off voltage 1 OV , 20°C(68°F) 1 .0 1.3 CJ> 0 0 ~ Charge 900 mA X1 .5hrs. --;-Temp. : 20°C(68°F) - 0.9 A ~ _,,.-/' I',.,......___ 18 A 0.8 07 - O 200 IX"- /r-,.... I""\\.' ~r--- 0.9 2 .7A ~ 400 5.4A 'l\ 11 600 800 1000 1200 1400 Discharge Capacity <mAh> 1600 1800 Fig.2: the cell terminal voltage at various discharge currents. At 18A, the average voltage is about 1 V. Electric propulsion technology has come a long way in the last few years. At left is the Geist 150 electric motor which employs samarium cobalt magnets while at right is the Hecktoplett 355/5 motor which has a rated power output of 860 watts (1300 watts input). Photo courtesy Moore Park Model Supplies, Armidale. nitely faces a life threatening situation, and it seemed to me at times that the middle of South America jungle, anacondas, piranhas and all would have been a much safer place to be. Such is the day-to-day-life of the R/C manufacturer or serviceman. Model cars, boats and aircraft do not call for such heavy duty or large capacity batteries but they do call for a reliable cell with a good capacity to weight ratio. Standard cell The standard cell for model car, boat and aircraft use is the 1.2Ah model but recent developments in battery and motor technology have introduced a host of new cell sizes into the R/C field. Indeed, the ever present demand for higher performance and thus lighter weight is forcing modellers back to the smaller cell sizes (800 and 900 mAh). This results in much higher capacity/discharge ratios and a subsequent increase in cell stress, lower terminal voltages and shorter cell life - see Fig.1 . Once again we are faced with determining the capacity/discharge ra- tios to establish which are the best cells for our particular application and again this calls for a precise understanding of what the cells will be called upon to deliver in operation. Electric motors for model propulsion fall broadly into four broad categories: 05 , 15, 25 and 40-size motors. These four styles come in a bewildering range of case configurations and armature windings and we'll delve more into this subject in a future article. The windings fall into two categories: stock and modified. One of the most popular sizes for electric cars is the 540 size case with the stock winding drawing about 23.5 amps free running and giving adequate performance on a 6-cell 1.2Ah battery pack for a 6-minute race time. The modified 540 draws about 2.54.5 amps free running and on a 1.2Ah battery is raced for 5 minutes. Electric flight The situation for model aircraft is quite different, with enormous powers being involved in the top level contest models. A little sports model using the 540 stock motor will require about 110 watts per kilogram. A 7-cell battery (ie, 8.4 volts nominal) driving a 540 fitted with a 9-inch (diameter) x 4 (pitch) prop will deliver about 8000 rpm. This is sufficient for models weighing up to 1.5kg. The loaded current on this prop is approximately 20 amps. Compare this with the high performance contest model fitted with one of the exotic European motors. All figures are quoted from the "Electric Flight Newsletter" from Moore Park Model Supplies. The model was fitted with a Hecktoplett 355/5 wind motor and a 27cell SCR/N 900mAh battery pack. An electronic speed control was fitted as a throttle device and the performance figures are given as follows: prop Bally 13 x 7; revs 11,200 at 51 amps. This is equivalent to a power input of about 1400 watts! Compare this to another of the expensive German motors (Geist) and some idea of the power involved is quite clearly indicated: prop 13 x 7; revs 11,500 at 72 amps. Notice also the improved efficiency of the first motor, which of course, was the most expensive and the heaviest. Now I would like to draw your attention to the cell capacity/load ratios (BOC) which leave absolutely no doubt that here we are dealing with high rate discharge and that the batteries involved had better be good. Notice also the tremendous wattages involved. At these currents, the average terminal voltage is about 1 volt per cell (Fig.1) giving 2 7V x 72 amps (1.944kW). Electric propulsion technology has come a long way in the past few years. The electronic speed controller alone is a major development in its own right. No weight is quoted for this model but a brief description of the test flight stated that it climbed like a scalded cat and was well up under the cloud base in 25 seconds. I would estimate the weight to be around the 3.5kg mark which works out at around 550 watts per kilogram. That's equivalent to a power to weight ratio of 550 kilowatts per tonne! It makes our 110 watts per kilogram sports model look like a kitten. The average speeds of this type of model are around 120150km/h. NOVEMBER 1990 105 r~~~===~====~s,~s,=+====::i:..----.-_-+·--_-_-_-_-+·--_-_-_-_-+·--_-_-_-_-_~·---~-~~--,--~ ..... Temperature: 20 ' C(68" F) 10 , 8 1 - - -- " " ~ - - + - -- ""'----""'~- voltage: 1.0V /cell Charge: Full charge +--+---+--- - - + - - -- - + - - - - - + E n d "' Q. E < 2 -...... 800 C Cl) 600 ::, (.) Cl) ...Cl 400 Cll ~ (.) Ill c 200 1 50SC E 1 40SCR/140SCRC 1 30SC R/130SCRC 120SGRP / 120SCPC I u, ' a.I +-~- ~i----+-.-,----J',,,.--f---f"oc+-'~~g~~:s11OOAS/1 OOC 100 E' ml : I -~---+---+-..._,--,...,.-__---l",ci80AAR 80 1------1--- + -- - - ii f - - -- - f - - - - , - -- - - - - t -- 60 I - + - - - t - --+-----"'-.:l--- - -_,,,..c -___,,,,i.:--+---.::--f'.....,_-+-+--""'c-"""',-,,..,;:-1 7QAR - +---+-----+--'- -- -+---'----------'-..__..,,l,---+----l-""...-""""'""60AA / 60SC i60SCR/60AS I 50AA / 50AAR I !----- - - + - - - - - - r------- - - - - >----+----+ . ! +--- - - -------t-- .. - -- 40 !----- - - + - - - - - + -- - · ··----ri - - · - +- ' I . l . -- - - ----+--- ~-~---- t - - ------+-->o.-i 30A AE 27AA ~ ---- · -7-- 20 I __L --- - - - - - -- + - - -- +--+~"lo.---+---P-...+--3'1180AAA 1 5 N/ 15NC 11AA 1 10 10 15 I◄,________ 20 40 1.5 4 2 6 8 10 -- - ·- Hours Minutes Typical duration time Fig.3: this graph of discharge current vs duration time can be used to aid cell selection. The cell types listed down the right hand side are from Panasonic. The batteries involved are of course very good and battery technology has leapt ahead in recent years. Fig.3 shows a graph used to aid in cell selection, while Fig.4 is a discharge characteristic graph for the Panasonic P-90 SCR cell. Fig.2 is a high rate discharge graph showing the terminal voltages at various discharge currents. Notice that the terminal voltage shows an average 1V at 18 amps, a long way short of the 72 amps in the model quoted above. This raises a terribly important point in the selection of electric flight batteries, that being the compromise 106 SILICON CHIP between using less cells of a higher capacity or more cells of a lower capacity. Reference to Fig.2 shows quite clearly that the higher the capacity/ load ratio, the lower the terminal voltage. Thus, in a 20-30 cell pack, great gains can be had by using less cells of a higher capacity and more suitable construction. The "Electric Flight Newsletter" confirms this when it states that a change from 30/800mAh cells (AR) to 27/900 SCR cells gave quite a large increase in 00mph. In this particular case, we see improved cell construe- tion and a better capacity/load ratio combining to give a result well in advance of what could be expected. Cell reversal Incidentally, running large numbers of cells in series like this is not really recommended as it increases the risk of cell reversal in the weaker cells, especially in applications where very deep discharge at very high rates is anticipated. Panasonic recommend that a low voltage cut-off be provided for packs of this type. It is for this reason that some specialist firms provide what they call "pushed cells". These are battery packs in which all of the cells have 1 ooi:=====i===:::i:==i=::::i=+=+=+=++==----~---~ C t, arg e : 900 mA X 1.5 hr ... s. 1--- - 1--- + -......_.--+---+-+-++- 1--c..._ ~ - -+--+--+--+- +-++++-~ Tern p ;A_ 20' C -+1-+-+-+-+-< 0 .8V ~~ 100 ? i- 80 0 --- .... ....... A B -~ 60 - ' ~ 40 u g. u C/10 x 15 hrs· Charge. Discharge C/5 Cut ott voltage . 1.0V 20 0 - 20 -10 - 4 - 14 ;;; a, :i '= E , t---------------+---+--+--++++ k --+----+--i, a, E .= 0) :".' ro .c +--+--+--+-+-+; 0 32 10 50 20 68 30 86 ·"'0 ~a::::J=::l=t:::1::tW !--- -+ --+--+ 1----+--+--+-+--t-+++-!- - -- M-+-+-1-+-++-H ~a, ~ --..:-- 1.2 -- - - ---- -- - Ol ~ 1 .1 -L . ' - - ' " \ I \ I 0 .9 - " R" type "S" type P-80AAR _ P-100AAS_ · 0 2 4 3 Discharge time (hours> I - ' I I 1 .0 - - ·c Charge: 1 C x 1.5hrs, 20 °C(68°F) Discharge: 0.3A, 20'C(68 ' F) > I-------- 60 140 'F I 1.3 --+-- 50 122 Fig.5: the effect of temperate on cell capacity. If the temperature during charge & discharge is high, the capacity decreases. 1 .4 u I - -- 40 104 + Fig.6: the "S" series Panasonic cells have 40% greater capacity than comparable "R" type cells. I 0 · 1 ':--1---'---'---'---'---'-'-...1...J..-'-:10:-----L--'--L-L--"-L....J...J...J100 Discharge Current IAI been matched as closely as possible in capacity to eliminate the problem of the weak cells collapsing in advance of other cells in the pack. In addition, a pushed pack is selected for maximum capacity, so a nominal l.2Ah pack may deliver in excess of l.5Ah. A "matched pack" is only graded on capacity. As you may have guessed by now, nicad cells in this type of application receive a hammering and fail reasonably often, hence all of the precautions in an effort to eliminate failures while at the same time pushing performance to the limit. The prime culprit for any failures · that do occur is heat and this problem is appi;oached by the manufactures in several ways. First, they warn the customer of the dangers and shortcomings of exceeding the temperature limit. Panasonic state flatly that discharge temperature ranges are -20° C to +65 °C and that because service life will be decreased by repeated discharges at extreme temperatures, discharges between 20°C and 30°C are recommended. Fig.5 shows the effect of temperature on cell capacity. Fig.4 (left): discharge characteristics for the Panasonic P-90 SCR cell. Now one sure way to raise the temperature of a cell is to subject it to extremely high rates of charge and discharge, exactly what modellers are now doing. Manufacturers go to great lengths to produce special cell types which will stand these extremely high charge and discharge currents. The Panasonic "R" type cell, designed for rapid charge, has a specially improved negative plate for example, with an increased gas absorption characteristic. It can be subjected to controlled charging at the 1C rate; thus a rapid charge in 1 to 1.5 hours is possible. The emphasis on the word "controlled" is mine because I often see modellers trying to cram a full charge into a cell in 15 minutes or less. This will result in excessive heating, and premature cell failure. The moral here is to use several packs and the longest charge time possible. For those with the necessary funds or dedication, there are the high capacity "E" series cells. These combine a high capacity density positive plate with a high capacity paste negative plate, resulting in a 20-30'¼, capacity increase over standard types. Next are the "RIP" high rate discharge and rapid charge cells which feature edge welded plates and terminals. This results in a sharp reduction in internal resistance and a subsequent improvement of the voltage characteristics during high discharge rates. This is combined with excellent gas absorption for high rate charging. Finally, we have the "S" series cells which feature a high density positive plate made of foam nickel and a paste negative plate. The resulting battery has a capacity 40% higher than the comparable "R" cell - see Fig.6. Again , it can be fully recharged in one hour as with the "R" type. All in all, there is now an impressive array of batteries, showing the great strides in battery technology during recent years. Without this technology Blectric flight models would still be just a dream. Acknowledgement The author thanks Malclom Wilson of Premier Batteries Pty Ltd, Chipping Norton, NSW and also "Electric Flight Newsletter" from Moore Park Model Supplies. ~ NOVEMBER 1990 107