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Battery Testing for RC |
R. W. Stuart The purpose of this paper is to help you check your R C batteries to avoid battery related problems during the flying season. Trouble comes from good batteries not properly charged and from bad batteries, no matter how they are charged - or both! All of the tests presume that a physical check has been completed; no broken wires, no warty or bumpy insulations and no green corrosion spots exist. The battery should have a 24 hour slow (1/10 C) charge. Modern R C systems use rechargeable NiCad batteries which are composed of nickle-cadmium cells. All NiCad cells are 1.2 VDC nominal, but they vary in capacity (amount of current they can deliver) depending on the area of the nickle and cadmium in chemical contact in the cell-- which dictates the physical size and cost of the cell. Most systems use 500 milliampere-hour cells which means the cell can deliver half an ampere for one hour or quarter ampere for two hours. We have two properties of a cell; the voltage (volts) which depends on the two metals of the cell, and the current (amps) which mirrors the area of the two metals. Obviously, the uncharged cell has no voltage and no current. Since current is also a function of connected load, the absolute current capability of the cell is expressed in ampere-hours as opposed to the current being delivered at any instant which is amperes. In our case instead of using 0.5 Ahr we are talking about the equivalent 500 mahr cell. A 500 mahr cell can deliver 100 ma for 5 hours after which it is dead. This does not mean that it was useable for R C for 5 hours. The system went down long before the battery died and the battery died when any included cell went dead. You and I must develop a sense of how much of that 500 mahr we can consume before the demons of electronics put the big thumb on us. The airborne pack (battery) is four 500 mahr cells in series producing a voltage of 4.8 VDC nominal with 500 mahr capability. Remember that only part of the 500 mahr is yours. If a cell fails the pack looses 25% capability and will provide only marginal to failing operation. It then becomes very important to spot a cell early on its way to failure and one way to do this is to test for the 500 mahr capacity of the battery. If a battery shows only 400 mahr, it is suspect and should be repaired or replaced. Battery repair will not be discussed in this paper. The receiver with four servos at idle uses about 40 ma. A good battery would hold this load up for 10 hours or so, but if a servo gets tweaked it draws much more current to move and four servos all moving at once may draw 200 to 300 ma for a time. Stalling servos consume even more current. The point is that we know very little about the airborne electrical load, so must be very conservative about how long we think the airborne battery will dependably behave. By looking at the scale on an Expanded Scale Voltmeter (ESV) we get an idea of the required conservatism to safely use the batteries. The top end is 5.8 VDC but the most you can get into a battery is 5.7 VDC and part of this is surface charge and will decay to 5.6 VDC which is the highest useable voltage you will see on a nominal 4.8 VDC pack. The yellow or caution band on the ESV is from 4.75 VDC to 4.80 VDC with 4.8 VDC to 4.75 VDC red. Note that you are being cautioned at the nominal voltage rating of the battery------ how can you get more conservative! I propose two different tests be used. The first is a capacity test of the battery designed to show the absolute mahr capability of the battery so we can anticipate failing cells. The mahr test to be performed once or twice a year. The second test is a charge condition test- very transient and changes from flight to flight. The charge condition test uses an ESV associated with a load. This should be a field test along with range testing prior to each day of flying and can also be useful as the number of flights for a given pack accumulates without charging. When we show the charge condition test you can see that it tells you how you are getting along, but does not show a pending catastrophic cell failure.
Table No. 1- Rcvr mahr capability
battery test - use 10 ohm, 5 watt load
Time min V-ana ma mahr summ
----------------------------------------------
8:15A 00 5.50 -- -- --
8:20 05 5.25 537.5 44.80 44.80
8.25 10 5.10 517.5 43.12 87.92
8:30 15 5.01 505.5 42.12 130.04
8:35 20 5.00 500.5 41.71 171.75
8:40 25 4.98 499.0 41.58 213.33
8:45 1/2 hr 4.97 497.0 41.46 254.79
8:50 35 4.95 496.0 41.33 296.12
8:55 40 4.89 492.0 41.00 337.12
9:00 45 4.80 484.5 40.38 377.50
9:05 50 4.75 477.5 39.79 417.29
9:10 55 4.60 467.5 38.96 456.25
9:15 1 hr 4.43 451.5 37.63 493.88
9:20 65 3.70 406.5 33.99 527.76
9:25 70 2.50 310.0 25.83 553.59
9:25 4.6 Disconnect load
9:26 4.7 Load disconnected
9:27 4.8 Load disconnected
9:28 4.8 Load disconnected
9:29 4.8 Load disconnected
9:30 3.0 Connect load
9:31 2.2 Load connected
9:32 0 Load connected
______________________________________________________________
Notes for Table No. 1: min is elapsed time, V-ana is voltage using a
standard meter because the ESV will not read the low voltages, ma is
the average milliamp load over the interval- calculated by averaging
the voltages beginning and ending the interval and dividing the average
voltage by 10 (the load ohms) to get the average current over the
interval, mahr is the average interval current times the duration of
the interval (1/12 hour or 5 min), summ is cumulative mahr at each five
minute interval. 4.8 VDC is the nominal battery voltage. You may use
longer intervals if you wish, but the accuracy is decreased. Consider
two readings, one at 8:15 and one at 9:25. The elapsed time is 1 1/6
hours and the average current is 537 ma plus 310 ma divided by 2 or 423
ma average. 423 ma for 1 1/6 hours is 493 mahr, where the 5 minute
interval calculations produced 553 mahr for the same battery and same
test--- 11% error. We are testing the ultimate current delivery of the
battery and take the battery to its total discharge. The battery
delivers a very satisfactory 553 mahr and shows no cell failure.
The activity from 9:25 to 9:31 points out two critical characteristics of the NiCad system. First, the battery voltage under load drops very fast at the end of the charge so there is little time between almost failing and failing due to charge. Avoid this area by never operating below some safe voltage while under load. This will be defined as we look at the field type testing using the ESV with its load. The second characteristic is that the voltage recovers some when the load is removed. In the failing airborne situation do not crank move after move into the flight- make ever so slight heading changes and try to get back to the field. The daily and flight line receive battery checks depend on an Expanded Scale Voltmeter (ESV) composed of a standard cheap analog voltmeter driven by a solid state circuit which turns on at 4.5 VDC (with zero voltage output) and drives up to 3VDC output with 5.8 VDC input. 3 VDC is the range of the cheapie meter and that range is calibrated as 4.5 VDC to 5.8 VDC with numbers you can hardly read and colors to indicate good, warning, and bad. The meter assembly also has a means of loading the battery with meter only, 200 ma and meter, or 500 ma and meter. I do not think that the 200 ma and 500 ma load is accurate. Some ESV are multimeters with ranges to cover 6, 8, and 10 cell batteries as well as the 4 cell batteries. Your ESV may be slightly different, but is basically as described above. The test is fast and simple: 1) connect battery to meter, 2) read the no-load battery voltage, 3) mentally or actually record no load voltage, 4) switch in the load, 5) mentally or actually record the loaded battery voltage, 6) disconnect load and observe battery recovery, 7) compare the results with actual or mental records from the calibration test, 8) decide on the present state of the battery and act accordingly. Are you beginning to notice how much of this battery business depends on you with your good judgement and experience? As an example I will tabulate the results of an extended, but not a mahr, calibrating test of a receive battery.
Table No. 2: Rcvr batt charge condition calibrating test
Time Elapsed Time Load ESV Mahr
9:07A 0 hr None 5.72 VDC None
9:08 0 200 ma 5.60 None
9:38 1/2 200 4.95 100 mahr
10:08 1 200 4.80 200
10:38 1 1/2 200 4.60 300
Interpretation of Table No. 2 Results: At time 9:07 we see the totally charged battery with transient surface charge. By the time you drive to the field the surface charge will be gone. At 9:08 the load is connected and we are operating; at 10:38 we have operated at 200 ma load for 1 1/2 hours, consumed 300 mahr of the charge and have reached the red portion of the ESV, but the system is still operational. This is 9 flights of 10 minutes each and that is plenty. Operation becomes questionable, but you never run full bore for 1 1/2 hours-- you fly 5 to 8 minutes and then exercise your mouth for the next 20 minutes; this saves the batteries and gives them a chance to "recover". They never regain charge during "recovery" but have a chance to get rid of some chemical concentrations and heat that inhibit the ability to deliver current. Any time you run a momentary load current check with the ESV it places your battery condition (voltage) along the range (Table No. 2 above) from zero to 300 mahr consumed and gives you a pretty good idea of the present charge condition of the battery. You may elect to use quarter hour increments for your calibration to provide better definition of decreasing volts with increasing battery charge consumption. Transmitter checks should be concerned with ultimate battery capability and the momentary charge condition of the transmit battery. The transmit battery is composed of eight 500 mahr cells in series- a battery at 9.6 VDC nominal and 500 mahr capacity. The FUTABA - 6 Ch- FM transmitter draws 190 ma whenever it is on (no matter how much you crank the sticks) and has a battery condition meter built in. The meter is marked in % battery charge. The transmitter battery is tested for ultimate capability (total time) by simply turning it on and recording the transmitter meter readings over a period of time with the antenna stubbed or extended; whichever gives the lower meter reading. Since the load is constant, simply recording the time will define how many flights are safe as far as the transmitter is concerned. Most transmitter times are about twice the receiver battery time. Keep watching the xmtr meter.
Table No. 3- Xmtr test using xmtr meter
and antenna extended xmtr as load ( 190 ma)
Time elapsed time meter
------- -------------- -------
9:04A 0 100% ++++
9:34 0.5 hr 100% +++
10:04 1.0 hr 100% +++
10:34 1.5 hr 100% ++
11:04 2.0 hr 100% ++
11:34 2.5 hr 100% +
Break----------------------------------
5:00P 2.5 hr 100% +
5:30 3.0 hr 100%
6:00 3.5 hr 90%
This test shows that the battery still had current left after 3.5 hours which calculates to 665 ma, so obviously the xmtr was not using the 190 ma as published by FUTABA- something less. It may be that I forgot to extend the antenna which would change loading. The system was still operational across the basement and would run the receiver and servos with no difficulty. Two continuous hours of operation is many flights. For this specific radio I would not fly after the meter got close to 100%- say 100%+. Examine your own system, record the results and establish a point beyond which you feel you become unsafe-- stop ahead of this point. The test also shows that the battery can deliver 500 or more mahr before it starts to slow down. Next year when you run this long test, if it fails at 400 mahr, consider that cells have failed, and should be corrected. The day after day and flight after flight test is a simple examination of that xmtr meter- by the long xmtr test you know where trouble will start- watch it constantly and do not pull that "just one more flight" stuff because it may well be!! The receive system fell apart six flights ago anyway. The xmtr has an advantage in that the load is constant while operating and you can almost constantly observe the meter. Do you look at your transmit meter each time you move to the flight line? Again, your judgement and experience are important to reasonable battery use. Don't forget to extend your antenna, and do not push your luck!! |
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