Keep in Touch Series

Assault and Battery

Yes, your batteries can assault your model and cause grievous bodily harm.  I’m not kidding.  In reading the model aviation journals, it is quite clear that the main reason for non-human related crashes is radio failure.  Further, radio failure is caused by two main factors:

1.         Wiring, connectors and other installation related problems

2.         Battery failure

The jury appears to be out on which of these is the main culprit.  But, nevertheless, each is a major cause of aircraft injuries.

Most folks use NiCad batteries for their transmitter (TX) and receiver (RX).  A smart choice, of course.  But it’s only smart if we are smart enough to properly manage those NiCads.  I have a technical degree, but not in battery technology, so, although I write this and I reasonably understand the details that make rechargeable batteries work, I’m really only summarizing the various writings of others in the hope that this article will help you not lose a plane one day.

Factoids:

NiCad - Nickel Cadmium.  NiCads generally do not have the same attributes as found in a non-rechargeable cell, such as a Duracell alkaline.  Alkalines have a very long shelf life and they will output more instantaneous power, size for size, than a NiCad, but, of course, they are not rechargeable.  So, why are NiCads used?  Economics, really.  They work out much cheaper to use because they can be recharged.  While this is true, please do not forget that NiCads do have some undesirable characteristics.  One of which is that they don’t last forever.  After recycling (charging/discharging) many times (usually measured in the hundreds) their performance drops off.  In some cases a cell will become totally defective, rendering the pack unusable.  And, if this happens and it is not noticed, it tends to render your plane unusable and un-repairable too.  If you are really unlucky, this could happen much sooner rather than later.  So check your batteries right from when they are new.

Shelf/Charge life - for NiCads is very short relative to Alkalines (Non- rechargeable).  Alkalines can have a shelf life measured in years - a NiCad has a shelf life (How long a charge will last) measured in days. What this means to the R/C flyer is simple.  RECHARGE YOUR BATTERIES (TX AND RX) FOR 16 HOURS IMMEDIATELY BEFORE YOU FLY.  No “Ifs”, “Ands” or “Buts”.  Just do it or don’t fly....  A plane that has no radio is not only costly for you, it is a serious danger to everyone at the field and to neighbors within the plane’s flying range.

Voltage - the nominal voltage of a NiCad cell is about 1.2 volts.  That means that a four cell RX pack will be 4.8volts and eight cell TX pack will be 9.6 volts.  If you fly with the voltage much below the nominal voltage - better start thinking about what you next plane will be!!!!  So, periodically CHECK THE VOLTAGES AT THE FIELD BETWEEN FLIGHTS.  A dead or dying cell will only show up as a low voltage after a few minutes flight time.  I’ll explain later.  One irritating aspect of a NiCad is the one attribute that it has that makes it so appealing to us modelers.  For the first 95% of its charge life, it stays at or near its nominal voltage under load.  So, by the time you see a drop off of voltage it’s often too late, because the charge will literally crumble to nothing within a few minutes.  If you don’t land your plane real fast, it’s off to the hobby shop for you.

Amps - is the measure of flow of electrical current.  Using the analogy of flowing water; amps is to electrical current as gallons per minute is to flowing water.  Examples: An electric heater and an electric shaver both use the same voltage (110volts) but the heater draws many more amps of current than the shaver.  A 60watt electric light bulb draws more current than a 40 watt bulb, and it gives off more light too.

MilliAmps - One milliAmp (1mA) is one thousandthorizontal stabiliser of one amp.  Therefore 500 milliAmps (500mA) is a half of one amp.

AmpHours (milliAmpHours) - is the TYPICAL time (according to the manufacturers) that a battery will offer acceptable output for a given load or current draw.  A typical/stock RX pack is 500 milliAmpHours.  This means that if its output current is constant at a half amp (500 mA) the pack will give an acceptable output for one hour.  At a quarter amp (250 mA), the output will last for two hours.  OK?  Well not really - What’s ”Acceptable”?????.  Also, this is the figure measured by the manufacturer in a laboratory.  At four hundred feet after 30 minutes of flying in very cold, or very hot conditions, the lab results just might not cut it.  After all,  it’s your plane up there, not theirs.  So, don’t rely on this potentially bogus factiod and “Calculate” how much more flying time you have -- just “‘cos it says so in the book”.

So, what is the surefire way of checking the batteries?

The only certain way is to simulate actual load conditions for the duration of the charge, and measure the battery’s performance.  (For you folks that have battery cyclers, you’re all set.  You can stop reading now).   That means log the voltage under continual load at preset intervals until the battery is discharged.  This list of voltages can be plotted on a graph to show you the “Discharge curve” of YOUR battery.  If you do this  when the battery is new, you will have created a “Benchmark”.  A benchmark is a set of initial results that you can use to compare with later results to indicate if any deterioration has occurred over time.  If you repeat the test - exactly - periodically (say every week or month depending on how much you fly) any significant deviation from the benchmark will indicate a problem.  (REMEMBER, THE LOAD MUST REMAIN ON THE BATTERY THE WHOLE TIME DURING THE TEST)

The test!  In order for this type of comparison to work, the first and subsequent tests must be identical.  The graph below shows my batteries when they have just been charged for 16 hours.  So next time, I’ll do the test after a full charge also.  Obviously, you must use the same loading device.  An expanded scale meter with a built-in load is ideal for this.

Notice the high voltage peak at T zero “Overcharge”.  That is a characteristic of a NiCad battery following a full charge.  The rapid dip to nominal voltage in this case is to be expected.  Even if a pack has a dead cell, directly after a full charge the pack could indicate nominal or above nominal voltage.  This is due to the expected “Overcharge” condition just mentioned.  A dead cell may recover some when it is off load, but it will begin to take effect within a few seconds of applying the load.  So, when making an instantaneous field measurement, leave the load on for at least 20 seconds or more.

Lastly.  I know you all don’t have computers, so you can easily plot this graph on paper by drawing equally distant graph lines, placing dots corresponding to the intersections of test times and the voltage measured.  Then connect the dots.  Alternatively, if you take your own measurements every 15 minutes intervals (intervals must be identical). Bring them to a general meeting and I will plot them for you by the following general meeting or sooner -- what a deal.

Even more lastly.  It can be tricky measuring the batteries in a Futaba Conquest TX, and maybe some others too.  They (Futaba) put a diode in series with the charging jack.  This allows charging current to enter into the TX but, in the event of an external short-circuit,  current will not flow out of the TX.  Thus protecting the battery pack.  This means you can not use the charging jack to connect to the battery.  So, you can do one of two things:

1.         You can open the back of the TX and disconnect the pack from the electronics and use the connector on the battery wires for access to the battery pack.

2.         You can permanently short out the diode using fuse wire (less than 1 amp blow point).  This will allow you to use the charging jack to gain access to the battery and it will still provide battery protection in the event of an external short circuit.  You will need to make a special cable that connects the charging jack to the expanded scale voltmeter.  (DON’T PUT PROBES INTO THE CHARGING JACK)

Both methods are fraught with dangers if you’re new to this.  Method two should not be attempted unless you are adept with soldering printed circuit modules and you know exactly where to make the modification.  Method two will also void your warranty, and method one may void the warranty if you cause any damage.  If you don’t want to do either 1. or 2. to your TX, here’s a less difficult, and less accurate, way to do it.  Since the meter on your TX is a voltmeter you can use it in a timed load test as follows;

1.         Recharge the TX battery pack as normal (16 hours).

2.         Remove the TX from the charger.

3.         Fully extend the TX antenna (DO THIS OR YOU CAN DAMAGE YOUR TRANSMITTER).

4.         Turn on the TX and note the meter reading (Although it’s really measuring voltage it may be shown as a simple number or a percentage).

5.         Leave the TX switched on and take a meter reading every 15 minutes.

6.         When the meter reading reaches the “Red line”, terminate the test and recharge the TX.

7.         Plot all the meter readings on a graph as before.

Because the TX’s meter is not an expanded scale, and because it is small, this method is much less accurate than when using proper test equipment -- but it’s better than not doing anything at all.  Other TXs may be different.

There is one other benefit of doing this test routinely.  When NiCads are partially cycled, such as under normal fly conditions.  And when they are partially cycled to about the same level of discharge each time, the cells may adopt a “Memory”.  In this condition, the cell becomes accustomed to the partial discharge and it assumes that this is a complete discharge.  In other words, after a time, the battery will not allow current to be draw beyond its new discharge point - your 500mAhr battery has become a 250 mAhr battery.  Periodic full cycling as described in the tests above will ensure that the cells do not adopt a memory.