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(Article submitted by Peter
Fundarek & Walter Gray)
The following advice is for the care and maintenance of battery packs for
electric flight motors. It has been put together from a number of
different sources but primarily from members of the Electric Model Flyers of
Southern Ontario (EMFSO).
Due to the nature of their use, motor battery packs are treated very differently
from battery packs used to power the radio system. Radio batteries are
typically required to provide a discharge current of approximately 250-600
milliamperes (mA) at any given time while motor battery packs generally operate
in the range of 10 Amperes (A) for very small or slow planes to up to 30-40
Amperes. While most receiver packs will be re-charged at 60 mA or so for
12-16 hours, motor battery packs are designed to be charged at a rate of 2-5 A
for 15-30 minutes. Due to the differences in their intended use, the
management of motor batteries is different from those of radio batteries.
All further references to batteries will involve only those designed for use as
electric motor batteries.
In general, the best batteries for electric flight are those manufactured by
Sanyo. They have a low internal resistance and are designed to
supply the high current required for this application. There are generally
two types for our purposes, SCR and SCRC batteries. Be sure to specify one
of these types when purchasing batteries or the pre-manufactured packs.
Nicad batteries are made by rolling together two plates with a perforated
insulator in between. The insulator allows the current to flow from one
plate to another, thus making the battery functional. Sometimes, due to
use, small 'bridges' form across the insulator and connect the two plates. This
then shorts out the cell and makes it useless. In motor batteries, the
high charging rates tend to blow out these bridges thereby allowing the battery
to last longer. However, they are still susceptible to having the battery
contents dry out due to prolonged trickle charging.
At the positive terminal of the battery is a small re-sealable vent. If
the pressure builds up inside of the battery case, the vent will open and allow
the gases to escape. Normally the battery will only develop pressure
when charging and then only if overcharged. If the cell vents then it will
lose some capacity and may be permanently damaged if the vent does not re-seal
properly. Therefore it is important to watch cells carefully while
charging to make sure that they do not become overcharged. An
overcharge condition with venting can happen within one minute of fully charging
the battery pack.
Battery capacity is measured in terms of mAh and normally batteries are
available in a wide range of capacities from 1000 mAh to 1700 mAh. The
higher the capacity of the battery, the heavier it is so there is some weight
penalty with high capacity batteries. There are 2000 mAh batteries
on the market but they are special versions of the 1700 mAh battery and I am not
convinced at this time that they are worth the extra weight or value. The
total motor-on time will depend on the amount of current that your system draws
and the capacity of your battery pack. They are related according to the
following equation:
Time (min) = (pack size x60)/current draw (mA)
For a 1700 mAh pack on a system drawing 15 A (15000 mA), the equation becomes:
Time (min) = (1700 X60)/15000 mA = 6.8 minutes.
Obviously, the lower the current draw, the longer the run time of the motor.
Current draw can be reduced by using a gear drive on the motor, using a speed
controller or using a smaller prop. Speed controllers work because
although full power may be required to get airborne, once there, you can
throttle back and reduce the current flow.
There are many different chargers on the market with most providing charge rates
of 2-5 A for the duration of the charge cycle. As the battery is charged,
the internal resistance changes and the voltage measured at the battery slowly
increases. When the battery is fully charged, the voltage reading will
peak and then start to go down. Note that this decrease in voltage
is only about 50 mV so you need a good digital voltmeter and you must watch the
reading at all times. If the battery is charged beyond this point, the
power supplied to the battery will be simply converted into heat and the battery
will vent (through the holes at the positive terminal) or more likely, explode.
Therefore, it is very important not to try and charge the battery beyond full
capacity.
The simplest chargers use a timer to turn off the power after a selected
charging time. However, the amount of current that they put through
the battery depends on the initial charge state of the battery. These
types of chargers must never be left unattended while charging as it is very
easy to overcharge the battery. The instructions with these chargers tell
you to set the timer for a specific time period when charging batteries of a
certain capacity. However, the total charging time will depend on the
initial charge state of the batteries and their capacity. If partially
charged cells are put on for a full charge then the cells will be overcharged
and vent. Therefore, always discharge the pack to a consistent level
and then charge up the batteries. Note also that this will probably be
conservative in the amount of charge and the batteries will never get to full
capacity. A better way when using these chargers, is to use a digital
voltmeter to watch the voltage at the battery terminals. As the
batteries are charged, the voltage reading will slowly rise. When they are fully
charged, the voltage will then start to slowly decrease. When you notice
that the voltage has gone down 50 mV, you have reached the peak and the charging
should be stopped. What you are in effect doing is making yourself a peak
detector charger. However, this method, while useful, requires a lot
of time and a constant watch over the charging voltage. As noted in the
earlier part, a time of one minute can separate the full charge from a vented
overcharge.
More sophisticated (and more expensive) chargers are available which
automatically detect the peak and shut off the charger. For this type of
charger, known as a peak detector charger, the only control is a push button to
start the charging cycle. The best peak detector chargers will also supply
a constant current to the battery pack thus ensuring that the charging takes
place in the least amount of time. The initial charge state of the
batteries is not as critical with a peak detector as the voltage peak will be
the same regardless of the initial charge. You can even put a fully
charged pack on the peak detector charger just before flying to get the maximum
capacity of the batteries.
Most chargers will automatically switch to trickle charge mode when the charging
cycle is completed. Motor battery packs should not be charged up to
capacity using the trickle charger. They need the full amperage push from
the charger to maintain their capacity. However the battery pack can be
left on trickle charge overnight to top up the batteries. Be careful not
to leave it on trickle charge for too long though as it can dry out the
batteries with a reduction in useful life and capacity as a result.
Battery packs should be trickle charged once per week when not being used to
maintain their condition and capacity. It is best to use a timer for this
to make sure that they are not kept on the trickle charge for too long though.
A couple of hours at a time should be sufficient.
When charging the batteries, it is good practice to charge them from the same
level. That is, the pack should always be charged when drained down
to a consistent amount. This provides the maximum exercise for the pack
and will maintain it in good condition. As a general rule, do not
discharge battery packs below 1 volt per cell. In other words, the
voltage of the pack during discharge should not go below the number of cells.
For a 7 cell pack, that means the voltage while discharging should not be taken
below 7 volts. This will avoid discharging any one cell too much and
damaging it. In a multi-cell pack, it is possible for one cell to become
so discharged that it will take on a reverse charge and then the pack will be
damaged.
Battery packs can be purchased as an assembled unit or they can be made up from
individual batteries. There are advantages and disadvantages to each
option. Assembled packs are pre-packaged and do not require any skill to
use other than proper care. However, the connector that is included
may not be the one you want and you may not be able to purchase a pack with the
number of cells that you want. Assembling your own is a bit tricky because
you are applying a considerable amount of heat to a battery that does not
appreciate it and if you are not careful in discharging the cells first, you can
cause a short circuit that could damage the batteries (or worse). However,
you can assemble packs in any configuration that you want and with any size
capacity. You can also include the connector of choice.
I started out with some pre-fabricated packs but ended up disassembling them and
making them up into my own units. The main reason for this is that I had a
bunch of 6 cell packs and needed some 7 cell packs to power the gear drive on
the Electra. However, new packs that I have purchased were made up from
individual cells that I bought.
Before making up the packs, you have to decide how the batteries are going to be
situated. There are two options, end to end or side by side. I
prefer the latter as it is easier to make packs of any number of cells this way.
Also to change the number of cells in a pack is a simple matter of cutting one
off the end. Finally it also allows you access to each individual cell for
maintenance and testing. The main disadvantage is that the cells are more
susceptible to being shorted and thus can be hazardous in that regard.
However, with care, I have not found this to be a problem.
Make sure that all your cells are completely discharged before starting.
Discharge them one at a time through a 0.5 ohm resistor first and then short
them out to remove the remaining charge. It is very easy to accidently
short cells during soldering and you want to avoid damaging the cells (or
yourself).
If you decide to set up the pack with the batteries side by side, it is easier
to solder them if they are all glued together. For this I use 'Shoe Goo'
which is available at Canadian Tire and similar stores. It remains
slightly flexible and is easy to cut when necessary. It also comes cleanly
off the battery with a bit of work. Position the cells as you wish
and then apply glue to one cell at a time and press it to its neighbour.
Make sure that each cell is oriented opposite that of its immediate neighbour.
That is, the positive terminal end of one battery should be beside the negative
terminal of the battery beside it. This will make for a short, easy
connection. Tape the whole set in place (I work on a 12" by 12"
ceramic tile) and wait 24 hours for it to set up. If you are making up
cell packs with the cells end to end, you cannot glue them together beforehand.
The main thing to remember when making up packs is to keep the batteries as cool
as possible. Heat is the enemy here and if the battery gets too much heat
it can vent, just as it would do in an overcharge situation. Keep a bottle
of refrigerant or other canned electronic component cooler handy and spray the
heated area after removing the soldering gun. This also helps to keep the
heat from building up in the battery.
I will only discuss soldering side by side packs here are end to end soldering
requires a bit more skill and technique.
When soldering the cells together, use a high wattage iron, at least 100 watts
or more (a soldering gun is ideal). Heat the location to be soldered and,
using rosin core solder, apply a little bit to cover the area, effectively
creating a solder 'button'. Immediately remove the soldering gun as
soon as the solder appears to adhere to the battery. Repeat on the other
terminal and for each of the batteries to be soldered. This provides
an initial contact that makes the rest of the soldering easier. Use
heavy wire to connect the batteries. Woven wire strap of 1/4" width
is good although the use of two pieces of #14 gauge copper house wire has also
been advocated. Whichever you use, apply a bit of solder to it as well to
tin it, before you attach it to the battery. Then, to attach the
connectors, position the tinned connector over the solder button and then heat
the connection so that the solder on the connector and on the battery melts and
joins. Immediately remove the soldering gun and spray with coolant.
Feel the battery to make sure that it is not too hot before soldering the other
terminal.
When you have finished connecting all of the batteries, connect the positive
lead wire to the remaining positive terminal and the negative lead wire to the
negative terminal. One terminal will be at one end of the pack and
the other at the other end of the pack. Place the whole pack in some large
diameter shrink wrap material and heat to tighten the plastic. This will
effectively insulate the ends of the batteries making them less susceptible to
shorting out.
This is but a brief description of the soldering process and you should make
sure that you are comfortable with the procedure before starting. However,
it can be done and it is an effective way to get the size and configuration of
battery pack that your system requires.
Rechargeable batteries require a certain amount of care in handling and
charging. If a few simple precautions are followed, they will last
for many years of charging and recharging.
Minimize the physical stresses on the batteries. Fortunately, due to the
nature of electric flight, there is little vibration which will harm the
batteries. However, take care not to drop or otherwise damage the battery
packs. Occasionally check them for signs of denting or damage, especially
following a 'hard' landing. Keep them free of dirt and debris as this can
short out the battery contacts if it becomes wet.
Exercise the batteries regularly to maintain full capacity. Between flying
sessions, trickle charge the batteries for one or two hours per week after you
have charged them to full capacity on the regular charger. Do not leave
them on the trickle charge continuously as that will dry out the contents of the
battery. On a regular basis, especially if they are not regularly used,
discharge the pack to a level of one volt per cell (i.e. 7 volts as measured
under load on a 7 cell pack) and then charge them back up again. The
discharging can be done through some charger units, by using the motor on the
plane (watch the propeller) or by discharging through a lamp or series of
resistors. I have built a unit which uses a series of resistors to
provide 5, 10 or 15 amp discharge rates. Fans provide cooling air for the
battery and the resistors (they do get hot) and the discharge rate is reduced as
the battery voltage drops. Whichever method you use, monitor the voltage
and make sure that it does not drop below the magic number.
When you have used the pack, if it has not been fully used, it is best to
discharge the pack (as described above) to allow for a full recharging cycle.
This will ensure that the batteries maintain their maximum capacity.
To check a battery pack for a faulty cell, charge it up to full capacity and
measure the voltage of the battery with no load or charger attached. Set
the battery pack aside for 24 hours or more and then measure the voltage again.
It should be relatively close to what it was before. A significant
decrease in voltage means one of the cells is faulty. Measure the voltage
on each cell individually to determine the faulty one. It may be possible
to recover this cell if you can charge it by itself first.
After the flying season is finished, motor battery packs are best stored with no
charge on them. Discharge each pack to about one volt per cell.
Then, if you have access to both ends of each cell (an advantage to having made
up packs with side by side mounting) you can discharge each cell in turn through
a 0.5 ohm resistor. I use test leads with alligator clips on each end.
I splice the resistor in the middle and have several of these so that I can clip
one lead across each cell in a pack, thereby shortening the process. Use a
20 watt resistor as there is a fair amount of current discharging through these
resistors. Leave the shorting resistor on overnight to drain the last of
the current and then remove all clips and store the battery for the winter.
Allow enough time for a full charge when the flying season is up and running
again.
When discharging for the season, it is a good practice to check the capacity of
the battery packs. This is done by discharging them at a constant current
and measuring the time taken for the voltage to drop to one volt per cell.
Knowing the discharge current and the amount of time taken allows you to
calculate the capacity of the pack in mAh. Keeping a record of this will
allow you to monitor the health of a battery pack and to note developing
problems. Note that a pack may not reach its indicated capacity until it
has been through several discharge/recharge cycles. A well used pack may
indicate a capacity higher than the indicated amount and this is perfectly
normal. A well used pack with a capacity well below the indicated amount
should be checked for a bad cell. If none are found, the pack should be
'exercised' by taking it through a few cycles. If it still does not show
signs of improving, accept the fact that the pack may be on its way out and
start saving for another.
(Article submitted by Peter Fundarek
& Walter Gray) | 
