BATTERY INFORMATION
THE CHARGING OF NiCad AND NiMh BATTERIES
by Jerry Houser
This discussion is intended
to present some practical information with regard to the
charging of NiCad (Nickel Cadmium) and NiMh (Nickel Metal
Hydride) batteries for use in electric powered aircraft.
While some of the principals are similar for Li-Poly batteries,
many of the principles are not; therefore, this is not
intended to apply to Li-Poly or Li-Ion batteries.
Li-Poly batteries require specific considerations that will NOT
be addressed in this discussion.
The term “cell applies to
an individual cell and not a group of cells. Connecting
two or more cells in series creates a battery”.
One milliamp (ma) is 1
/1000th of an amp of current. In other words
1000 ma = 1 amp.
One millivolt (mv) is
1/1000th of a volt. In other words 1000 mv = 1
volt.
BATTERY CAPACITY. The
manufacturer specifies a battery capacity for a particular cell.
This capacity is usually stated in “mahr” (milliamp hours).
For example, a 2000 mahr cell (or battery) should be able to
deliver 200 ma (milliamps) of current for approximately 10 hours
(200 ma X
10 hours = 2000 mahr).
It would also follow that this cell could deliver 100 ma for 20
hours (100 ma X 20 hours = 2000 mahr). However, our
electric aircraft tend to draw a lot more current. If an
aircraft were to draw 20 amps (20,000ma), the battery would only
be good for about 1/10th (6 minutes) of an hour of
flight time (20,000 ma X 0.1 hours = 2000 mahr). While
this is nice in theory, batteries are usually rated at the 10
hour (200 ma) or 20 hour (100 ma) current draw rates. When
one draws more than this amount of current out of a battery, the
capacity drops. In other words, the 2000 mahr battery will
not actually deliver that much capacity when we are drawing 20
amps from it. At a current draw of 20 amps, the 2000 mahr
battery will probably only last about 5 ½ minutes instead of 6
minutes. Its capacity will be more like 1800 mahr
instead of the rated 2000 mahr.
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CHARGE RATES. There are 3 common terms used when
charging NiCad and NiMh batteries. They are slow, fast,
and quick.
-A slow charge is commonly made at 1/10th C (C is the
capacity of the battery).
In this example we are
using a 2000 mahr battery so we would want to charge it at 200
ma for a slow charge. Charging is not 100% efficient, so
we would need to charge our 2000 mahr battery for about 14 hours
(40% longer than expected) to bring a fully discharged battery
back up to a full charge. It is recommended that the first
charge for a new battery be done at this rate. It is also
recommended that this be repeated after a battery has received
25-50 quick charges – this slow charge process will tend to
“rebalance” the cells.
-A fast charge is commonly made at about 1/3rd C. In
this example we are using a 2000 mahr battery so we would want to charge
it at 667 ma for about 4 hours.
-A quick charge is commonly made at somewhere between 1C and
3C. To charge our 2000 mahr battery at 2C, we would use a charge
current of 4000 ma for a little over ½ hour.
As the rate of charge increases
from 1/10th C to 1C, the charging efficiency gets
better. When charging at 1C, a fully discharged battery
will be charged in slightly over 1 hour.
Batteries are designed for a
particular maximum charge rate. While all batteries may be
charged at the slow rate, some batteries are not intended to be
charged at the fast or quick rates. Attempting to charge
“slow rate” batteries at a faster rate could result in very
(dangerously) hot batteries and damage to the cells in the
battery.
Some manufacturers state that NiCad
batteries may be charged at rates of up to 3C while NiMh
batteries should not be charged at a rate of over 2C. At
the field, there is a tendency to charge at the maximum possible
rate so that another flight can be made without having to wait
so long for the battery to be recharged. However, the
faster one charges a battery, the shorter will be the life of
the battery in terms of the number of cycles before the battery
won’t hold much of a charge. A cycle is one charge and one
discharge. For this reason, many pilots charge NiCad
batteries at no more than 2C and NiMh batteries at no more than
1 1/3C (4/3C). A good solution would be to have a
second battery pack so that one will not be tempted to charge at
quite so high of a rate/current at the field.
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It is common for a battery that is
being quick charged to get slightly warm to the feel -
especially near the end of the charge as the battery is reaching
its fully charged state. However, a battery that is very
warm early on during the charge or one which gets very hot near
the end of the charge indicates trouble. Either the charge
rate is too great for that battery or some of the cells in the
battery are going bad. Very warm temperatures during
charge should be avoided.
As a NiCad or NiMh battery reaches
its fully charged state, its voltage will peak (reach a maximum)
after which the voltage will fall slightly. This is the
characteristic that a “peak” detection charger looks for to
signal that the battery is fully charged and that the charger
should shut off (or move into trickle mode). As soon as
the voltage starts to decrease slightly from its peak (maximum
voltage) the charger knows that the battery is fully charged.
Many chargers allow the user to set the value for the voltage
decrease after the peak in order to signal the charger to quit
charging. The chargers usually ask for this value in
mv (millivolts, 1/1000th of a volt) per cell.
Most users enter 5 mv/cell for NiCad batteries and 3 mv/cell for
NiMh batteries. NiCad batteries tend to have a slightly
greater voltage decrease as they reach their fully charged state
than do NiMh batteries. Setting this value too high
results in the charger not shutting off quite as soon after the
cell reaches it fully charged state – the extra energy being put
into the cell turns into heat which is not good for the battery
and will likely reduce the number of cycles available from the
battery before it becomes unusable. Setting this value too
low may cause the charger to think that the battery is fully
charged when it isn’t and the charger will “false peak” and shut
off too soon. One may need to experiment with this to get
the setting correct for a specific battery pack.
It is an advantage to have a
charger that will not only allow one to program various
parameters into “battery memories” but also one that has a
digital display so that one can monitor the progress of the
charge. Two items are of particular interest during the
charging of a battery – voltage and the amount of charge put
back into the battery.
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As a battery becomes fully charged,
it will read its maximum voltage which is followed by a very
slight decrease in voltage that signals the charger to shut off
(or go into trickle mode). NiCad cells peak at about 1.55
volts per cell (a 10 cell battery would peak at about 15.50
volts) while NiMh cells peak at about 1.60 volts per cell (a 10
cell battery would peak at about 16.00 volts). Both of
these voltages assume that one is charging at 1C. If the
charge rate is lower, the peak voltage will be slightly lower.
Monitoring the voltage not only gives one an indication of
approximately where the battery is in terms of reaching its
fully charged state, but also gives an indication of a problem
with one or more cells in the battery pack. An unusually
high voltage reading may mean that there is a defective cell(s)
in the battery pack while a low reading may mean that there is a
shorted cell(s) in the battery pack.
The amount of charge put back into
the battery indicates (approximately) how much energy was used
on the previous flight. Remember that charging is not 100%
efficient, so if one puts 1500 mahr (as indicated on the
chargers readout) back into a battery, it means that probably
1300 mahr to 1400 mahr were actually used during the flight and
put back into the battery. Charging is about 90% efficient
when charging at 1C and only about 70-75% efficient when
charging at 1/10th C. Also remember that a 2000
mahr battery is only going to provide about 1800 mahr of
capacity when drawing high currents like we do with our
aircraft.
As an example, one makes a flight
with a 2000 mahr battery. After landing, this battery is
charged at 1C (2000 ma). When the charger shuts off, its
display indicates that 1400 mahr were put back into the battery.
What this really means is that the 2000 mahr battery should be
considered only a 1800 mahr battery at the high currents that we
are drawing and that about 1300 mahr of energy was used during
the flight and replaced during the charge cycle (remember
charging efficiency). Considering this and
monitoring the amount of energy put back into the battery during
charge, one can determine approximately how much energy was left
in the battery upon landing and one can also make sure that the
charger didn’t “false peak” and go out of the charge mode before
it should have. Let’s assume that a “normal” flight
requires that we put about 1400 mahr of energy back into the
battery during charge. We have just landed after making a
“normal” flight and put the battery on the charger. About
20 minutes later, the charger indicates that the battery is
fully charged although we notice that the display on the charger
indicates that the battery accepted only 850 mahr of energy (550
mahr less than usual after a normal flight). Something is
wrong – the most likely thing would be that the charger “false
peaked” and terminated the charge cycle too soon). If this
happens, feel the battery for warmth. If the battery is
quite warm, then it may be fully charged – but if it is not
warm, press the charge button again to re-start another charge
cycle. The two charge cycles together should equal about
1400 mahr of energy being put back into the battery.
If this “false peaking” occurs often, the voltage decrease after
reaching peak may need to be re-programmed and increased
from 5mv to 6-7mv for a NiCad battery or from 3mv to 4-5mv for a
NiMh battery. Repeated false peaking may also be caused by
a cell in a battery going bad or from a badly imbalanced battery
pack – try slow charging it to restore the balance.
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Some pilots program multiple charge
routines into a charger for one particular battery.
Routine 1: Set to charge the
NiMh battery at 1.5 C. This would be used to recharge the
battery at the field after a flight and prior to another flight.
This rate gives a fast field charge to the battery.
Routine 2: Set to charge the
NiMh battery at 1C with a programmed cutoff at 1200 mahr.
This could be used after the last flight of the day. Note
that the charge rate is a bit slower so as to prolong the life
of the battery and that it is set to cut off at 1200 mahr (200
mahr before where the charger normally cuts off when charging
the battery after a normal flight). This means that the battery
is not quite fully charged. This battery is now ready for
a very brief peak charge just prior to flying on the next trip
to the field. Using this method, the next day one needs
only a few minutes to peak charge the battery (using routine 1)
for the first flight of the day. Charging the battery just
before use warms the battery up a little which causes it to put
out a little higher voltage at the beginning of the flight.
During the flight, the battery will remain warm due to the high
rate of discharge. It is normal for a battery to be warm
(but not hot) after a flight. Fully charging the
battery after the last flight of the day and then attempting to
charge it again the next day just before a flight seems to
shorten the life of the battery.
Routine 3: Set to charge the
NiMh at 1/10th C. This is the slow charge that
should be performed on a brand new battery as well as after
25-50 fast charges (to rebalance the battery pack).
Please remember that there are many
variables in batteries including the manner in which they were
manufactured. Therefore, slight differences may be noted
between brands of batteries.
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