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A Primer on Batteries
by Dallas Carter
Ok, I'm the new kid on the block, and by no means an expert, but I have had a
lot of past experience with batteries. Of the various disciplines that are
applied to this sport, it is one of the most misunderstood. I will try to
present some of the basics here as they apply to wet fueled aircraft.
Electrics have their own peculiarities, and whereas many of the principles
still apply, there are some things that are specific to electric
applications that I am not as up on. I will defer to the “E” crowd for
better definition of those particulars. I have also not had any experience
with Lithium-Ion batteries. I doubt that, for the present, many of us will
have occasion to use these anyway. My focus here will be on Nickel Cadmium
(NiCad) batteries with some comments about Nickel Metal Hydride (NiMH).
This is a rather lengthy article and is by no means the whole story.
Without putting you to sleep or exceeding the boundaries of my own personal
knowledge and experience, I have titled each paragraph so that if you have a
specific interest or question, you can skim the paragraph titles and zip
right to it.
Battery Rating and Terms
Let's first settle on some terms of reference. These batteries have a
voltage rating of 1.2 volts per cell. That is the potential
of the cell. Each cell is
rated in Ma/Hr or milliamps per hour. That is the capacity of
the cell. As we add cells in series, or end to end (+ to - to + to - etc.)
we increase the potential, or voltage of the pack. We must keep the
capacity equal on all cells. If we don't, we will wind up causing damage
to the unmatched cells. The capacity of the pack will remain the same as a
single cell under this configuration. To increase capacity (using cells of
the same capacity) we must connect them in parallel (+ to + and - to
minus). Again, we want to use cells of the same capacity. These
paralleled cells can be configured cell to cell or pack to pack. That is to
say, we can parallel two 4.8 volt packs or connect four pairs of 1.2 volt
cells in series to achieve the same voltage and capacity. Connecting
batteries of equal capacity in parallel doubles the capacity while keeping
the voltage constant.
Battery Sizing
What's the right voltage and capacity for my application? This is a
question that requires some thought and often, trade offs. The points to
consider are size, weight, load and servo speed. Ok, I threw in another
term, load. That simply refers to the drain in milliamps that is placed on
the battery pack. As an example, a 500 Ma/Hr pack with a 250 milliamp load
would last about two hours. This is fairly representative of a small
trainer. As we outfit larger craft, we start adding load. As an example, a
small plane with eight standard servos might draw as much as 500 milliamps
and reduce the operating time to one hour. Weight and size are also
considerations. Aircraft performance is often directly linked to weight.
There is often limited space to mount the batteries. As voltage and
capacity of the pack are increased, size and weight also go up. As we build
bigger and faster planes, we use larger servos. For maximum maneuverability
we want the servos to operate faster. Bigger means more load and faster
means more voltage. This is when we start to see 6 volt packs as opposed to
the 4.8 volt packs. The higher voltage gives us more servo speed, however;
it also increases load. As a
result, a 6 volt pack of the same capacity as a 4.8 volt pack will not last
as long.
What's Happened to My Memory?
Is there a problem if I use a high capacity battery pack in my small
trainer? Isn't bigger better? Well, not necessarily. When we are talking
NiCad, we must consider another term called memory.
This memory phenomena is something that we teach a battery pack.
If we don't use most of the capacity of a battery occasionally, it becomes
lazy and develops what is called memory. Take the 500 Ma/Hr pack and
consider that if you continuously only use it for one 10 minute flight at
250 mils, you will only pull about 40 mils out of the pack leaving nearly
460 mils unused. After this is continuously repeated, the battery will get
lazy and will lose capacity. It will only want to deliver a low capacity.
Let’s say that you use your plane more and draw 3 or 400 mils from the pack
during most flying sessions. That is a good load to capacity ratio. If you
then stick a 2300 Ma/Hr pack in there, you are back to the low percentage of
capacity problem and you have added considerable weight and size to your
craft. You will note that the electric flyers always run their battery
packs down after each flight. This helps to ensure that the battery does
not develop a memory and limit itself from charging fully. There are a
couple of other steps that can be taken that will minimize the memory
problem which will be discussed later under cycling and storage.
Why Dual Battery Packs?
I notice that many larger planes use dual battery packs. Why is that?
Larger planes generally have larger loads. As a result, they require packs
with larger capacity. In some cases, the capacity is so large that they use
multiple receivers to split the loads. In other cases, they just want to
increase capacity while providing some redundancy. There are two separate
schools of thought on exactly how to accomplish this and provide the desired
redundancy. I won't go into the particulars as it is often a volatile
subject and in my opinion there is no clear cut correct answer. There are
pros and cons to each approach (ie. to electrically isolate or not to
isolate the batteries).
Charging Rate
Fast charge or slow charge, which is best and how much? Generally speaking,
battery experts agree that rapid charging is best for your battery packs.
It helps to overcome some of the memory issues by generating a small amount
of heat in the pack. The exception is when the pack is new. The first
charge cycle should be at a slow rate. Slow rate is generally accepted to
be 1/10 of the battery capacity. That is to say that for the typical 600
Ma/Hr pack, it's a charge rate of 60 milliamps. There is an efficiency
factor when recharging a battery pack. The general rule is capacity times
1.6. Therefore, at 60 Ma/Hr, it would take 16 hours to fully charge the
pack. Most plug-in charger modules do not detect when the pack is fully
charged and require human intervention to limit the charge cycle. Some
folks use timers with these modules to control the time. The only problem
with this is that over time, if you have not used the majority of the
batteries capacity, you will wind up constantly over charging the pack.
This is one of the causes of short life in the battery pack. The more
sophisticated chargers have micro-processors in them that look at how much
charge the battery is accepting. As the pack reaches full charge, the
amount of current (milliamps) that it takes in is reduced. After a bit, the
charging current starts to increase again. This little current hump that is
caused by the physics of batteries, is called the charging peak. The
microprocessor in the charger detects this peak and reduces the charging
current to what’s called a trickle or float
level. Both of these terms mean the same thing. The trickle charge is just
a few milliamps, not enough to cause damaging heat to the pack, but enough
to maintain a fully charged condition. These chargers are known as "Peak
Detection" chargers and are what most serious enthusiasts use. Fast or
quick charging requires some care in order to prevent damaging your pack.
The KILLER for batteries is excessive heat. The maximum charge rate for a
NiCad pack is generally accepted to be about 2C. This means twice the
battery capacity. The exception to this rule is for NiCad cells with an "R"
suffix. These are designed for rapid charge and can be charged at 4C. For
NiMH it's about 1C. I just have NiCad packs, and generally set my quick
charger to 2C. Some chargers have their micro-processors make the decision,
others have a meter and current control to manually make the selection.
Here the peak detection becomes a very important feature. For a 600 Ma/Hr
pack, a 2C charge should be at 1.2 amps (1200 mils). If the pack had been
fully discharged, it would take one half of an hour (30 minutes) times the
1.6 factor, or about 50 minutes. The peak detector will drop the charge
rate once it detects a peak. At 4C this would come out to be about 25
minutes.
Flying Time and Battery Endurance
How much flying time do I get from a charge? There are a number of issues
involved here, transmitter battery condition, receiver battery condition,
and load based on your flying parameters. More servo operation means a
heavier load. Checking your transmitter and receiver battery condition
prior to flight is the best way to ensure that you have enough power for a
flight. Most experts (notice how I dodge responsibility here) will say that
a battery level of a few tenths above the battery rating (under load) is
adequate. Two things to explain here. It is important to measure the
battery level under load. Several model meters have loads built in so that
you read the battery level with a standard load applied. This load is
generally 250 mils. The battery will indicate a higher voltage with no
load, and will give you a false sense of security. The "few tenths of a
volt" above rating comes from the characteristics of NiCad and NiMH
batteries. You will have noticed by now that dry cell (flashlight D, C, AA
and AAA) batteries are rated at 1.5 volts and NiMH and NiCad are rated at
1.2 volts per cell. If you measure the voltage of a new dry cell it will be
1.5 or 1.6 volts. It will drain down in voltage at a relatively constant
rate. NiCad and NiMH are rated to deliver 1.2 volts at a given rate. That
would be Ma/Hr, or to put it another way, a 600 Ma/Hr cell would provide a
nominal 1.2 volts at 600 milliamps for one hour, or 300 milliamps for two
hours and so on. The actual voltage per cell at full charge is actually
closer to 1.5 volts. The characteristic to which I refer is that as the
cell is discharged in the early stages of it's useful charge, it will
steadily drop until it reaches 1.2 volts. The voltage level will then level
off and remain at essentially 1.2 volts for the life of the charge. As the
capacity of the cell is depleted, it will then start to go down at a more
rapid rate. As a result of this phenomenon, it can be seen that as long as
the voltage in each cell is slightly above 1.2 volts, the majority of the
capacity remains in the cell. As it drops below 1.2 volts, it will rapidly
be depleted.
Performance Testing
A more accurate determination can be made by checking to see how much
capacity has been taken from the battery during your flight. This requires
some advance work with a battery cycler. Go through a couple of charge and
discharge cycles recording the Ma and discharge time. A cycler will
discharge your pack to some predetermined level, normally 1.1 volts per
cell, and record the capacity and time. This is all done at some constant
load. Mine is done at either 250 or 500 Ma/Hr and is user set. After you
have cycled your battery pack a couple of times and record the readings, you
will be able to determine the nominal capacity of your pack. Let's say for
example that your nominal battery capacity is 588 Ma and it lasted for 144
minutes at 250 Ma/Hr. Charge the battery and head for the field. Fly your
plane in your typical manner for the normal period of time. Take the plane
back to the shop and put the battery on the cycler. This time, don't charge
it, just go through the discharge cycle. Let's say that when the battery is
discharged that the cycler reads 429.2 Ma. This shows the capacity and time
(at 250 Ma) left in your pack. If you subtract these numbers from the
nominal capacity recorded earlier, you will see that you used 58.8 Ma. If
your flight lasted 15 minutes, multiply that by 4 and you will come up with
a load of 235.2 Ma/Hr. This means that you can get a lot of flying time out
of that battery pack. Remember that this is just an estimate, and you
should still check battery level before each flight. Just sitting around,
the battery level will drain down slowly.
Checking Battery Condition
When should I replace my batteries? I try to cycle my batteries twice a
year. This way I have a pretty good idea of how the battery life is holding
out. I find that when cycling a battery, it should be taken through two
complete cycles. This tends to remove some of the memory that has
developed. I also quick charge the battery to capacity and then discharge
it on the cycler. This seems to also restore some of the life in a battery
pack. A rule of thumb is that when the capacity of a pack decreases to 80
percent of it's rating, it's time to replace it. It may actually still have
plenty of power to fly several flights per day, but it is an indication that
the pack is dying, and the cost of a new pack is far less than the cost of a
replacing a downed plane.
Pro's and Cons of NiCad vs. NiMH
These two types of batteries have similar characteristics, with NiMH costing
slightly more. NiMH batteries are typically smaller, lighter and longer
lasting. As an example, a 600 Ma NiCad might weigh 21 grams in aa AA size
package whereas a 600 Ma NiMH cell might weigh 13 grams and be in a AAA
package. The NiMH is less susceptible to developing memory, but is more
sensitive to heat. Thus, maximum charging rate is reduced and the use of
peak detection more critical. Manufactures claim that NiMH cells provide
almost twice the service of NiCad. Although NiCad cells are heavier and
larger for the same capacity, they can be charged at a higher rate and can
also deliver power more quickly. This is of critical importance to electric
flyers.
Discharging and Storing Batteries
What about storage or winterizing? When a battery is not going to be used
for an extended period, like a few months or over the winter or has been
taken out of service, it should be discharged. The best way to do this is
with a battery cycler. The important thing here is that the battery should
NEVER be discharged to 0 volts. It is recommended that a pack is discharged
until each cell reaches 1.1 volts. That would be a total of 4.4 volts for a
4 cell pack, 5.5 volts for a 6 volt pack and 8.8 volts for an eight cell
pack. The actual voltage is not that critical, but you want to get down to
just below the cell rating of 1.2 volts. You NEVER want to get to 0 volts.
The reason is that as batteries age, the individual cells age at different
rates. Therefore, as a really good cell reaches 0 volts, a deteriorating
cell may actually go negative. When you attempt to charge a pack that has a
cell that has gone negative, the reversed voltage causes that cell to want
to charge in reverse. The tremendous instantaneous heat generated in this
case can cause the cell to explode. That's another reason to charge new,
stored, or battery packs that have been left on and drained, to be charged
at 1/10 C. Stored packs or those that are in equipment that has been left
on and drained, should be monitored very closely for the first hour or so of
charging to ensure that no individual cell starts heating up. If it does,
turn the charger off and either dispose of the pack or locate and replace
the bad cell.
Who's to Blame for This?
I hope that this information is found to be useful. I welcome your comments
and or corrections. If you have additional questions, I would be pleased to
try to shed more light on the subject. I can be contacted at < sacrc@sacrc.com>.
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