**Lithium Ion Polymer
Applications**

**If they are 4.2 volts a
cell when full, how do we apply that voltage?**

The first thing to
understand is how many volts you'll need to fly your model. Lithium cells
are 3 volts per cell when empty and 4.2 volts per cell when full.
Generally a 2 cell lithium polymer pack will best simulate a 7 cell NiCad or 7
cell low resistance NiMH pack or a 8 cell typical NiMH pack. A 3
cell lithium is much like a 10 cell NiCad/HV NiMH or 11 cell typical
NiMH. So, if you’re replacing an 8 cell 720 NiMH pack in your
S-280 or S-300 Bird, we're going to need a 2 cell pack. If you’re
replacing a 10 cell 600 AE NiCad pack in a 7.2V S-400 Ship, we're going to need
a 3 cell pack. With NiMH or NiCad cells we can make nice small
incremental jumps in our voltage. A bird that doesn't fly properly
on a 7 cell NiCad 600 AE pack can often be fixed by adding a cell.

**How much current (amps)
can I pull from a Lithium?**

Rather than trying to
learn the individual discharge amp limits of every cell, it's a good idea to
learn the "formula". It's very simple really. All of the
Kokam cells listed with Radical RC can handle continuous discharge rates from 3
to 4C. "C" is capacity. So, if we pick a cell out,
let’s say the 2500, we know it should perform well if we limit long full
throttle current to 7.5 to 10 amp (please note, there are 1000 mAh in 1
amp) In other words, 3 or 4 X it's capacity (shown in mAh). Learn the
formula, then you can look at any cell and know what to expect. An
example to look at is a ZAGI, it is very common for these to be flown at or
near full throttle for the full duration of the flight. If we need to
support 10 amps on a constant basis we need to find a cell that when its
capacity is multiplied by 3 or 4 results in 10 amps. So, the
2500x4=10000 or 10 amps, the 3300x3=9900 or 9.9 amps, 3300x4=13200 or 13.2
amps. You might ask "which is the better number to use when I
multiply the capacity, 3 or 4"? Generally, if you know the model
will be at absolute full throttle ALL the time, lean heavily towards the cell
that when multiplied by 3 equals the current you need. If the model
will see mostly high throttle, average of 80% throttle, then you can use slightly
smaller cells and use the factor of 4X capacity.

**Can they deliver more
current for shorter periods?**

Yes! This is
the good news. Most models fall into the "Fun-Fly" or
"Stunt" category. When the model gets an extreme amount of
throttle control (very little full throttle and then only very short bursts) we
can use a factor of 7C. An example of this is our Edge 540. On the
BD301 it pulls about 8 amps from a 2 cell Li-Poly pack. Our
favorite pack for this model is in the 2000 mAh range. However, we've flown
in many times on an 1120 2 cell pack. Some quick math, 8amps/1.120(amp
capacity of pack) =7.14. So, were flying this particular pack just
over 7xCapacity or 7C! The model will still hover for 30 seconds at a
shot a time and provides a very satisfying aerobatic flight. The
pack works fine and no noticeable drop off. But, keep in mind this is a
close in high power to weight 3D fun-fly model. The throttle is moving
all the time and is seldom at full throttle for very long. Another
example is an E3D. At 40 amps, we can get away with a pack in the 5700 to
7500 mAh range easily. This type of model naturally is not held at full
throttle for long periods.

**What about my Heli?**

This is an interesting
challenge. Unlike a fun-fly airplane, a heli has a long hard drain
and the battery never gets any off time or very low throttle time.
We need to pick a lithium capacity that fits in the 3-4C range for best results
here. If you’re flying 10 cells now and hover current is 20 amps (20,000mah),
we need a pack of 5000mah to 6600 mAh range to support this. If we want
to do some more aggressive flying we need a larger pack. Bear in mind
that your hover current with the Lithium pack will be reduced somewhat due to
lowering the weight of your machine. If we design the pack to handle your
machine at its heavy NiCad or NiMH current then we've also built in a little
buffer.

**How much current is my
Heli or Airplane pulling?**

Nothing beats an Astro
Watt meter for taking actual static amp readings. Short of that, there
is another way to figure your "Average" current. First,
understand you don't get the full capacity of your pack in a flight with the
only exception being very slow floaters like thermal sail planes.
Let’s presume first that we only get about 90% of out capacity out most
of the time. You can adjust this number if you've measured differently in
your setup. Our example model has a 1700 mAh pack in it. It
performs well for 8 minutes. We need to get 90% of our capacity as a
starting place. 90% of 1700(1700X.9) is 1530. Now, the pack
is rated in Milli-Amp-Hours. Hours is the "KEY" word here. We
need to convert our 8 minutes of flight time to it's fraction of
hours. 60/8=7.5. So, we're flying on 1530 mAh of
energy for 1/7.5th of an hour. 1530 mAh X 7.5=11,475mah or 11.475
amps of average current.

If we're flying 6 minutes
on 3000 mAh cells how much is our average current? 3000 X .9=2700 mAh
energy used. 60 minutes/6 minutes flying time=10 or 1/10th of an
hour. 2700x10=27,000mah or 27amps average current.

I've personally owned a 10
cell fun-fly model propped to 50 amps at full throttle on a 1700 size
NiCad. The model would fly pretty easily (with lots of aerobatics) for 8
minutes. My average current as figured from above would have been
11.475 amps. The same model on a 3000 NiMH pack would fly about 12
minutes. Average current with the heavier pack is 13.5amps. The
model pulled more current on average due the heavier pack. You might also
assume I was more "careful" with the 1700 because I knew it was in
the airplane and could be depleted faster.

Can you figure the size
and cell count of the Li-Poly pack I would need to fly the above example?

Max current 50 amps on 10
cells. 50/7=7.14Amp-Per-Hour or 7140 mAh minimum pack size. This
would be absolute minimum! We could make that up from 3 2500 3 cell packs
in parallel (7500 mAh total capacity). We'd have voltage similar to
a 10 cell NiCad or NiMH pack and would weigh about 15.75oz, about 71% of
the 3000 NiMH weight.

The "Average Discharge
Rate Approximation" formula is (Capacity X .9) X (60 / Flight
Minutes) =Average Discharge Rate in Milli-Amp-Hours.

**Don't be Greedy, Look
for about 80%.**

Near the top I mention
flying my Edge 540 profile model on an 1120 pack at 8 amps as a good 7C
example. In truth I like the model better with about a 2000 mAh
pack in it. Performance is the same but flying time is more satisfying
also the high power part of the pack is much longer and overall it suits the
model and "my tastes" better. Generally the best packs work out
to be around 80% of the weight of the best NiCad and NiMH packs for a given
application. 80% weight is just a rule of thumb, if you’re
best pack is 70% or 90% of the best NiMH pack then all is well. If however,
you are considering a pack that is 50% or 120% of the NiCad or NiMH pack you've
got a solid indication to double check your math and be sure you’re not
overlooking or miscalculating something.

**How do I understand
Parallel and Series packs?**

An example of Parallel
packs that probably everybody has seen is a heavy duty pickup with two 12V
batteries in it. This is used to extend the electrical work
capacity of heavy duty work trucks such as plow trucks and other trucks that
run hydraulics or cable winches. Often a pleasure boat will have
two batteries in it to make it more certain that it will start when your 10
miles off shore. The two batteries are connected negative to negative
(black to black or - to -) and plus to plus (red to red or + to +).
Rather obviously we know big trucks and nice boats are still 12 volt
systems. When you connect batteries up negative to negative and
positive to positive you increase the capacity. This is to say we
can do the same job 2 X as long. The voltage stays the same. Below
is a drawing of a typical lithium pack that is 2S-2P (2 in series, 2 in
parallel) making the 1020 cells into an 8.4V pack of 2040 mAh capacity?

An example of Series is your
transmitter pack. It is probably made up of cells that are 600 mAh
in capacity. If you strip off the cover you can read the actual capacity
printed on the cells. These cells are connected to each other
Positive to Negative to Positive to Negative and so on. When
cells are connected thusly the capacity stays the same and the voltage increases.
A TX pack is usually 8 cells (1.2V per cell) and 9.6V.
The savvy readers will have already figured out that our truck example above
also includes a "series" battery. The 12V lead acid
battery in your car or truck is really 6 each 2 volt cells in series.

Series and Parallel
battery packs must contain all cells of exact same capacity and they should be
of the same brand. If not they won't run down equally.

**How long does it take
to charge a Lithium battery?**

Roughly you can figure
Empty Capacity in mAh/charger output + 1/2 hour. So, if we're charging an
empty 2 cell 2100 (empty is 6V) pack at 400 mAh it will take about 5 3/4
hours. If you charge at the maximum rate of Capacity X 1 then it will
take about 1.5 hours. At Capacity X 1 (2100 mAh in this example) it will
take about 1 hour to fill the pack to 90% and an additional 30 minutes to pack
the last 10% of the charge. There is no good way to charge them
faster.

**Lithium batteries are
slower to charge than NiCad and NiMH cells then. :-(**

**NO! **Striking statement but the answer
is no. It takes longer (or as long) to charge a NiMH or NiCad of similar
weight and job. Remember, in electric flight, most of our
successful lithium packs fly the models for roughly 3 - 10 minute flights.
Our Edge 540 flies about 5 minutes on a 350 NiCad which takes 15 minutes to
recharge. About 10 minutes on a 720 NiMH pack taking about 35 minutes to
recharge. It flies about 30 minutes (with the same vigor!) on the
2100 Lithium which takes 1 1/2 hours to recharge. In the
"flying" time it takes to run down the lithium, you've got to charge
the NiCad 6 times taking about 1.5 hours, You've got to charge the NiMH 3 times
taking about 1 hour and 45 minutes. So, in the real word, it takes
no more time to charge a Li-poly pack than it does to charge any other type of
common pack as long as the weights and jobs are similar.

Yes, it takes 3x as long
to charge a 720mah lithium cell as it does to charge a 720mah NiMH cell but
this is minutia as the lithium and NiMH cell of the same capacity can't do the
same job. We're typically running a Lithium of 3X the capacity as the
best NiMH cells. Even though we can charge a NiMH cell at capacity
X 2 giving a 35 minute charge and we can only charge a lithium at Capacity X 1
giving a 90 minute charge what you must remember is the Lithium pack is 3X the
capacity often. In the Case of the 720 (Edge 540 example above) the max
charge rate is 1.4 amps; the 2100 lithium cell can accept a charge rate of 2.1
amps! Both do the same job but the Lithium is being charged at HIGHER
amperage due to it's being 3X as big. So, the lithium pack will accept
10minutes flying time worth of charge faster!

**I wish I could add less
than 4.2 volts to a Lithium Pack. How do I match my 8 cell NiCad machine
with a Lithium?**

You’re correct in
remembering that a 2 cell Lithium is more directly related to a 7 cell
NiCad. If we run a 2 cell Lithium "AND" change nothing
else we'll have less power presuming the pack weight is the same.
If we run a 3 cell lithium then the voltage will be so high that our current
will be more than we had planned for the setup. How do we unravel this
riddle? First, understand that you're probably getting a small weight
reduction. This will make up for some of our reduced prop RPM form the
lower voltage of the 2 cell lithium pack. What we are left with is
learning to become better students of gear ratio and prop selection. It's
likely you'll need to reduce the gear ratio for example, drop from 4:1 to 3.5:1
to get your RPM back where it was with the 8 cell NiCad. Another
alternative (simpler) is to choose a prop with a little more pitch.
If you were running a 6 pitch prop, you might get your performance back with a
7 pitch on the lithium pack. It's really not that complex but it is a
concern you'll have to address more with the limited number of voltage
selections in lithium packs.

**Improve your skills.**

Most of our electric
models do not fly at full throttle the whole time. In fact, it is a mark of
an experienced pilot to be able to make the model look good and do many
different maneuvers at many different speeds. Learn to fly with throttle
control. The throttle is variable. At times when I watch people I
wonder if their speed controls are really like the Space Shuttle Boosters, once
you turn it on it goes at full blast till it is empty. ;-)
Use only what you need and you'll have much better flying models, lighter
battery packs and longer flights. Some simple examples are: don't use
full throttle on the down side of a loop, when you’re flying down or
lowering your altitude do it with the motor off or at a reduced
RPM. Take some time in your flights to do some low speed
aerobatics. Practice these things a little at a time and soon you'll be
showing the full range of your model's capabilities in every
flight. I think you'll find it enjoyable and it will certainly
enrich the quality of your electric experience.