All about Lithium Polymer Batteries
Hi Red,
I have a question about LiPo batteries. I don't think I'm
the only one either because nobody I've asked seems
to know. Perhaps you could talk about how they are
rated. For example I know what 3S means (3 in series = 11.1
volts) but I don't know what 3S1P means, could you
explain the basics?
Thanks, Marty
Thank you very much for the great question
Marty! I am sure you are not the only one
either, as the electric industry as a whole is
really not that well defined when it comes to
explaining its various ratings and acronyms. It
can be very difficult, for instance when
converting an IC (internal combustion)
powered model to E power, to spec an equal
performing power system. More about this
later, but that should be really how you think of
this type of thing, as system. (motor, ESC,
battery and prop all have a great deal with the
“power” your system will produce) Thankfully
the battery nomenclature is pretty well defined
and easy to follow once you understand the
system.
On to your question! By definition all batteries
are made up of individual cells, whether they
are NiCad, Nickel Metal, Lipo or any other
chemistry. A battery is a cluster of cells wired
together either in Series and/or in Parallel.
Let’s take a simple 2100 maH 11.1v pack as
an example and look how it is defined. The
2100 is the total capacity of energy the pack is
able to deliver effectively rated in
milliamp/hours. If you have a system that
draws 30 amps you can expect to fly for about
7 minutes. (Provided the 30 amp draw falls
under the packs “C” rating, more later!)
The 11.1v rating as you noted comes from the
fact that this pack is wired in Series as the 3
individual cells are wired in such a way (pos &
neg soldered together) that their individual
voltages (3.7v) are added together make to
the 11.1v rating.
Parallel (pos & pos/neg & neg soldered
together) wiring is used when a larger capacity
is required. The largest typical single cell size
is around 2700 maH (there are other larger
single cells, but they are rare and don’t really
apply in this example). Parallel wiring sums
the capacities together. So if you buy a 4200
maH pack it likely is two 11.1v 2100 packs
wired in Series/Parallel or 3S2P configuration.
That means there are 6 individual cells wired
into two 3S packs to get 11.1v, and those two
11.1v packs wired together to get in parallel to
get a capacity of 4200 maH. So for any battery
the first number indicates how many cells are
needed to get the desired voltage and the
second number is how many packs of cells at
the desired voltage are added together to get
the desired capacity. You can stack any
number of cells together to get the desired
voltage and capacity. The only rule is all cells
must be of the same starting capacity. (i.e all
2100 cells) It is not uncommon to see up to
10S4P setups in larger aircraft.
I suppose some of the confusion is that a
3S1P battery really only is referred to as a 3S
battery leaving out the 1P part of the
designation. By default it is a 1P battery.
Which is fine as 1P batteries are all most
people fly with anyway. The 2P (or 3,4,5 or
more) is not really seen very much at all.
Earlier I mentioned C rating. The C refers to
capacity, and the rating is used to tell how
quickly the pack can safely discharge current.
Using the same 2100 maH pack as an
example, one might have a 15C rating while
another may have a 35C rating or more. The
reason it is important to pay attention to this is
because it is possible to permanently damage
your packs if your system exceeds the C
rating of the pack. How it works is simple. You
take the C rating and multiply it by the
capacity to get the maximum available current
the pack can deliver safely. Let’s say our 2100
pack is rated at 15C, you multiply 2100 x 15 =
31.5 maximum amps continuous. So for our
example 30 amp setup mentioned above, this
battery would suffice but I would consider it a
bit too close for comfort. You can see that the
35C battery with its 73+ amps continuous
available is a much better solution. Most
batteries also have a burst C rating, most of
the time it doubles the continuous rating. The
burst rating lets you know that if you have a
hotter system and your battery is on the edge
of being adequate, that you can exceed the
normal C rating for short periods of time (i.e.
take-off, hovering etc.) but you should not
exceed more than 10 seconds or so at this
higher current draw.
In normal operation, LiPo batteries should
never be brought down below 3v per cell (9v
for a 3S pack). Doing so can permanently
damage the cell and can lead to poor
performance and shorter run times. In extreme
cases they can even swell and catch fire. Most
ESC’s have this 3v limit set at the factory, but
it is possible to disable it on some controllers
like the Castle series for instance. At any rate,
flying until you hit the low voltage cut off each
time is not the best way to preserve your
packs. It’s best to land when you notice the
power getting soft.
Balancing is always a good idea, you cannot
over balance. I balance every time I charge.
This will maintain your packs to the highest
possible standard. I have packs that are going
on 5 years old and still perform as new.
Balancing becomes even more appealing
when you are running multiple parallel packs
together as the different performance
characteristics of each individual cell will
unbalance them rather quickly.
Well Marty I hope this answers your question,
and thanks for sending it in.
Email Red at
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with your comments and suggestions.
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