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This is part of a series of articles put here, written by me to try and
explain electic flight to a newcomer. These articles are taken from the
spad combat
forum
in the electric section.
Battery
Now here is something that confuses a lot of people. This is easier to
understand if you have a working knowledge of basic electronics, if
not, I'll try my best to explain anyway. This article is
mostly about lithium polymer battery packs.
You might see a lipo pack listed as something like "10C 11.1v 1500mah
3s2p". Now you're probably thinking, what the hell does all that mean.
Well, let's break it down.
Now, in electronics, amps is the rate at which a charge flows. That
means it's basically charge per second. More amps generally means more
power. If amps = charge/time then it must follow that amps x time =
charge. The total capacity of a battery is a measure of how long it can
supply power for, before running out. More capacity ALWAYS keeps a
motor going for longer than less capacity if the throttle settings are
the same.
Capacity is measured in amphours, or milliamphours (mah). This is the
amount of milliamps (1/1000s of an amp) the battery could supply for an
hour. A 2000mah battery will supply 2000milliamps, or 2 amps for one
hour. This follows that the battery would supply 4 amps for half an
hour, or 8 amps for 15 minutes, or 16 amps for seven and a half
minutes... you get the idea? However, obviously most batteries would
struggle to supply say, 300 amps for half a second. This is where the
max discharge rate comes in, more on that later...
Anyway, this makes it easy to find out how long a battery will last. If
your motor draws an average of 10 amps, and you have a 1500mah battery,
it will last 1.5amphours / 10 amps = 0.15 hours, which is 9 minutes.
Geddit?
The voltage is pretty easy, it's just the driving force. Imagine
voltage as a pump, and amp hours as the water capacity of a tank. More
volts pumps the "water" (electricity) at higher speeds and higher
power, but uses up the capacity faster. Ok?
Now, the 3s2p bit. Lithium packs are made up of cells. Each cell has a
nominal voltage of 3.7 volts. In real life, this fluctuates depending
on state of charge/discharge. This is what a 3s2p pack looks like:
"One end of circuit"---Cell---Cell---Cell---"Other end of circuit"
---Cell---Cell---Cell---
Different voltages can be made using different amounts of cells in
series (series means in a row, one in front of the other. In this case,
the voltages of each component add up to give the total voltage). So
two cells in series will give 3.7+3.7 = 7.4 volts. Now, this is when
there are two cells in series. If we have three cells in series, that
will give 3.7+3.7+3.7 = 11.1 volts. How does this relate to 3s2p? Well,
if you look at the first bit, 3s means "three packs in Series."
Here's our example pack again: "10C 11.1v 1500mah 3s2p"
Now, the 2p bit. Well, cells can be put in parallel as well, which
means they work alongside each other instead of in a row. Now when you
have cells in parallel, it doesn't change the voltage, but it DOES
change the capacity (the total amount of charge the cells hold). So if
one cell has a capacity of 750mah, then two cells in parallel have a
total capacity of 1500mah. The 2p bit means that there are two cells in
parallel.
Remember, this is important, cells in parallel add capacity, cells in
series add voltage!
Lastly, the C bit. This bit is just as important as all the other bits.
Our example pack was 10C. The C rating determines maximum discharge
rate, in amps. If you exceed this by using a larger motor than you
should do, the battery pack could be damaged, or more likely, will just
not be able to supply those amps. To find the maximum amount of amps a
pack can supply, multiply the C number by the total number of amp hours
in a pack.
This pack has 10C and 1500mah, that's 1.5 amp hours. Now, 10 x 1.5 =
15. Therefore, this pack can supply a maximum of 15 amps. Simple eh?
Disclaimer
I don't claim to be an expert. All of this information is offered on an
"as is" basis. All text here is hereby released under the GNU FREE
DOCUMENTATION LICENSE for anybody to use, copy or alter, commercially
or otherwise, as long as, AND ONLY AS LONG AS THIS TEXT IS PRESERVED.
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