2 or 3 cell lipo
10-29-2007 | 10:21 AM
#3
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From: Texas Hill Country
RE: 2 or 3 cell lipo
Yes the voltage is the difference, but if the motor is designed for 11 volts, it will provide much more thrust with a three cell Lipo compared to a two cell Lipo.
10-29-2007 | 05:03 PM
#4
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From: Ames, IA
RE: 2 or 3 cell lipo
As mentioned, the voltage increases from 7.4 volts to 11.1 volts, but also remember that this means your power system will draw more current so if it is rated for 2s you will need to use a smaller prop to use it with 3s. Obviously the pack will be heavier then a equal capacity 2s pack, but it will also have proportionally more total energy.
Ryan Lefevre
www.CommonSenseRC.com
The Go To Guys For Electric Power
Ryan Lefevre
www.CommonSenseRC.com
The Go To Guys For Electric Power
12-19-2007 | 12:46 PM
#5
My Feedback: (2)
RE: 2 or 3 cell lipo
AMPS vs. VOLTS vs. C
By Ed Anderson
aeajr on the forums
This brief discussion is intended to clear up a few terms and concepts
around electricity as it applies to electric airplanes.
Think of electricity like water. Volts = pressure Amps = flow
Volts is like pounds per square inch, psi. Says nothing about how much
water is flowing, just how hard it is being pushed. You can have 100 psi
with zero water flow.
Amps is flow, like gallons per hour. You can have flow at low pressure and
you can have flow at high pressure.
Amp hours is how much flow can be sustained for how long. It is used as a
way of measuring how much electricity is in the battery. Like how many
gallons of gas in your tank. It is a capacity number. Says nothing about
flow or pressure, it is about capacity.
Amps and mili amps? We are just moving the decimal point around.
1 amp (short for ampere) = 1000 miliamps (mili means 1/1000 amps)
Examples
So a 7 cell NIMH or NICD pack provides 8.4V (pressure).
The motor will draw electricity from the pack at a certain flow rate, or
amps.
If you have a have a 650 mili amp hour pack, it can deliver a flow of .650
amps (650 miliamps) for one hour. If you draw it out faster, it
doesn't last as long. So your motor might pull 6.5 amps for 1/10 of an
hour, or about 6 minutes.
A 1100 mah pack has double the capacity of the 650 mah pack, so it should
last "about" twice as long.
What is C in relation to batteries?
C ratings are simply a way of talking about charge and discharge rates for
batteries.
1C, = 1 time the rated mah capacity of the battery. So if you charge your
650 mah pack at 1C, you charge it a 650 miliamps, or .650 amps.
1C on a 1100 pack would be 1.1 amps.
2 C on your 1100 pack would be 2.2 amps
Motor batteries are often rated in Discharge C and charge C.
So a 1100 mah pack (1.1 amp hour) might be rated for 10C discharge, so you
can pull 11 amps ( flow ) without damaging the battery.
Then it might be rated at 2C charge rate (flow), so you charge it at 2.2
amps (2200 mah)
How did I do? Things clearing up?
If you have a 500 mah pack - any kind - and it is rated at 16C that means it
can deliver 8 amps.
If you have a 1000 mah pack - any kind - and it is rated at 8C that means it
can deliver 8 amps.
If you have a 1000 mah pack - any kind - and it is rated at 12C that means
it can deliver 12 amps
If you have a 1500 mah pack - any kind - and it is rate at 8C that means it
can deliver 12 amps
If you have a 1500 mah pack - any kind - and it is rated at 20 C that means
it can deliver 30 amps.
If you have a 3000 mah pack - any kind - and it is rated at 10 C that means
it can deliver 30 amps.
So, if you need 12 amps you can use a pack with a higher C rating or a pack
with a higher mah rating to get to needed amp delivery level.
One last point. Motor batteries vs. receiver batteries
Some batteries can sustain high discharge rates. Others can not.
Those used as transmitter/receiver packs typically are made for low flow/amp
rates while those made for motor packs can sustain higher rates.
Having a 600 mah pack does not tell you if it is a motor pack that can put
out 6 amps, or if it is a transmitter/receiver pack that would be damaged if
you tried to pull power at 6 amps. It is enough to say that they are
different.
Clearly a motor pack could be used for a transmitter/receiver job, but a
transmitter/receiver pack should not generally be used as a motor pack.
It is best to size your battery packs so they run somewhat below their
maximum C rating. You will stress them less and they will last longer. For
example, if your motor needs a pack that can deliver 10 amps, getting a 1000
mah pack that is rated for 10C ( 10 amps ) will meet the spec, but it is
running at its limit. A 15 C rated 1000 mah pack would be better, or
perhaps a 1300 mah 10 C pack. In either of these cases, the pack will be
less stressed and should handle the load much better over the long term.
Other Resources
Sizing Electric Power Systems -
http://www.rccyberflyer.com/forum/showthread.php?t=272
Basics:
http://www.modelaircraft.org/mag/FTGU/Part8/index.html
Lithium Batteries
http://www.*********.org/lithium_bat...eakthrough.htm
Lithium Balancers and Balancing Chargers
http://www.rcgroups.com/forums/showthread.php?t=599287
New Electric Flyer FAQs
http://www.ezonemag.com/pages/faq/a105.shtml
A series of posts on electric power system basics
http://www.wattflyer.com/forums/showthread.php?t=1933
http://www.rcgroups.com/forums/showthread.php?t=417868
MotoCalc
MotoCalc will tell you everything you need to know: Amps, Volts, Watts, RPM,
Thrust, Rate of Climb, and much more! It is a popular tool for predicting
the proper motor, prop, battery pack for electric planes.
http://www.motocalc.com/
The Great Electric Motor Test
http://www.flyingmodels.org/motortest/index_e.htm
Electric Motors Described
http://adamone.rchomepage.com/guide5.htm
By Ed Anderson
aeajr on the forums
This brief discussion is intended to clear up a few terms and concepts
around electricity as it applies to electric airplanes.
Think of electricity like water. Volts = pressure Amps = flow
Volts is like pounds per square inch, psi. Says nothing about how much
water is flowing, just how hard it is being pushed. You can have 100 psi
with zero water flow.
Amps is flow, like gallons per hour. You can have flow at low pressure and
you can have flow at high pressure.
Amp hours is how much flow can be sustained for how long. It is used as a
way of measuring how much electricity is in the battery. Like how many
gallons of gas in your tank. It is a capacity number. Says nothing about
flow or pressure, it is about capacity.
Amps and mili amps? We are just moving the decimal point around.
1 amp (short for ampere) = 1000 miliamps (mili means 1/1000 amps)
Examples
So a 7 cell NIMH or NICD pack provides 8.4V (pressure).
The motor will draw electricity from the pack at a certain flow rate, or
amps.
If you have a have a 650 mili amp hour pack, it can deliver a flow of .650
amps (650 miliamps) for one hour. If you draw it out faster, it
doesn't last as long. So your motor might pull 6.5 amps for 1/10 of an
hour, or about 6 minutes.
A 1100 mah pack has double the capacity of the 650 mah pack, so it should
last "about" twice as long.
What is C in relation to batteries?
C ratings are simply a way of talking about charge and discharge rates for
batteries.
1C, = 1 time the rated mah capacity of the battery. So if you charge your
650 mah pack at 1C, you charge it a 650 miliamps, or .650 amps.
1C on a 1100 pack would be 1.1 amps.
2 C on your 1100 pack would be 2.2 amps
Motor batteries are often rated in Discharge C and charge C.
So a 1100 mah pack (1.1 amp hour) might be rated for 10C discharge, so you
can pull 11 amps ( flow ) without damaging the battery.
Then it might be rated at 2C charge rate (flow), so you charge it at 2.2
amps (2200 mah)
How did I do? Things clearing up?
If you have a 500 mah pack - any kind - and it is rated at 16C that means it
can deliver 8 amps.
If you have a 1000 mah pack - any kind - and it is rated at 8C that means it
can deliver 8 amps.
If you have a 1000 mah pack - any kind - and it is rated at 12C that means
it can deliver 12 amps
If you have a 1500 mah pack - any kind - and it is rate at 8C that means it
can deliver 12 amps
If you have a 1500 mah pack - any kind - and it is rated at 20 C that means
it can deliver 30 amps.
If you have a 3000 mah pack - any kind - and it is rated at 10 C that means
it can deliver 30 amps.
So, if you need 12 amps you can use a pack with a higher C rating or a pack
with a higher mah rating to get to needed amp delivery level.
One last point. Motor batteries vs. receiver batteries
Some batteries can sustain high discharge rates. Others can not.
Those used as transmitter/receiver packs typically are made for low flow/amp
rates while those made for motor packs can sustain higher rates.
Having a 600 mah pack does not tell you if it is a motor pack that can put
out 6 amps, or if it is a transmitter/receiver pack that would be damaged if
you tried to pull power at 6 amps. It is enough to say that they are
different.
Clearly a motor pack could be used for a transmitter/receiver job, but a
transmitter/receiver pack should not generally be used as a motor pack.
It is best to size your battery packs so they run somewhat below their
maximum C rating. You will stress them less and they will last longer. For
example, if your motor needs a pack that can deliver 10 amps, getting a 1000
mah pack that is rated for 10C ( 10 amps ) will meet the spec, but it is
running at its limit. A 15 C rated 1000 mah pack would be better, or
perhaps a 1300 mah 10 C pack. In either of these cases, the pack will be
less stressed and should handle the load much better over the long term.
Other Resources
Sizing Electric Power Systems -
http://www.rccyberflyer.com/forum/showthread.php?t=272
Basics:
http://www.modelaircraft.org/mag/FTGU/Part8/index.html
Lithium Batteries
http://www.*********.org/lithium_bat...eakthrough.htm
Lithium Balancers and Balancing Chargers
http://www.rcgroups.com/forums/showthread.php?t=599287
New Electric Flyer FAQs
http://www.ezonemag.com/pages/faq/a105.shtml
A series of posts on electric power system basics
http://www.wattflyer.com/forums/showthread.php?t=1933
http://www.rcgroups.com/forums/showthread.php?t=417868
MotoCalc
MotoCalc will tell you everything you need to know: Amps, Volts, Watts, RPM,
Thrust, Rate of Climb, and much more! It is a popular tool for predicting
the proper motor, prop, battery pack for electric planes.
http://www.motocalc.com/
The Great Electric Motor Test
http://www.flyingmodels.org/motortest/index_e.htm
Electric Motors Described
http://adamone.rchomepage.com/guide5.htm
12-20-2007 | 04:38 PM
#6
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Joined: Oct 2006
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From: CanberraACT, AUSTRALIA
RE: 2 or 3 cell lipo
One thing Ed's article didn't mention (although no doubt it is in the other links he posted) is the relationships between volts, amps and watts.
power = volts x amps (and we measure power in watts so watts=volts x amps).
Thinking of the water analogy again imagine a big old wooden waterwheel (the sort that might have powered a sawmill in times gone by). Now imagine hitting that waterwheel with a supersoaker water pistol - very high pressure, but low flow (so high voltage, low current) - the wheel would hardly move. Then imagine the gently babbling creek feeding the water wheel and it turning slowly (so low voltage, high current). Finally imagine a fire hose hitting the wheel and it spinning quite quickly (so, high voltage, high current).
power = volts x amps (and we measure power in watts so watts=volts x amps).
Thinking of the water analogy again imagine a big old wooden waterwheel (the sort that might have powered a sawmill in times gone by). Now imagine hitting that waterwheel with a supersoaker water pistol - very high pressure, but low flow (so high voltage, low current) - the wheel would hardly move. Then imagine the gently babbling creek feeding the water wheel and it turning slowly (so low voltage, high current). Finally imagine a fire hose hitting the wheel and it spinning quite quickly (so, high voltage, high current).
