This note 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)
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. (16X.5 amps)
If you have a 1000 mah pack - any kind - and it is rated at 8C that means it
can deliver 8 amps. (8X1 amp)
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 deliver 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.
So, 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/reciever job, but a
transmitter/reciever pack should not generally be used as a motor pack.
Basics:
http://www.modelaircraft.org/mag/FTGU/Part8/index.html
Lithium Batteries
http://www.*********.org/lithium_bat...eakthrough.htm