

Sep 19, 2005, 08:27 AM  
LI, New York, USA
Joined Mar 2003
24,689 Posts

First the background:
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. 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 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.rchobbies.org/lithium_bat...eakthrough.htm New Electric Flyer FAQs http://www.ezonemag.com/pages/faq/a105.shtml 
Sep 20, 2005, 09:18 AM  
As far away from RCGroups and the AMA as possible!
Joined Aug 2004
6,601 Posts

Wow  Aeajr did a GREAT job in explaining everything. How long did it take you to type all that?!
ElSeeker  If you'd like to play with some numbers, try this web site  http://brantuas.com/ezcalc/dma1.asp 
Sep 20, 2005, 10:46 AM  
LI, New York, USA
Joined Mar 2003
24,689 Posts

Quote:
It took a while, but this is a question that comes up often. I thought I would take the time to work out a good answer that is clear and simple. I hope I accomplished that. I now have it saved so, when the next person asks, this is a cut and paste post. I will evolve it over time so I can help other newbie electic flyers understand this topic. If anyone has other resources or links that would be good addons to this writeup on motors/amps/props, I would love to see them. I am always trying to help the new guys. 

Sep 20, 2005, 01:48 PM  
LI, New York, USA
Joined Mar 2003
24,689 Posts

Quote:
One key point is that each motor typically has a wattage limit either expressed as amps at a given voltage or watts ( voltsXamps). If you exceed the wattage capacity of the motor you will either shorten its life or destroy it. Likewise your wattage targets will tell you what you need to get out of your battery pack. If you need 15 amps at 9.6 volts from your pack and it can only deliver 12 amps without damage either your setup will perform poorly, your batteries will be damaged, or both. This is a VERY simplified expression of a very complex world, but you get the idea. If you visit this link you will see a table for the GWS 350C motor/gearbox setup. They chart amps at volts based on gearing and prop. This is the best practical illustration of the ideas expressed here. http://www.gws.com.tw/english/produc...em/eps350c.htm In most cases the maker of the model or motor will advise on a motor/prop/volts/amps recommendation. Use those as your starting point. I hope this has been helpful. 

Sep 20, 2005, 01:51 PM  
LI, New York, USA
Joined Mar 2003
24,689 Posts

Quote:


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