Oct 06, 2011, 04:25 PM Registered User United States, NC, Raleigh Joined Aug 2011 35 Posts Question LiPo's and ESC ratings Hi all, Hoping the huge knowledge base in here can answer a couple of newbie questions.... If a battery is listed as a 1000mAh 7.4V 20C, am I correct in assuming: 1. 1000mAh = the amount of power stored in the battery when fully charged, ergo the higher this number leads to longer flight times? 2. How/where does the voltage value come into play? 3. 20C = the output in amps? 4. an ESC has to have equal or better capacity (ie 20amp or higher) to function without burning out? I'm sure it's a lot more complicated than this, and I could be completely on the wrong track...but I'm just trying to get my head around this new addiction...(damn - my missus is gunna hate me shortly!!) Last edited by rkathner; Oct 06, 2011 at 04:31 PM.
Oct 06, 2011, 06:06 PM
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New Rochelle, NY
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Quote:
 Originally Posted by rkathner If a battery is listed as a 1000mAh 7.4V 20C, am I correct in assuming: 1. 1000mAh = the amount of power stored in the battery when fully charged, ergo the higher this number leads to longer flight times?
In general yes...

Quote:
 Originally Posted by rkathner 2. How/where does the voltage value come into play?
Volts is the total voltage of combined cells in the pack.. A 7.4 1000 is a 2S1P or two cells and 1 pack.. Voltage relates to power system requirements, such as the motor, size of the plane, etc.. Electricity has two major components: volts and amps or current.. Volts multiplied by amps equals watts which is power output, like horsepower.. Lots to study there..

Quote:
 Originally Posted by rkathner 3. 20C = the output in amps?
No the C rating is the rating for max current if the cells that has to do with the internal resistance of the cells.. You determine the max amps by multiplying C x mah or size of the pack.. So a 1000mah pack with a 20C rating would be 20 x 1000 or 20 amps -- a 20C 2000mah pack would be 20C x 2000 or 40 amps..

Quote:
 Originally Posted by rkathner 4. an ESC has to have equal or better capacity (ie 20amp or higher) to function without burning out?
Each component must be rated based on the amount of power pulled by the prop which is a function of the speed of the prop in RPMs and the size and pitch of the prop.. You must start with the prop/thrust and work back through the rest of the power system to supply the needed power.

Jim
Last edited by ny_hawk; Oct 06, 2011 at 06:28 PM.
 Oct 06, 2011, 06:32 PM Registered User United States, NC, Raleigh Joined Aug 2011 35 Posts Much appreciated Jim. This clears up a lot of things for me - I have been reading the threads on power systems, but my head started to spin!! lol Your info has given me a good starting point to learn more! Rob
Oct 06, 2011, 06:55 PM
Registered User
New Rochelle, NY
Joined Jun 2008
671 Posts
Quote:
 Originally Posted by rkathner Much appreciated Jim. This clears up a lot of things for me - I have been reading the threads on power systems, but my head started to spin!! lol Your info has given me a good starting point to learn more! Rob
No problem... A prop calculator will help too.. Power systems are based on what you want a given air frame to do, how you want it to perform.. So you need to know what plane and what you want it to do first..

That takes you to watts per pound, where you want about a minimum of 75 watts per pound for tame flying and more, up to say 150 watts per pound for more powerful performance.. Then speed, determined in part by the pitch of the prop.. Once you know the total watts you can put together a power system that will supply that..

Here's a simple calculator..

http://personal.osi.hu/fuzesisz/strc_eng/index.htm

Jim
 Oct 06, 2011, 07:00 PM Registered User United States, NJ, Frenchtown Joined Mar 2003 9,687 Posts Another very good power calculator
Oct 06, 2011, 07:14 PM
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United States, AZ, Mesa
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Quote:
 Originally Posted by rkathner Much appreciated Jim. This clears up a lot of things for me - I have been reading the threads on power systems, but my head started to spin!! lol Your info has given me a good starting point to learn more! Rob
Yeah he just gave you the basics. I'm a science type person and know my physics fairly well and I could write another couple pages just on batteries, off the top of my head. However, for me, just knowing the basic principles, what all the units mean, and the special things about Lipo batteries (which don't need an apostrophe BTW) - that gives me just about all I need.

SO... I would suggest reading and doing your best to understand the content at the following links...
http://en.wikipedia.org/wiki/Voltage
http://en.wikipedia.org/wiki/Amperage
http://en.wikipedia.org/wiki/Power_(physics)

And understand what the units mean. One of my biggest peeves is misuse of these units - they have very clear definitions, but people get current and charge capacity and power all mixed up.

Volts - it's the force behind the electrical flow
Amps - it's an amount of electric charge per unit time, 1 coulomb of charge per second
Amp-hours - it's an amount of electric charge. (period) Since you have taken the amount of charge per unit of time, and multiplied by an amount of time, you are left with the amount of charge. Also called "charge capacity".

Milli-Amp-hours (mAh - small m, big A, little h) is just 1/1000th of an Amp-hour. It's not amps per hour or anything like that. 4Ah (4000mAh) is 4A for a period of 1h, or 2A for 2h, or 20A for a 12 minute flight. So, if you had a plane which could fly on 4 Amps and carry a 4Ah battery (unlikely), it could fly for 1 hour.

C-rating - it's the current capacity of the battery. It is how much current (in Amps) can we pull from the battery without causing damage. It is expressed as a function of the charge capacity. Simply multiply the C number by the mAh number to get the max Amperage the battery can handle.

I use a lot of 35C batteries - let's say I have one which is 1000mAh and one which is 2000mAh, otherwise they are the same. Regardless of the voltage of this battery pack, I can pull 35 amps from the small one and 70 amps from the bigger one. Even if they were 3.7V batteries. Think on that a while, I'm sure you have some questions now
 Oct 07, 2011, 11:47 AM Registered User United States, NC, Raleigh Joined Aug 2011 35 Posts Thanks again all! Jasmine - Thanks for the info! (I'm a musician - NOT a science type person )
 Oct 07, 2011, 12:52 PM Rocket Programmer United States, AZ, Mesa Joined Jul 2007 25,946 Posts That stuff often goes hand in hand, music being a language and a form of math I play lots of instruments actually, but none of them very well.
Oct 08, 2011, 02:35 AM
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Letchworth, Great Britain (UK)
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Quote:
 Originally Posted by jasmine2501 ... C-rating - it's the current capacity of the battery. It is how much current (in Amps) can we pull from the battery without causing damage. It is expressed as a function of the charge capacity. Simply multiply the C number by the mAh number to get the max Amperage the battery can handle. ...
As an engineer to a science-type person, we must remember that the units either side of an equation must be the same. So C-rating is not the current capacity of the battery -- it is a multiplier (units: 1/time) which when multiplied by the mAh capacity of the battery gives the current capability of the battery in mA
Oct 08, 2011, 09:48 AM
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Quote:
 Originally Posted by abenn As an engineer to a science-type person, we must remember that the units either side of an equation must be the same. So C-rating is not the current capacity of the battery -- it is a multiplier (units: 1/time) which when multiplied by the mAh capacity of the battery gives the current capability of the battery in mA
Yeah I didn't want to get too deep into that, but that is one thing that bugs me. We had a demo about batteries at our club meeting this month, and there was so many slight inaccuracies that I'm not sure if I should post the video or not. But one thing that was funny about it was he had the RX battery hooked up to some light bulbs and he marked the current going through the bulbs as "2000 mAh" and then he even said it wrong too "these bulbs are using 2000 milli-amp-hours" - grrrr...

And here's a good question of the day... if I had a 35C 3-cell 2200mAh battery, how much power does it have? hehe...

(the bulbs are using 2 Amps - if you left them on for an hour then they would have used 2000 milli-amp-hours)
Oct 08, 2011, 02:23 PM
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Letchworth, Great Britain (UK)
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Quote:
 Originally Posted by jasmine2501 Yeah I didn't want to get too deep into that, but that is one thing that bugs me. ...
Yes, in practice it's easier to stick solely to the numbers, without worrying about what the units actually are.

Quote:
 Originally Posted by jasmine2501 ... And here's a good question of the day... if I had a 35C 3-cell 2200mAh battery, how much power does it have? hehe... ...
None; (a) it doesn't "have" power, (b) it's disconnected