From Rudy

I case some one else was wondering about the same thing about "C" rating's
Hi Larry,


The C-Rating is used to identify a battery pack's charge and discharge requirements. The C ratting for charging is usually 1-C (a 4000 mAh battery can be charged at a 4000 mAh rate for one hour to be fully charged) Now many batteries can be charged at up to a 4-C rate, which means you can charge the above battery at a 16,000 mAh rate (16 amps.), which will charge the battery in approx. 15 min. :-)


The C ratting for DISCHARGE (which is your question) is the maximum amps you can pull from the battery without damaging it. A 4000 mAh battery pack with a 20C rating can discharge at a maximum rate of 80 amps (4000 x 20 = 80,000 mAh (80 amps) up until it is fully discharged.

Our example 4,000 mAh, ( 4 amp), battery would be fully discharged if it used a steady 4 amps for 1 full hour. If our example 4 amp battery used its max 20C rate (80 amps) at a steady rate, it would be fully discharged in 3 minutes. We average around 20 to 30 amp discharge rate for our flights. So you can see why a 4 amp 20C battery is enough for a mission (providing a little extra safety margin.).  :-) 


This is a very important # for ERC, less important for us in ECL. In ERC we often pull very high amounts from the battery at peak times so we need a battery that can give us the needed HIGH amp draw for short bursts.

In ECL we have peaks (for a short time when we do tight tall maneuvers) of only 30 to 50 amps. This means that a 15C ratting is enough for our mission. I always use 20C (or higher) because I often use the same batteries in my ERC planes and I also like the extra "headroom" they give me. The higher C rating batteries are more expensive to build and they weigh more.

If you use too low of a C rating then when the motor (under high load) DEMANDS more amps from the battery the battery will NOT be able to deliver the needed power even though the GOV in the ESC will keep demanding it. One or more things will happen: The motor will NOT be able to maintain the desired, set, RPM. The battery will overheat and be damaged. The ESC will overheat and be damaged. ......... Or ALL THREE :-( 


You can think of it as a "muscle ratting". If you need to lift a heavy weight very fast, then you need X size muscle to do it. But you don't want to waste the weight, time, expense, food, etc. to build a 25X size muscle to do a 15X size job. ;-)

The above should give you some insight into why it is important to choose a battery that is larger than one would think so that we have some extra "headroom" for the battery near the end of our CLPA flight when it has the most demand for the tough maneuvers. The EXC in gov. mode will DEMAND the needed HIGH C amps when needed. If the battery is near depletion it may not be able to provide the needed spikes of amps and the system will not have the power JUST when you need it most! ;-)

I hope this helps a little. If not, please feel free to ask for more info. Via Ph or email.

I look forward to seeing you at the Meet and Meat, and or the GS meet in Oct. :-)