Originally Posted by
DirtRacer68
I would be interested in anything that could help determine runtime. How does the final drive ratio effect the runtime and motor temps?
I use 2 equations to get me pretty close to my needs before even buying a lipo using the max amp rating of the motor.
It is just to get a good estimate but not to hold your numbers as absolute.
But anyone who tries this will see it can be used as a very good tool to estimate what lipo size (mah) for a specific runtime.
Amp/hr x 200 / current = minimum runtime
Amp/hr x 250 / current = maximum runtime
*(Amp/hr = mah / 1000)
Use the minimum and maximum to put you in a way better position in knowing the size lipo you need.
(*this system works best with 2s for 1/10 scale and 4s for 1/8 scale)
The last thing to understand about my system is that the runtime number that you will get is only to take your lipos down to nominal voltage (3.7v per cell), not any further, so with this being said there is actually more runtime for those who set their lvc lower than 3.7v per cell.
Now when using HV setups with low kv motors simply start with a motor that will give around 30,000-35,000rpm's for your 2s or 4s application (*see note above).
Now that you know the Amp/hr for that runtime use the known Amp/hr number in the following equation:
Amp/hr x nominal voltage of pack / nominal voltage of the HV pack
ex:
9 = (Amp/hr)
9 x (4s=14.8v)
9 x 14.8 / (5s=18.5v)
9 x 14.8 / 18.5 = Amp/hr