I have a 3" drone frame, flight controller that can take 3S or 4S LiPo, and 1507 4200KV motors. How do I find the right capacity of battery to purchase? Should I purchase 4S 850mAh or 4S 1500mAh? And why?
As @JacobB suggested in his comment, first check all the parts of your aircraft that connect directly to the battery can accept 3S or 4S voltages (usually the flight-controller and esc.) This may immediately constrain your choice.
First you need to calculate the maximum weight your drone can lift; you can work backwards from this question:
- Get the thrust per motor for your drone
- Multiply this by the number of motors
- Divide by two
- Weigh the drone without the battery and subtract from number above
This will give you a target maximum weight for your battery. You can then choose the highest capacity battery that falls in this range.
More expensive batteries will give slightly higher mAh ratings at slightly lower weights - to work out what is good cost/flight time balance in your situation, you can estimate the flight time for a given mAh rating with the following formula:
Endurance = ( (0.8 * mAh) / AverageAmpsDraw ) * 60 [source]
My normal procedure is to see what similar models use. A quick google indicates that 4s800 is common for 3" frames. A 4s1500 is commonly used on 4 and even 5" quads, it's almost double the size probably be too heavy.
However, you can use significantly large or smaller batteries, depending on your flying style. A smaller battery will reduce weight and give you better power-to-weight, a large battery will give you more flight time (assuming it fits on the frame, and isn't too heavy to lift).
Ideally you'd be able to borrow a range of batteries from your friends to try. If not, I suggest buying one battery that is about 20% smaller and one that is 20% larger (as well as one the recommended size, so perhaps 600, 800 and 1000) and trying them to see what you like best. If you find you prefer the bigger one, try another that's 20% bigger again.
This sort of size difference is large enough to notice but small enough that you'll still use these batteries, and it's nice to have a bit of variety occasionally.