I’m building a heavy quadcopter using the multiwii fc. I’m designing it for heavy lift and the longer the flight time the better. The motors I’m using are 870kv run by 50a esc’s. Each motor has a max current of 32a. Does this mean i will need to select a battery capable of delivering 4x32= 128a? Is there a 4s capable of delivering this much current?
That max current is your peak number and while having a battery that can put out that much is nice, you should rarely need to max them. Having a battery than can put out 4x20 is a better average. As for ESCs, unless you plan to even lower KV motors in the near future, that seems a touch overkill, but no harm done in that.
For battery size and flight length, my average amp draw is 6x20a, 120a, and my 4s 16000mah 10c nominal battery has more than enough in it for slow cruising flights with minimal heat buildup. If you upgrade to bigger, more power hungry motors, you would eventually need to upgrade to a bigger battery. My build is designed for camera and FPV/Photography using a gopro, with a flight time of around 30 some odd minutes, wind depending.
I recommend that you figure your average weight you will be lifting and reverse figure your overall needs while leaving some weight for fighting sudden winds.
What is peak number? If I was to run it 8000mah 4s 20c could I expect 15m+ runtime? Would the battery handle this load ok?
Your Peak number is your maximum input. That is the maximum it can take before burning out or risking failure. You never run your quad at peak/full throttle for more than a few seconds and some of the calculations are based on peak, while others are the average.
I calculate my numbers at average draw. If peak is 35 and average is 20, i do my numbers at 23 or so so i have some wiggle room in my battery and total take off weight.
Look at your motor specs and you should see a Peak or Max Amp number and an Average amp number. Your average draw is what your motors will be drawing at 50% throttle, which is what you should be setting your hover/no vertical motion point at. As for your question, you could run that drone with those motors and ESCs with that battery for an average of 15 minutes.
Yeah for heavy lift drones, what @vwdstriker said is good and helpfull, but if you wanted to understand abit more check out our community guide on choosing the best battery for your drone which goes into abit more details:
Would it also be possible to run each motor/esc with it’s own battery then?
Lets say a 2200mah 4s 40c lightweight battery or wouldn’t this idea be practicle?
I’m considering this as an option to limit costs also, as i think it could be beneficial on maintenance as if a cell broke down i wouldn’t have a massive cost associated with a high capacity battery. I could then just change out a £20 battery as opposed to a £100 + battery?
Yes that is entirely possible and could be a good idea if something did break, but you will need to connect all 4 batteries in parallel. The only thing is that if using them all isolated it gets hard to monitor the voltages for all batteries so this method probably increases the likely hood of over discharging a cell. This is because each motor does not use exactly the same amount of battery during a flight. At-least if they are in parallel, this issue is mitigated somewhat.