here is 120A PDB and power sensor 120A!
When I design and manufacture a UAV the first thing I do (after obviously ascertaining what the UAV is being used for) is decide on a power train i.e, motor first. From what you say you are using a U3 (decent choice, I also use a lot of T-Motor parts) so going by the data for the 700KV unit looks like you are limited to 3s or 4s. That makes it straightforward. There is a consensus that the higher series packs are more efficient i.e. 4s is more efficient than a 3s pack etc. So I would run with that, a 4s with a decent discharge rating i.e. >25C. That being so you have a good choice of packs to go for. This is only what I do, others may have a different opinion but that's life!
The issue is you have not stated what props you plan on using 10x4" for 3s or 8x4" for 4s 2200 mah or larger battery 30c minimum !
First you need to determine the maximum power required for the weight of the machine; thrust and full speed movement.
Determine the thrust required to hover (defeat gravity); how much total weight is to be lifted. Thrust for each motor = weight /4.
The thrust to hover (and current draw) will be a fraction of the maximum power.
You can use the ecalc.ch web site to determine batter, motor, props...
That U3 motor has a max current draw of 25A. Note that is MAXIMUM CONTINUOUS;ie, a full throttle climb or acceleration or flat out continuous movement. While the motor may have a 25A rating, typically these products are OVERRATED and it will probably start to smoke if you continuously run 25A. I'd de-rate it 20%, lets say 20A is safe. So total through the PDB will be 80A.
At hover, only a fraction of that will be drawn. The high current draw only occurs in a short duration high speed acceleration and max speed continuous movement; how fast do you need to go?
Determine the desired max rate of climb and/or desired max forward speed, and that will determine how much power is needed.
Most of the PDBs have a burst rating for max power climb/acceleration; check the "continuous" rating.
Add some extra power capacity for reliability; perhaps 20%.
So you might have 5A per motor at hover (20A draw continuous total). 20A at full throttle climb (80A burst total).
Finally all of this is limited by the voltage and C rating of the battery. You'll never draw 80A burst for very long if the battery is too small. I've used 40C(80C burst) 4S batteries with Tiger MS2814-10 770V. (28A ) $32 each! Similar to U3, but MUCH cheaper.Granted the newer U3 has a bit higher wattage rating.
Note that a higher battery voltage will be more efficient and reduce the current through the circuit(and the rating of the PDB required). While a 4S would be good, a 6S (21V nominal) might be best. Definitely don't want to use a 3S!
Note the U13 is only rated for up 4S(14,4) volts.Perhaps select a higher voltage motor. At $100/motor, there are many out there!
Hope this helps!
FYI: Take a look at the Tiger motor specs. It will show thrust per motor for various prop sizes at different voltages(3S,4S,etc.). Additionally, I prefer hard case batteries. I know there is a bit of added weight, but the resistance to puncture in a crash is worth it! With the side benefit of being ROAR approved! You can also run two batteries in parallel to increase the C rating.
Oops forgot to mention an PDB! Take a look at Tiger's F55A Pro. Pretty impressive design...
Do you know my power board ?