So I’m building a quadcopter, and I want to be able to use the same battery to power both the escs and the flight controller.
However, the escs are connected to the PDB which is connected to the main plug of the lipo. I was wondering if it was safe to then use wires 1 and 3 of the balance plug to get 7.4V, and power the flight controller with that?
Yes, this is fine, provided you don't draw current in excess of the normal balancing current that the balance wires and connector are rated for. That shouldn't be a problem if it's just running logic and radios, might be more of an issue if you were wanting to run some high-powered servos.
Note that it it's important to only draw from the outer balance wires which are directly connected to the overall pack + and -. If you were to tap off intermediate wires (to get a lower voltage) you'd quickly unbalance the pack.
A betaflight F4 flight controller can draw 1.5A max @ 5V
If your main load is 10A that's only 15% mismatched loads. If 30A then only 5% , maybe ok ... but maybe 25% shorter battery lifespan. ( charge cycles)
If so, the problem is those cells will deplete 15% faster.
The only thing that kills a battery pack faster than excess load current is cell imbalance under heavy loads. The reason is even though they may be 0.1% matched the best case when new, the weaker cell drains first and this accelerates the cell capacity faster and faster ( capacity runaway effect) so when it becomes 10% it will increase its ESR at the same time resulting in more cell heat and faster ageing rate and thus faster cell capacity mismatch and lower cell voltage.
So if it is a low percentage DoD is not as critical but as imbalance degrades DoD and CV level of charge 4.2 vs 4.1 or 4.0 becomes more critical to get life charge cycle rating. Of course its only money and life expectancy of charge cycles. Why? because individual cell under voltage duration and cell overvoltage duration accelerates "cell death"
Cell balancers are usually passive and limited to some power level which is a small percentage of total Watt capacity and normally limited to CC range for charging not CV. But if you have fancy "flying-inductor SMPS bridge cell-balancer" ( my name), no worries. Patented but expensive.
Use full cell voltage and a 5V buck converter or whatever is needed for the flight controller.
