Batteries: mAh is the wrong number

Charge is not energy

Milliamp-hours measure charge. Charge alone cannot tell you how much work a pack will do, because the same charge at twice the voltage is twice the energy. Energy is watt-hours:

Wh = (capacity in mAh ÷ 1000) × nominal voltage

A 4S 1500 mAh pack is 22.2 Wh. A 6S 1300 mAh pack, which sounds smaller, is 28.9 Wh: 30% more energy in a pack with a lower headline number. Comparing mAh across cell counts is comparing nothing at all.

This catalogue therefore stores watt-hours and sorts by Wh/kg, which is energy per unit of mass you have to carry. That is the figure that decides endurance.

Cell count, chemistry and voltage

Cells in series add voltage. The per-cell voltage depends on chemistry, and getting it wrong throws the energy calculation out badly:

Chemistry	Nominal	Charged
LiPo	3.7 V	4.2 V
Li-ion	3.6 V	4.2 V
LiFePO4	3.2 V	3.65 V
NiMH	1.2 V	1.45 V

A 6S LiFePO4 pack is 19.2 V, not 22.2 V. Treating it as LiPo overstates its energy by 15%.

What a C rating really says

C is a multiplier on capacity that gives maximum discharge current:

max current = capacity in Ah × C rating

A 1300 mAh 100C pack claims 130 A. Whether it delivers that without the voltage collapsing is a different question, and the honest answer is that C ratings are the least reliable number on a battery label. They are not independently tested and the incentive runs one way.

What is real is internal resistance, in milliohms per cell, which very few vendors publish. Voltage sag under load is resistance times current, and sag is what you actually feel: a pack that reads 22.2 V at rest and 18 V under full throttle has lost you both power and flight time.

Treat C as an upper bound written by the marketing department, and buy on brand reputation and measured reviews for anything that matters.

Sizing a pack

Three constraints, all of which must hold.

1. Voltage: cell count matches what the motor and ESC are rated for.
2. Current: capacity times C exceeds the sum of all motors at maximum, with margin. Four motors at 35 A is 140 A.
3. Energy: watt-hours divided by hover power gives flight time. More capacity is more weight, which is more hover power, so there is an optimum rather than a maximum. Past it, you are carrying battery to carry battery.

That optimum is usually where the pack is 30 to 40% of all-up weight. Beyond half, you are almost certainly better off with a smaller pack.

Practical care

Charge to storage voltage, about 3.8 V per cell, if a pack will sit more than a few days. Do not run below 3.5 V per cell under load. Both of these cost you nothing and roughly double useful pack life.

Lithium packs are a fire risk when damaged, over-charged or punctured. Charge in a bag or a metal tin, on a surface that will not burn, and never leave a charger unattended.