Batteries and safety

Why this matters

The battery is at once the drone’s energy source, its heaviest part and its most dangerous part. Too small a pack overheats and sags until a failsafe triggers during a climb; too large a pack adds weight until flight time stops improving. This module is about choosing a pack that fits the job, and understanding what the label numbers do and do not say. Charging, storage and transport in detail were covered in UAT 321.

Building packs from cells

Packs are written xSyP: 6S1P, for example, is six cells in series, one string.

• Series (S) adds voltage: one LiPo cell is 3.7 V nominal, 4.2 V fully charged
• Parallel (P) adds capacity and deliverable current
• Energy equals nominal voltage times capacity

C rating and maximum current

In Grepow’s explanation, the C rating is discharge current divided by capacity, so a 10 Ah pack rated 25C can deliver $25\times 10=250$ A continuously, as the manufacturer claims. Grepow itself warns that some sellers quote C ratings from short pulse discharges, so the number is a manufacturer’s claim: use it with margin and check the pack’s actual temperature.

Internal resistance

Every battery has an internal resistance $R$. Drawing current $I$ lowers the terminal voltage by $IR$ and generates heat $I^{2}R$ inside the pack. Heat grows with the square of current: double the current, four times the heat.

cells, cap_ah, c_rate, r_pack = 6, 10.0, 25, 0.018
v_nominal, v_full = 3.7 * cells, 4.2 * cells
print(f"{cells}S: nominal {v_nominal:.1f} V, full {v_full:.1f} V, energy {v_nominal * cap_ah:.0f} Wh, "
      f"claimed max {c_rate * cap_ah:.0f} A")
for label, amps in (("hover", 25), ("climb", 80)):
    drop = amps * r_pack
    print(f"{label:<5} {amps:>3} A: drop {drop:.2f} V -> {v_full - drop:.2f} V at the terminals, "
          f"heat {amps ** 2 * r_pack:.1f} W inside the pack")

6S: nominal 22.2 V, full 25.2 V, energy 222 Wh, claimed max 250 A
hover  25 A: drop 0.45 V -> 24.75 V at the terminals, heat 11.2 W inside the pack
climb  80 A: drop 1.44 V -> 23.76 V at the terminals, heat 115.2 W inside the pack

LiPo or Li-ion

• LiPo delivers high current well, suiting hard acceleration or heavy lifting. For example, a Tattu 4S 10000 mAh pack holds 148 Wh at 935 g, about 158 Wh/kg for the whole pack
• Cylindrical Li-ion, such as the Molicel P45B 21700 cell, holds 4.5 Ah and 16.2 Wh, delivers 45 A continuously (10C) and reaches 242 Wh/kg for the bare cell, suiting long flights at moderate current

These two figures cannot be compared directly, because the first includes the pack’s wiring and casing and the second is a bare cell. A real choice compares finished packs under the actual current.

Module lab

Common mistakes

Summary

• Series adds voltage, parallel adds capacity, and energy is nominal voltage times capacity
• The C rating gives the maximum current as claimed by the manufacturer; use it with margin
• Internal resistance causes an $IR$ voltage sag and $I^{2}R$ heating
• LiPo delivers high current well; Li-ion gives high energy per mass; choose by the job

Check your understanding

1. A 4S2P pack uses 3.7 V 5 Ah cells. What are its voltage, capacity and energy?
2. A 5 Ah pack is rated 40C. What maximum current does the manufacturer claim?
3. With 20 mΩ and 50 A, what are the voltage drop and heat?
4. If current rises from 25 to 50 A, by how many times does pack heating increase?
5. Why can the P45B cell’s 242 Wh/kg not be compared directly with a LiPo pack’s 158 Wh/kg?