Batteries and records

Why this matters

The battery is the fastest-wearing and most dangerous part of a drone. A lithium battery that is overcharged, punctured or swollen can catch fire and is hard to extinguish. An organisation with dozens of packs must know each pack’s history, and which ones to retire before they retire themselves in the air.

Voltage per cell

A lithium-polymer (LiPo) pack consists of several cells in series; 6S, for example, means 6 cells. Care is therefore thought of in voltage per cell.

• Nominal 3.7 V per cell, used to calculate energy
• Fully charged 4.2 V per cell. High-voltage packs (LiHV) charge to 4.35 V or higher as the manufacturer specifies; the charger must be set to the right type
• Storage about 3.7–3.85 V per cell. If a pack will not be used for several days, do not store it fully charged; most chargers have a storage mode
• In flight the measured voltage is lower than the resting voltage because of internal resistance. PX4’s default treats 3.6 V per cell under load as “empty”

The battery register

The Thai CAAT guidance for developing an operations manual requires a battery record for every charge. A good register shows each pack’s wear trend, such as falling measured capacity and rising internal resistance.

import pandas as pd

packs = pd.read_csv("battery_log.csv")
packs["capacity_pct"] = packs["measured_mAh"] / packs["rated_mAh"] * 100


def status(p):
    if p["swelling"] == "yes":
        return "remove: swelling"
    if p["capacity_pct"] < 80:
        return "remove: capacity"
    notes = []
    if p["ir_mohm_pack"] > 30:
        notes.append("watch: resistance")
    if not 3.70 <= p["storage_v_per_cell"] <= 3.85:
        notes.append("fix storage voltage")
    return "; ".join(notes) or "ok"


packs["status"] = packs.apply(status, axis=1)
cols = ["pack_id", "cycles", "capacity_pct", "ir_mohm_pack", "storage_v_per_cell", "status"]
print(packs[cols].round({"capacity_pct": 1}).to_string(index=False))
print(f"correlation of cycles with capacity: {packs['cycles'].corr(packs['capacity_pct']):.2f}")

pack_id  cycles  capacity_pct  ir_mohm_pack  storage_v_per_cell                                 status
 B6S-01      12          99.2          16.8                3.82                                     ok
 B6S-02      48          97.4          18.1                3.80                                     ok
 B6S-03      95          92.8          21.5                3.84                                     ok
 B6S-04     160          86.2          27.9                3.79                                     ok
 B6S-05     210          77.2          38.4                3.81                       remove: swelling
 B6S-06      74          95.6          19.6                4.18                    fix storage voltage
 B6S-07     133          90.4          24.2                3.78                                     ok
 B6S-08     188          80.4          33.0                3.52 watch: resistance; fix storage voltage
correlation of cycles with capacity: -0.98

Carrying batteries by air

Travelling to field work by air must follow IATA’s lithium battery guidance (the International Air Transport Association), which classifies batteries by energy in Wh; airlines may impose stricter rules. In summary, for spare batteries carried by passengers:

Spare batteries must be in carry-on baggage only, never in checked baggage, and each must be protected from short circuit, for example in a pouch or with covered terminals. Lithium-ion batteries shipped on their own as cargo must be at a state of charge not exceeding 30%.

def iata_category(volts, mah):
    wh = volts * mah / 1000
    if wh <= 100:
        rule = "carry-on spare, no approval needed"
    elif wh <= 160:
        rule = "carry-on spare with airline approval, max 2"
    else:
        rule = "not allowed with passengers; ship as cargo"
    return wh, rule


for name, volts, mah in [("4S 5200 mAh", 14.8, 5200), ("6S 5000 mAh", 22.2, 5000), ("12S 16000 mAh", 44.4, 16000)]:
    wh, rule = iata_category(volts, mah)
    print(f"{name:<14} {wh:6.1f} Wh -> {rule}")

4S 5200 mAh      77.0 Wh -> carry-on spare, no approval needed
6S 5000 mAh     111.0 Wh -> carry-on spare with airline approval, max 2
12S 16000 mAh   710.4 Wh -> not allowed with passengers; ship as cargo

The 6S 5000 mAh battery of this course’s drone is 111 Wh, so it needs airline approval and is limited to 2 per person. The rules can change every year; check the latest IATA guidance and the airline’s rules before every trip.

The records chain and readiness

Each kind of record must refer to the others by code. For example, flight F-0412 used battery B6S-03 with pre-flight check PF-0412, and found defect DEF-021, closed by job card JC-017. When someone asks what this battery has been through, the documents answer at once. The CAAT guidance requires pre-flight and post-flight checks and records kept for at least 3 years.

Module lab

Common mistakes

Summary

• LiPo cells are 3.7 V nominal and 4.2 V full, stored at about 3.7–3.85 V; LiHV charges higher as the manufacturer specifies
• The battery register tracks capacity, resistance, cycles and condition, and a swollen battery must be retired at once
• Energy in Wh is nominal voltage times Ah, and determines how batteries may travel by air under IATA
• All records must refer to each other, and readiness must be confirmed with evidence in every field

Check your understanding

1. What is the total voltage of a fully charged 6S pack?
2. How many Wh is a 22.2 V 4500 mAh battery, and how can it be carried on a flight as a spare?
3. Rated capacity is 5000 mAh and measured capacity 3900 mAh. What percentage remains?
4. A battery will not be used for two weeks. At about what voltage per cell should it be stored?
5. An aircraft has passed its pre-flight check, but no one knows whether a due bolt-torque check has been done. What is its readiness status?