8.2 Battery Terminology, Types, Charging & Storage

Battery Language Before Battery Decisions

A battery converts chemical energy into electrical energy. The UAS draws current; voltage under load supplies electronics and motors. Controller percentage is an estimate affected by temperature, cell condition, current demand, and calibration.

Capacity

Capacity is stated in amp-hours (Ah) or milliamp-hours (mAh). A 5,000 mAh pack is 5 Ah. It describes charge, not guaranteed flight time. Mass, wind, cold, ageing, and manoeuvre can consume usable energy faster.

C-rate

A C-rate expresses current relative to capacity. For a 5 Ah pack, 1C is 5 A, 2C is 10 A, and 10C is 50 A. Discharge rating and permitted charge rate are different. Do not use a high advertised discharge C-rate as a charge instruction.

Voltage and cell count

Lithium packs are often labelled 4S or 6S. Series cells add voltage while capacity remains that of the string. A charger set to the wrong chemistry or cell count can overcharge cells and cause fire.

Memory effect

Classic memory effect is mainly associated with particular nickel-cadmium use patterns and is often used loosely for nickel-based batteries. It is not the normal principle of LiPo or lithium-ion packs. Never deep-discharge lithium to “erase memory”; this can damage the pack.

Battery Types

Chemistries are not interchangeable. Never substitute solely because the connector fits or voltage looks similar. Mass, current, management, dimensions, software, and balance matter.

Correct Charging and Use

Use the specified charger, chemistry, cell count, polarity, and current. Balance-charge multicell lithium packs where required so individual cells stay within range. Charge on a non-flammable surface in a supervised ventilated area away from escape routes and combustibles.

Inspect for swelling, dents, puncture, corrosion, damaged wire, loose connector, leakage, heat, or imbalance. Secure the pack and include its mass in MTOM and CG.

Allow a warm pack to cool before charging. Do not fly into damaging deep discharge. Set warnings and landing reserve from real loaded performance.

Storage and Transport

Follow the manufacturer’s recommended storage state and temperature. Common lithium practice uses partial storage charge rather than leaving packs full or empty for long periods, but the exact target belongs to the manufacturer.

Store cool and dry with terminals protected from short circuit, away from combustibles and direct sun. Fire-resistant containment can reduce secondary risk but cannot make an unsafe pack safe. Do not leave packs heat-soaking in a vehicle.

A crash-damaged pack may ignite later. Isolate it safely and use manufacturer and local hazardous-waste guidance. Do not put damaged lithium in ordinary waste.

Combining the Terms

A 6S 5 Ah pack with a high discharge rating can still be unsafe because one cell is imbalanced, the pouch is swollen, or the UAS does not approve it. More capacity can increase endurance but also mass, CG shift, and impact energy. A high state-of-charge display can collapse under cold load.

The A2 answer is system-based: correct type, healthy cells, approved configuration, known loaded performance, and reserve.

Why LiPo Knowledge Is on the A2 Flight-Performance Syllabus

Most Open-category multirotors use lithium polymer (LiPo) or related lithium battery packs. Batteries are not a side hobby topic — they are a flight performance system. Capacity, voltage sag, temperature, and cell health decide whether you complete a Near People sortie, trigger a low-battery failsafe over a pavement, or suffer a mid-air power collapse.

UAS.OPEN.030 does not publish a chemistry textbook, but RAE(PC) papers routinely test practical LiPo safety and performance because poor battery discipline is a common cause of flyaways, forced landings, and fire after a crash. Treat this section as examinable airmanship, not optional workshop lore.

Storage Charge — Partial, Not Full

Industry practice taught for safety and competency exams is clear:

• Do not store LiPo packs fully charged for long periods.
• Store at a partial state of charge, typically about 40–60%.
• Many pilots and instructors use the storage voltage rule of thumb of about 3.8 V per cell (for conventional LiPo nominal 3.7 V/cell chemistry) when a charger’s “storage” mode is available.

Why partial storage matters:

Operational sequence for professional Near People work:

1. Store packs at storage charge in a suitable container between jobs.
2. Charge to flight voltage shortly before the mission, not days early “to be ready.”
3. After flying, allow packs to cool, then return them toward storage charge if they will not fly again soon.
4. Never leave a hot pack in a sealed car cabin after a high-current flight.

Exam distractors often claim “always store at 100% so you can launch immediately” or “store at 0% to be safest.” Both are wrong for standard LiPo care taught in remote-pilot safety contexts.

Charging Discipline (Performance + Safety)

Good charging practice supports both fire safety and predictable endurance:

• Use a charger matched to the pack chemistry and cell count.
• Set the correct cell count and charge current per manufacturer guidance (many training contexts teach conservative rates rather than maximum advertised charge).
• Prefer balance charging so individual cells stay aligned.
• Charge on a non-flammable surface; do not charge unattended for long periods in a wooden loft full of prop boxes.
• Do not charge a pack that is damaged, wet, or swollen.
• Allow packs to cool after flight before charging.

Unbalanced cells (one cell significantly lower or higher than siblings) reduce usable capacity and can cause premature low-voltage events. Near People, a sudden low-voltage cut while you are manoeuvring at 5–30 m from people is a ground-risk event, not only a battery event.

Cold Weather — Capacity Loss and Power Sag

Cold temperatures reduce available capacity and increase internal resistance. The aircraft may show a healthy percentage on the ground in a warm van, then suffer voltage sag under hover load outdoors. Sag means voltage collapses under current demand even if the pack is not empty in the amp-hour sense.

Near People implications:

• Planned 15-minute flights become 8–10 minute flights without the pilot noticing until the low-battery warning.
• Climb performance fades exactly when you need height after an abort.
• Aggressive camera moves draw current spikes that trip voltage protection or cause brown-out-like behaviour on sensitive electronics.
• RTH may start with less reserve than your warm-weather mental model assumes.

Cold-weather mitigations for A2 pilots

1. Keep packs warm (inside jacket, insulated bag, vehicle cabin) until immediately before flight — without cooking them on heaters.
2. Reduce expected endurance; plan extra reserve.
3. Avoid maximum payload on the coldest days if the combination of mass + cold sag is untested.
4. Hover-check power and telemetry in a safe area before entering the closest planned proximity to people.
5. Land earlier than your summer habits; do not “squeeze one more orbit.”

Cold weather is also a meteorology link: wind, density, and battery chemistry stack. Performance questions may combine a cold morning, a heavy camera, and a low-battery failsafe over a footpath.

Swollen or Puffed Cells — Retire, Do Not Fly

A swollen (puffed) LiPo has generated internal gas from damage, over-discharge, over-charge, impact, or age. Visual cues include a rounded pack, tight or stretched shrink wrap, and cells that no longer sit flat.

Hard rule taught for safety exams:

• Do not fly a swollen pack.
• Do not charge it like a normal pack for “one more job.”
• Retire it safely using accepted disposal practice (many clubs and training materials describe discharging to a safe empty state outdoors away from flammables, then taking the pack to an appropriate battery recycling/disposal route — follow local hazardous-waste rules and manufacturer guidance).

Why flying a puffed pack is unacceptable near people:

• Internal damage can progress to thermal runaway and fire in flight or after impact.
• Capacity and internal resistance are unpredictable — endurance telemetry becomes fiction.
• A mid-air power loss at low height near uninvolved persons is a severe ground-risk outcome.

If a pack swells after a crash or hard landing, quarantine it away from other batteries and flammable kit. Do not throw a puffed LiPo into household general waste.

Fire Risk and Transport Bags

LiPo fires are intense and self-oxygenating once thermal runaway starts. A2 candidates should know practical controls:

Transport bags are not magic armour against every failure, but they are standard professional practice and a frequent “best answer” on safety-style items when the stem asks how to carry spare packs to a site.

Never puncture a swollen pack to “let the gas out.” Never leave charging packs on a sofa. Never ignore the smell of electrolyte (sweet/solvent odour) after a crash.

Cell Balance Monitoring

Multicell packs (for example 4S, 6S) need individual cell health, not only pack voltage:

• A pack might show 22.2 V total while one cell is critically low and another is high.
• Imbalance reduces usable capacity and can trigger protective cutoffs early.
• Balance leads and charger balance ports exist so you can see per-cell voltages.

Pre-flight habit:

1. Check pack resting voltage and, when available, per-cell readings.
2. Reject packs with large cell deltas relative to manufacturer or training thresholds you use in practice.
3. After storage months, verify balance before the first Near People job of the season.

Low-Voltage Cutoffs and Failsafe Links

Flight controllers and ESCs implement low-voltage protection or battery failsafe logic so motors are not driven into damaging deep discharge — and so the aircraft attempts a controlled response before power collapses.

Links you must understand conceptually for A2 theory:

• Low battery warning thresholds should leave enough energy for a safe landing or RTH without overflying people as a default path (site selection + failsafe configuration — next section).
• Hard low-voltage cutoff that simply stops motors at low height near people is a catastrophe; modern systems prefer warned descent/RTH, but configurations differ by manufacturer.
• Aggressive flying + cold + heavy payload reaches cutoff earlier than brochure times.
• Battery failsafe mode (RTH vs land vs hover) is a configuration decision, not a mystery left to factory defaults you never read.

Performance planning means knowing your pack’s real endurance in the loaded, weather-specific configuration and setting battery failsafes accordingly before Near People operations.

Realistic Scenarios

Scenario A — weekend storage at 100%. A pilot charges six packs on Friday for a Sunday A2 job and leaves them full on a bedroom shelf. One pack is warm to the touch Saturday night. Correct mindset: long-term full storage is poor practice; isolate the warm pack, do not fly it, and adopt storage-charge habits. Full charge belongs near the flight, not as a multi-day default.

Scenario B — cold car park, heavy camera. Telemetry shows 60% at launch after a warm indoor charge reading. Three minutes into a hover near a shopfront, voltage sags and the controller warns early. The pilot should have planned reduced endurance, kept packs warm, and validated the payload in cold air far from people first.

Scenario C — puffed pack after a tip-over. One corner of the pack is rounded. A colleague says “it still shows 15.8 V, fly it.” Refuse. Retire the pack safely. Voltage alone does not clear structural cell damage.

Scenario D — unbalanced 6S. Five cells sit near 3.85 V in storage; one sits at 3.55 V. Flying invites early sag on the weak cell. Balance/investigate or retire; do not launch a Near People job on a clearly unbalanced pack.

Memory Table for Exam Day

Battery discipline is how you keep published flight times honest. Near People operations punish optimistic battery stories faster than empty-field recreational flying.