5.3 Lithium-Polymer Battery Management and Safety
Key Takeaways
- A LiPo cell is 3.7 V nominal, 4.2 V fully charged, about 3.8 V storage, and permanently damaged below about 3.0-3.3 V - never over-discharge.
- Multi-cell packs must be balance charged via the balance lead so no cell is over- or under-charged relative to the pack.
- A swollen or puffy LiPo is a fire precursor - retire it immediately; never charge, fly, or reuse a puffed pack.
- Charge in a fire-safe location (LiPo bag, non-flammable surface), never unattended, at the correct C rate (1C standard).
- C-rating gives the max discharge current (capacity in Ah multiplied by C); plan to land with 20-30% capacity remaining and always check per-cell voltage before flight.
Lithium-Polymer (LiPo) Battery Fundamentals
The LiPo (Lithium-Polymer) battery is the dominant power source for small and medium RPAS because of its high energy density and high discharge rate. It is also the most safety-critical component on the aircraft: mishandled LiPos can swell, catch fire, or explode. Understanding cell voltage and care is non-negotiable for a Remote Pilot Licence holder.
Cell Voltage and States
A LiPo cell has a nominal voltage of 3.7 V. The safe operating range and key states are:
| State | Voltage per cell | Notes |
|---|---|---|
| Fully charged | 4.2 V | Maximum safe voltage; do not exceed |
| Nominal | 3.7 V | Mid-range, in-use voltage |
| Storage voltage | ~3.8 V | Long-term storage level; safe for weeks or months |
| Low warning | ~3.5-3.6 V | Time to land soon |
| Damage floor | ~3.0-3.3 V | Below this, cells are permanently damaged |
| Resting after flight | ~3.7-3.8 V | Healthy post-flight level if not over-discharged |
Never over-discharge a LiPo below roughly 3.0-3.3 V per cell - the cell chemistry is damaged, capacity drops permanently, and the cell becomes unsafe. Most flight controllers have a low-voltage warning; respect it and land promptly.
Balance Charging
A multi-cell LiPo (for example 3S = 3 cells, 4S = 4 cells, 6S = 6 cells) must be balance charged using the balance lead. Balance charging monitors and equalises the voltage of each cell individually so that no cell is over-charged (which causes swelling or fire) or under-charged relative to the pack. Always use a balance charger and the balance lead - never fast-charge a multi-cell pack without balancing.
Storage Practice
If a LiPo will not be used for more than a few days, it should be brought to storage voltage (~3.8 V/cell) using the charger's storage mode. Storing a LiPo:
- Fully charged for long periods risks swelling, internal damage, and fire.
- Fully discharged risks dropping below the damage floor through self-discharge.
Store packs in a fire-safe location (LiPo bag, metal ammo box, or ceramic container) at room temperature, away from flammable materials.
Swelling and Puffy Cells - Retire Immediately
A swollen or puffy LiPo cell is a cell whose internal chemistry has begun to gas. This is irreversible and is a fire precursor. Retire a swollen pack immediately - do not attempt to recharge, discharge, or use it just once more. Discharge it safely (for example via a resistor or salt-water bath per local guidance) and dispose of it at a battery recycling point.
Fire-Safe Charging
- Charge in a fire-safe location: a LiPo fire bag, on a non-flammable surface, away from combustibles.
- Never charge unattended - most LiPo fires occur during charging.
- Use the correct charger and charge rate (1C is standard: a 2200 mAh pack charged at 2.2 A). Higher rates are possible only if the pack label permits.
- Charge in a place where, if a fire starts, it cannot spread - not on a carpeted floor near furniture.
C-Rating and Maximum Current
The C-rating states the maximum continuous discharge current. For a 5000 mAh pack rated 20C:
Max current = capacity (Ah) x C-rating = 5.0 Ah x 20 = 100 A.
Choose a pack whose C-rating comfortably exceeds the aircraft's peak current draw; too low a C-rating causes voltage sag, overheating, and premature failure under load. Burst ratings (for example 20C/30C burst) apply for only a few seconds.
Capacity (mAh) and Endurance Estimation
Capacity in mAh (milliamp-hours) sets how long the aircraft can fly. A rough endurance estimate:
Flight time (min) is approximately (Capacity mAh x 0.8 divided by average hover draw in mAh/min).
The 0.8 factor reserves about 20% - never fly a pack to empty. For example, a 5000 mAh pack with an average draw of 1000 mAh/min gives roughly (5000 x 0.8 / 1000) = 4.0 min to the reserve point - adjust for real draw measured in flight. Always plan to land with at least 20-30% capacity remaining.
Pre-Flight Voltage Check
Before every flight, check the per-cell voltage with a cell checker or the charger. A healthy fully-charged pack reads about 4.2 V/cell; a storage-charged pack (~3.8 V) should be topped up before flight. If any cell reads significantly lower than the others (more than about 0.05-0.1 V), the pack is unbalanced or damaged - do not fly it.
Series and Parallel Packs, and Voltage Sag
A pack's total voltage is the sum of its cells in series (a 4S pack reads about 16.8 V fully charged). Parallel packs add capacity at the same voltage. Under load, a healthy pack sags only a few tenths of a volt; a pack that sags sharply and recovers slowly is either under-spec for the aircraft's current draw or nearing the end of its cycle life. Persistent voltage sag, warmth after flight, or a cell that will not balance are all signs to ground the pack and investigate before the next flight.
Transport
Transport LiPos in a fire-proof bag with terminals insulated (never loose in a bag where contacts can short). Keep them away from metal objects and out of direct sun. For air transport, follow airline and CASA dangerous-goods rules - carry-on limits and terminal taping usually apply.
What is the correct storage voltage for a LiPo cell that will sit unused for several weeks?
A LiPo pack arrives at the pre-flight check visibly swollen. What is the correct action?