LiPo batteries in the field – charging, flying and transport

Charge a LiPo battery to 4.20 V per cell (LiHV to 4.35 V), end the flight before the voltage under load drops below approx. 3.5 V per cell, and leave it at 3.80–3.85 V per cell for storage. In the field, three more rules apply: protect the packs from cold and heat, charge only under supervision in a fireproof bag, and transport them with the contacts protected. If you are flying by plane, spare batteries go only in the cabin, and you keep within the watt-hour limits.

Voltages you need to know

A LiPo pack consists of cells connected in series – 4S means four cells, 6S six. All the thresholds below are given per cell, because that is how the charger shows them and how it is worth setting the alarm in the OSD.

The Gens Ace and Tattu pack manufacturer's instructions say plainly: do not exceed 4.2 V per cell for LiPo and do not discharge the pack under load below 3 V per cell, and in drones end the flight at approx. 3.5 V¹. LiHV packs are charged to 4.35 V per cell only in the charger's LiHV mode². Charging a LiHV pack in standard LiPo mode is not dangerous – you simply will not use its full capacity. The opposite mistake is dangerous: a standard LiPo charged in LiHV mode is overcharged, which is a straight road to swelling or fire.

How to prepare packs before going into the field

The day before you go, inspect every pack: is the wrap intact, are the balance leads not chafed, is the XT60 or XT30 connector firmly attached? A pack with a dent, a cut wrap or a clear bulge stays at home.

Charge with a balancer at 1C, i.e. 1.3 A for a 1300 mAh pack. The pack manufacturer's instructions recommend not exceeding 1C¹. If your pack's label gives a different value, follow the label. After charging, check the voltage difference between cells. If it grows with each cycle, or the charger shows a clearly higher internal resistance in one cell, mark that pack and keep an eye on it.

Charge the packs preferably on the day of flying or the day before. A full battery left for several days loses condition faster than one kept at storage charge.

How many packs to take and how to rotate them

Rotation matters more than the number of packs itself: fly, let the pack rest, charge, let it cool down again. After a flight and after charging, a pack needs a while to return to ambient temperature, so a plan with two packs and one charger will quickly grind to a halt. It is better to take more packs and fly at a calmer pace than to charge them at a higher current than recommended just because you cannot keep up.

Stick a number on each pack, and in your flight log record the pack number, flight time and voltage after landing. After a few weeks you will see which packs hold their performance and which are starting to fall behind – and you will retire them before they fail in the air.

Charging in the field – safety rules

In the field you usually charge from a car battery, an inverter or a generator. Whatever the source, the rules are the same:

• Wait until the pack has cooled to ambient temperature after the flight – do not connect a warm one to the charger.
• Put the charger and pack on a non-flammable surface – a concrete slab, a tile, a metal tray – never on dry grass, pine needles or a car seat.
• Charge in a fireproof bag or container and keep a fire extinguisher to hand.
• Do not leave charging unattended. The manufacturer's instructions list this among the basic rules¹.
• When parallel charging, connect only packs with the same number of cells and similar voltages. Check the voltages before plugging into the parallel board, not after.
• If a pack swells, heats up or hisses during charging, stop charging and move it to a safe place.

When charging from a car battery, keep an eye on its voltage. A drone charger draws a high current and can drain the battery so much that you cannot start the engine. Also check the charger's input voltage range and the condition of the power leads – thin wires and loose crocodile clips heat up at high current. Place the charger so that air can flow freely around it and do not cover it with anything.

In winter, do not charge packs taken straight from a freezing boot. Let them warm up first, and check the permitted charging temperature range in the pack's data sheet.

How to fly without destroying the battery

Voltage alarm in the OSD

In Betaflight, set the display of average voltage per cell and an alarm at around 3.5 V. During hard acceleration the voltage drops momentarily (so-called sag) and the alarm may flash for a second – that is not yet a reason to land. If, however, the voltage stays around the alarm level during calm flight, end the flight. After landing, the resting voltage should return to around 3.6–3.7 V per cell – if it is clearly lower, land earlier next time.

Cold

At low temperatures a pack delivers less energy and sag is greater. Keep the batteries in an insulated bag or an inside jacket pocket and take them out just before the flight. Fly the first few dozen seconds calmly – the pack will warm itself up.

Heat

In summer the biggest enemy is the sun. Do not leave packs on the car bonnet or in a hot car interior. According to the manufacturer's instructions, the pack temperature during discharge should not exceed 60 °C¹. If after a flight the pack is so hot that it is hard to hold in your hand, you are flying too hard for its capabilities or you have chosen too small a capacity.

After a crash

Inspect every pack that fell out of the drone or took a hit straight away. A dent, a cut wrap or a smell of electrolyte means the end of flying with that pack.

When a battery needs replacing

Retire a pack when:

• it is swollen or changes shape after a flight,
• it has a damaged wrap, a dent or chafed wires,
• after charging, one cell's voltage clearly differs from the others,
• flight time and voltage under load have dropped despite correct charging,
• it heats up much more than other packs of the same type.

Disconnect a damaged or swelling pack immediately and put it in a fireproof container¹. Do not puncture it or try to "rescue" it. Before you take it for disposal, you can discharge it with a discharger in a safe place outdoors, under supervision. Then tape over the contacts with insulating tape and take the pack to a battery collection point, e.g. a municipal recycling centre (in Poland, a PSZOK). Do not throw it in ordinary rubbish.

Storage between flights

If you do not intend to fly in the next few days, discharge or charge the packs to storage voltage – most chargers have a separate Storage programme for this. For longer storage the manufacturer recommends a temperature of approx. 23 ± 5 °C¹. Keep the packs away from flammable materials. A metal box or a LiPo battery container that is not airtight works best – in the event of ignition, the gases must have a way out.

Transport by car and by plane

By car

Carry packs at storage charge, in a fireproof bag or container, with the connectors protected. Do not leave them behind a window or in a sunny boot. Loose packs in a rucksack among tools are asking for a short circuit.

By plane

The rules for passengers are set out in IATA guidance, and the airline may have its own, stricter rules³:

• You carry spare lithium-ion batteries, including LiPo, only in cabin baggage, never in checked baggage.
• Packs up to 100 Wh do not require the airline's approval.
• Packs above 100 Wh and up to 160 Wh require the airline's approval, and you may have at most two spares per device.
• Packs above 160 Wh cannot be carried as a passenger – they have to go as cargo.
• Every pack must have its terminals protected: original packaging, tape over the connector or a separate pouch.
• IATA sets a limit of 20 spare batteries per person, and the airline may approve more.

You can calculate a pack's energy yourself: Wh = nominal voltage × capacity in Ah, where the nominal voltage is 3.7 V per cell for LiPo and 3.8 V for LiHV². A 6S 1300 mAh pack is 22.2 V × 1.3 Ah ≈ 28.9 Wh, and a 4S 1500 mAh pack is 14.8 V × 1.5 Ah = 22.2 Wh. Typical packs for 5″ drones are therefore well below 100 Wh, but with larger packs for long-range drones it is worth doing the sums.

IATA also notes that airlines prefer spare batteries to be charged below 25%. A pack at storage charge has considerably more, so before travelling discharge it using the charger's discharge programme, but do not take it into deep discharge. Pack each battery in a separate pouch.

Key points

• Fully charged is 4.20 V per cell (LiHV 4.35 V), storage 3.80–3.85 V, end of flight approx. 3.5 V under load.
• Charge with a balancer, at 1C, on a non-flammable surface, in a fireproof bag and never unattended.
• Protect packs from frost and sun – cold ones lose power, overheated ones lose lifespan.
• Retire a swollen or damaged pack immediately, secure it and take it to a collection point.
• On a plane, spare LiPos go only in the cabin, with the terminals protected: up to 100 Wh without approval, 100–160 Wh with the airline's approval.