LiPo batteries for drones

The lithium polymer (LiPo) battery is what makes modern FPV flight possible: it packs a huge amount of energy into a light pack and can dump it at the enormous currents motors demand. That same energy density makes LiPos genuinely dangerous if mishandled, so understanding both their ratings and their safety rules is non-negotiable.

Cells, S-count and voltage

A LiPo is built from one or more cells. Each cell has a nominal voltage of 3.7 V, charges fully to 4.2 V, and should not be drained below about 3.0 to 3.3 V under load.

Packs are described by their S number, the count of cells in series:

3S = 3 cells = 11.1 V nominal (12.6 V full)
4S = 4 cells = 14.8 V nominal (16.8 V full)
6S = 6 cells = 22.2 V nominal (25.2 V full)

More cells means more voltage, which (with the same motor) means more RPM and power.

Capacity: how much energy

Capacity is given in milliamp-hours (mAh) and tells you how much charge the pack holds. A 1500 mAh pack can, in theory, deliver 1500 mA for one hour, or 3000 mA for half an hour. Bigger capacity means longer flight time, but also more weight, so racers balance the two; a 4S 1300 to 1500 mAh pack is a common 5-inch choice.

C-rating: how fast it can deliver

Capacity tells you how much energy; the C-rating tells you how fast you can take it out. The maximum safe continuous discharge current is:

Max current = C-rating x capacity (in amp-hours)

So a 1500 mAh (1.5 Ah) 100C pack can supply 100 x 1.5 = 150 A continuously. This figure must comfortably exceed the quad’s peak demand. A related effect is voltage sag: under heavy throttle, internal resistance causes the pack voltage to dip temporarily. A higher C-rating means lower internal resistance and less sag, so more of the pack’s power is actually usable.

Keep capacity and C-rating distinct in your mind: capacity sets flight time, C-rating sets deliverable current. A good pack needs enough of both.

Storage and charging

LiPos do not like sitting fully charged or fully empty. For anything longer than a day or two, store them at about 3.8 V per cell (a 4S pack near 15.2 V). Most chargers have a storage mode that brings packs to this level automatically. Leaving cells at 4.2 V degrades them and is a leading cause of swelling.

When charging, always balance charge. The charger’s balance lead lets it monitor and equalise every cell, so none is overcharged. Charging cells unevenly stresses them and is a serious fire risk. A common, safe charge rate is 1C: a current equal to the capacity. For a 1300 mAh (1.3 Ah) pack, 1C is 1.3 A. Faster charging requires a pack explicitly rated for it.

Safety: treat LiPos with respect

This is the part that matters most. LiPos store a great deal of energy and can ignite, sometimes in a violent thermal runaway, if abused.

Key safety rules:

Never overcharge (above 4.2 V/cell) or over-discharge (below ~3.0 V/cell).

Always balance charge with a proper LiPo charger.

Never charge unattended; charge on a non-flammable surface or in a LiPo-safe bag.

Never puncture, crush or pierce a cell. The electrolyte is reactive and can ignite on contact with air.

Retire damaged packs. A pack that is swollen (puffed), punctured, or hard-crashed should be inspected and, if in doubt, disposed of safely rather than reused.

Stop immediately if a pack gets hot or starts to swell during charging.

A hard crash deserves special caution: even if a pack looks intact, a dented or impacted cell may be internally damaged and can fail later. When unsure, err on the side of retiring it.

Putting it together