The battery is 30–40% of your aircraft's mass, the most safety-critical component you ship, and the only consumable customers will buy for years. Treating it as a commodity purchase is how programs end up with aircraft that sag out of the sky in winter. Here's the working knowledge.
Chemistry choice: the only two that matter today
| Property | LiPo (pouch) | Li-ion (18650/21700) |
|---|---|---|
| Energy density (pack level) | 140–190 Wh/kg | 200–260 Wh/kg |
| Continuous discharge | High (genuine 10–30C) | Modest (1–10C by cell type) |
| Cold performance | Poor below ~5 °C unheated | Poor–moderate; heated packs common |
| Cycle life (realistic) | 150–300 | 300–500+ |
| Mechanical robustness | Soft pouch — needs enclosure | Steel cans — inherently sturdy |
| Pack design freedom | Any shape | Cylindrical granularity |
The decision rule: compute your hover (or cruise) current draw and divide by pack capacity — that's your working C-rate. Below ~4C average, Li-ion wins on endurance; above ~8C, LiPo is the only sane option; between, look at temperature envelope and cycle economics. Efficient large-prop aircraft (see propeller guide) often fall in Li-ion territory — one reason efficient aircraft compound their advantage.
C-ratings: institutionalised optimism
A "25C" 20 Ah hobby pack theoretically delivers 500 A continuously. It will not. Marketing C-ratings are unregulated; halve them, then check the cell datasheet. What actually matters:
- Cell-level continuous current from the manufacturer datasheet (e.g. a Molicel P42A: 45 A burst, ~30 A continuous with temperature limits).
- Voltage sag under your load: sag steals usable energy — a pack that drops 0.3 V/cell at hover current hits the low-voltage failsafe with charge still in it.
- Temperature at end of flight: packs landing above ~55 °C are ageing fast. Log it (smart batteries make this automatic).
Voltage architecture
Higher pack voltage means lower current for the same power: thinner wires, cooler ESCs, less sag. The industry has drifted upward — 6S for small professional aircraft, 12S–14S for 25 kg-class. Costs: HV components price higher, and above 60 V DC you enter shock-hazard territory with handling and design implications. Pick the highest voltage below 60 V your component ecosystem supports comfortably.
Smart batteries: worth it sooner than you think
A smart battery adds a BMS with a gas gauge, cell balancing, temperature logging, charge/discharge FETs and a data interface (SMBus/CAN) the flight controller can read. For anything you hand to customers, this converts "operator misjudged remaining charge" from your accident report into an interlock: accurate state-of-charge (not voltage guessing), state-of-health for retiring tired packs, enforced storage-voltage self-discharge, and serialized fleet history. For internal prototypes, standard packs plus a logging charger and disciplined labels get you 70% of the value at 20% of the cost.
Endurance arithmetic
Usable energy = capacity × voltage × usable fraction. Plan for a usable fraction of ~80% (landing reserve plus sag), then apply the mass spiral: added battery raises MTOW, which raises hover power ~1.5× proportionally. There is an optimal battery fraction beyond which endurance falls — the full derivation with worked examples is in the endurance calculation guide.
Lithium batteries are Class 9 dangerous goods: UN 38.3 test reports, state-of-charge limits (≤30% for air cargo), packaging rules. If you ship aircraft to customers, battery logistics is a workstream — plan certification cost and lead time (weeks, four figures per pack design) before the launch date does it for you. Storage discipline matters too: full packs stored warm lose capacity permanently; store at ~3.8 V/cell (LiPo) around 15–20 °C.
Fleet economics
A pack is a wear item with a cost per flight: pack price ÷ realistic cycles. A €400 pack lasting 200 cycles costs €2/flight — budget it in your service pricing. Retire packs on data (internal resistance rise, capacity fade past ~80%, sag at reference load), not on appearance; a puffed pack should have been retired two datapoints ago. And design the aircraft so pack swap takes seconds without tools — battery logistics, not flight time, sets your operational tempo on site.
Frequently asked questions
Should my drone use LiPo or Li-ion batteries?
Use LiPo when discharge rates are high (small aircraft, aggressive flight, high disk loading). Use Li-ion (18650/21700 packs) when hover power is modest relative to pack size and endurance matters most — Li-ion stores roughly 30–60% more energy per kilogram but delivers less current per cell.
Are drone battery C-ratings accurate?
Marketing C-ratings on hobby LiPo packs are routinely 2–4× optimistic. Treat them as a brand identifier, not an engineering number. Size packs from the cell datasheet's continuous discharge current, or from your own load testing with temperature logging.
How many charge cycles does a drone battery last?
Well-treated LiPo packs deliver 150–300 cycles before noticeable sag and capacity loss; Li-ion packs typically 300–500+. Charging to 100% only when needed, storing at ~3.8 V/cell, and keeping discharge under 60% of rated maximum all extend life significantly.