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Not with LetterAgent, and rarely by mail at all. Loose lithium cells are dangerous goods shipped alone: US postal rules allow individual lithium-ion cells domestically by surface mail only, in limited quantities, in original sealed packaging, with the proper battery mark and a surface-only notice. Loose cells rolling in a mailer are the case every carrier is strictest about.
| How the battery ships | Classification | What the rules say |
|---|---|---|
| Inside the device it powers (laptop, flashlight, vape) | UN3481: contained in equipment | The most lenient path; most postal services allow it within quantity limits |
| In the same box as the device, but outside it | UN3481: packed with equipment | Stricter; terminals must be protected against short circuits |
| Alone, no device at all (spare 18650s, power-bank cells) | UN3480: batteries shipped alone | The strictest path: surface only, original sealed packaging, marked and declared |
A lithium cell is a concentrated packet of energy with two exposed contacts. Touch both contacts with something conductive (a coin, a key, another cell's terminal) and the cell short-circuits: it dumps its energy as heat, heats up, and can vent, catch fire, or set off neighboring cells. That is why every set of rules starts with terminal protection: individual cells in non-conductive packaging, terminals taped or capped, cells separated so they cannot touch each other or anything metal. A plastic 18650 storage case, the kind that holds each cell in its own slot, is the right instinct; a zip-lock bag full of loose cells rattling against coins is the textbook failure. Power banks are always classified as spare batteries (UN3480) even though they look like gadgets, which surprises people who assume a power bank is equipment.
The details vary by country, but the shape is the same everywhere. In the US, postal rules allow individual lithium-ion cells domestically only by surface transportation, in limited quantities, and only in their originally sealed packaging: the cells you pulled out of a shrink-wrapped four-pack do not qualify. The package must carry the lithium-battery handling mark identifying them as UN3480, plus text stating surface mail only and forbidden aboard passenger aircraft. Air transport of loose cells is where the strictest rules live: international air rules treat shipped-alone lithium-ion cells as full dangerous goods with quantity limits, packaging standards, and a state-of-charge cap. In Canada, standalone lithium batteries are non-mailable through regular Canada Post parcel services, full stop. Internationally, loose cells (UN3480/UN3090) are banned from most postal streams entirely.
Do not. An undeclared lithium fire in a mail stream is discovered when a parcel starts smoking in a sorting facility or a cargo hold, and the penalties for undeclared dangerous goods are real on both sides of the border. Carriers also routinely X-ray and scan parcels, and battery-shaped dense objects in a package declared as something else draw attention. Beyond the legal risk, the physical risk is yours: a shorted 18650 in your own outgoing mail bin can burn before it ever leaves your house. The legitimate alternatives are simple. Ship the device with the battery installed, which falls under the lenient UN3481 rules. Buy new cells from a retailer that ships them properly. Or hand the cells to someone local: the restrictions that make mailing painful do not apply to driving a battery case across town.
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