The battery is the single component most likely to turn a medical device program into a recall. It is the highest energy density object in the enclosure, it degrades on a schedule nobody controls, it is the part most often changed by a contract manufacturer looking to save money, and it can injure a patient without any electrical fault elsewhere in the design.
The requirements are well defined. The catch is that most of them must be satisfied by decisions made very early, when the team is still choosing between a coin cell, a pouch pack, and a set of AA cells.
Primary or rechargeable is a clinical decision
Before chemistry, decide whether the device is replaceable-battery or rechargeable, because that decision follows from clinical use rather than from engineering preference.
Replaceable primary cells suit devices used intermittently, stored for long periods, or deployed where charging is unreliable. Alkaline AA and AAA cells are cheap, universally available, and shed nearly all of the lithium regulatory burden, at the cost of energy density and a user who must be trusted to change them.
Rechargeable packs suit devices used daily, devices with high peak current, and anything the user should not be opening. They create obligations: charge management, state-of-health tracking, an end-of-life replacement plan, and a service model. A device with an unreplaceable pack has a defined lifetime, and you need to say what happens at the end of it.
Lithium chemistry and the safety case
Most rechargeable medical devices use lithium-ion or lithium-polymer cells, and the dominant hazard is thermal runaway: an internal short, overcharge, or crush event that drives an exothermic reaction the pack cannot dissipate. The practical defenses are layered and each layer has to be justified in the risk file, not merely present.
- Cell selection. Lithium iron phosphate trades roughly a third of the energy density of a cobalt-oxide cell for a much higher thermal runaway onset temperature, a trade often worth making in a device worn against the body.
- Protection circuitry. A dedicated protection IC for overvoltage, undervoltage, overcurrent, and short circuit, independent of the main microcontroller, so a firmware fault cannot disable protection.
- Thermal sensing at the cell, with charge inhibited outside the manufacturer's temperature window. Charging a cold lithium cell plates metallic lithium and is a leading cause of latent internal shorts.
- Mechanical protection. Pouch cells need room to swell and must not be compressed by the enclosure or pierced by a screw boss.
- Single-fault safety. IEC 60601-1 expects the device to remain safe under any single fault, so no component failure may lead to a fire or an unacceptable surface temperature.
These are the same fundamentals covered in our guide to battery pack design for a product, with a higher standard of documentation attached to each one.
The standards you will actually be asked for
| Standard | What it covers | Who asks for it |
|---|---|---|
| IEC 62133-2 | Safety of rechargeable lithium cells and batteries for portable use | Notified bodies, test labs, most CMs |
| UN 38.3 | Transport safety: altitude, thermal cycling, vibration, shock, external short, impact, overcharge, forced discharge | Every carrier and freight forwarder |
| IEC 60601-1 | Battery-specific clauses within general medical electrical safety, including thermal limits, single-fault behavior, and charge circuit safety | FDA reviewers, notified bodies |
| UL 1642 | Lithium cell safety, widely used as a cell-level qualification | Cell vendors, US market |
| IEC 60086-4 | Safety of primary lithium batteries | Applies if you use lithium primaries |
Two notes. UN 38.3 testing is done on the cell and on the pack as shipped, and must be repeated if the pack configuration changes, including a cell supplier swap; our article on UN 38.3 testing covers the eight test sequences. And the medical electrical safety work sits on top of the battery standards rather than replacing them, as described in our overview of IEC 60601 electrical safety testing.
Medical-grade cells and traceability
There is no formal category called a medical-grade cell, but the phrase points at something real: cells from a controlled channel and a named factory line, with lot-level traceability, a change-control agreement, and a supplier who notifies you before altering construction. Open-market cells have none of that, and a silent construction change inside the same part number invalidates your qualification testing.
Under a quality system the pack is a controlled component: approved supplier, incoming inspection criteria, lot records tied to each device history record, and a change-control path requiring re-qualification rather than an email. Plan for obsolescence too, because cell formats disappear faster than medical devices do. Design so a second source is feasible, and read our article on what to do when a component goes end-of-life before committing to a custom cell.
Telling the user how much battery is left
IEC 60601-1 requires internally powered equipment to signal low capacity in time for the operator to act. A four-bar icon derived from raw voltage is not adequate for a lithium pack, whose voltage curve is flat across most of its usable range.
Use a coulomb-counting fuel gauge with a chemistry-matched model, correct for temperature, and re-learn capacity as the pack ages. Then decide what the device does when the estimate runs out mid-therapy: a controlled stop with an alarm, a reserve for graceful shutdown, or a hand-off to mains power. That behavior is a risk control and belongs in the hazard analysis described in our guide to ISO 14971 risk management. Extending runtime through firmware is usually cheaper than adding cells; our article on low-power firmware and sleep modes covers where the current goes.
Shipping, labeling, and the field
Lithium batteries are dangerous goods. Devices shipped with cells installed, with cells packed alongside, or as bare packs fall under different packing instructions, and air freight is more restrictive than ocean or ground. Standalone lithium-ion packs shipped by air must generally be at a state of charge no higher than 30 percent, and shipments require the lithium battery mark, a test summary available on request, and trained shipping staff.
The consequences reach further than logistics. A home-use device a patient may fly with should ship with guidance on carry-on rules, since airlines restrict spare packs to the cabin. Any replacement pack sold as a spare part is itself a dangerous goods shipment, a cost most teams discover after launch. Build these constraints into the design transfer plan rather than treating them as a shipping department problem.
Projects House selects, qualifies, and integrates battery systems for medical devices, from chemistry choice and protection architecture through IEC 62133 and UN 38.3 testing and the supplier controls a quality system requires. Bring us your device through our contact form.