Two layers of safety: the electronic and the mechanical
Battery safety relies on defence in depth, organised in two complementary levels:
- Active protection (electronic): the BMS (Battery Management System) and protection circuits. They continuously monitor each cell and cut or limit the current before a dangerous threshold is reached. This is the first, and the smartest, line of defence.
- Passive protection (built into the cell): mechanical devices and materials that react directly to pressure, temperature or current, with no need for electronics or software. This is the safety net that stays active even if the BMS fails.
That redundancy matters: an electronic component can burn out, a sensor can lie, firmware can have a bug. The passive devices, on the other hand, simply obey physics.
The CID: cutting the current during overcharge
The CID (Current Interrupt Device) is the main defence against overcharge. Its principle is purely mechanical and based on pressure.
When a cell is charged beyond its maximum voltage (typically 4.2 V for a common NMC chemistry), side reactions kick in and generate gas inside the casing. Internal pressure rises. The CID is a metal disc designed to deform at a precise threshold — on the order of 10 bar (about 145 psi) in a cylindrical cell. As it deforms, it permanently breaks the electrical connection between the inside of the cell and its terminal. The current is cut, the charge stops instantly.
Two important characteristics:
- It is irreversible: once triggered, the CID retires the cell for good — it becomes unusable. That is the price of safety.
- It acts before the vent: the CID is calibrated to trigger at a pressure below the one that would open the safety vent, so as to stop the problem before it escalates.
A 2021 study in the Journal of The Electrochemical Society (Underwriters Laboratories and Purdue University) confirmed the decisive role of the CID: in every cell-level overcharge test, the CID activated and prevented thermal runaway. Without it, the authors note, the cell temperature would rise uncontrollably to the point of fire.
The PTC: limiting the current during a short-circuit
The PTC (Positive Temperature Coefficient thermistor) protects against the other major hazard: the external short-circuit, when the two terminals are bridged by a conductor (a key, a tool, a wiring fault).
The PTC is a component whose resistance is low in normal operation but rises sharply as soon as its temperature climbs. A short-circuit draws a huge current that instantly heats the PTC: its resistance jumps, throttling the current and protecting the cell. Unlike the CID, the PTC is resettable: once the short is gone and the temperature drops, it returns to its low resistance.
The same 2021 study showed that the PTC effectively regulates the thermal response during an external short, protecting fresh and aged cells alike.
Two more safeguards
- The safety vent: if pressure keeps rising anyway, a vent (a deliberately weakened area of the casing) opens to release the gases in a controlled way and prevent the casing from bursting. On an 18650 cell it opens around 2.8 MPa (~406 psi).
- The shutdown separator: the film separating the two electrodes is designed to partially melt at around 130 °C, closing its pores and blocking the flow of lithium ions. The reaction stops on its own. This device exists in every cell format.
So what about prismatic and pouch cells?
This is a crucial question, because these devices are not present in every format — a point that is often overlooked.
| Device | Cylindrical | Prismatic | Pouch |
|---|---|---|---|
| PTC (short-circuit) | Yes, standard | Rare | No |
| Pressure CID (overcharge) | Yes, standard | Variants | Not standard |
| Safety vent | Yes | Yes | Via the seals |
| Shutdown separator | Yes | Yes | Yes |
| Reliance on the BMS | Medium | High | Very high |
In plain terms:
- Cylindrical cells (18650, 21700 — scooters, e-bikes, power tools, some EVs): the best natively protected format. Their rigid metal casing allows a precisely calibrated pressure CID, PTC and vent to be built in. This is the format studied in the research cited above.
- Prismatic cells (rigid metal can, larger EVs and storage): they have safety vents and CID variants (sometimes a thermal fuse), but the PTC is generally absent. They lean more on the vent and on the BMS.
- Pouch cells (flexible aluminium-laminate pouch — smartphones, drones, some EVs): because of limited space and their soft envelope, they carry neither a PTC nor a standard CID. They protect themselves through swelling (the pouch expands and can break the electrode contact; some designs add a “swelling CID”), through venting at the seals, and through the shutdown separator. As a result, they depend far more on the external protection circuit and the BMS.
Behaviour in a runaway differs too: a rigid cell (cylindrical or prismatic) may rupture or explode, whereas a pouch cell tends to swell and then catch fire.
What this means for a well-built pack
The lesson is clear: no single device is enough on its own. Safety comes from stacking independent barriers — BMS, CID, PTC, vent, separator — each covering the others' weaknesses. A format with little internal protection, like the pouch cell, demands a flawless BMS and protection circuit. A quality pack, whatever the format, combines healthy cells with reliable management electronics.
That is exactly the cirBATT approach: every second-life cell is tested (real capacity, internal resistance, safety), sorted, then reassembled with our proprietary BMS. We never rely on a single barrier.