Battery Fault Current: Why the Cell You Pick Decides Whether a Mistake Is Survivable

Every battery has an internal resistance, and that number, not the voltage on the label, decides what a wiring mistake costs you. Two cells can both read 3 V and behave completely differently when something on the board shorts: one sags to nothing, the other delivers enough current to destroy silicon. ## The same fault, two outcomes A coin cell such as a CR2032 has a high internal resistance — on the order of tens of ohms when fresh, rising steeply as it depletes. Datasheets rate them for continuous draws of a few milliamps and brief pulses ranging from roughly 5 mA to a few tens of mA depending on the manufacturer. Short one out and it can't hurt much: the cell's own resistance becomes the current limit and the voltage simply collapses. A CR123A is the same 3 V lithium chemistry in a far larger can, holding around 1300–1500 mAh with correspondingly low internal resistance. Typical ratings allow on the order of 1.5 A continuous and pulses of several amps. That is a hundred times the fault current a coin cell can supply, from a cell that reads exactly the same on a voltmeter. So a board designed around a coin cell has an accidental safety property: its power source is too weak to execute most mistakes. Substituting a physically larger cell of the same voltage silently removes that property. The classic failure sequence is that an external cell is fitted for testing, the polarity is wrong or a fault already exists, and the new supply now has the muscle to turn a survivable error into a dead chip — see Reverse-Polarity Damage in ICs: The Hot Chip and the Diode-Asymmetry Test. ## The general rule **Voltage determines whether the circuit works. Available current determines what a mistake costs.** The scale is wide: - Coin cells: milliamps. Self-limiting; faults brown out. - Cylindrical primaries (CR123A, AA): hundreds of milliamps to a few amps. Enough to cook a small IC, not usually enough to start a fire. - Lithium-ion cells and packs: many amps, and a shorted cell risks Thermal Runaway on top of the electrical damage. See Lithium-Ion Battery. - Lead-acid car batteries: hundreds to over a thousand amps into a dead short. This is why a dropped spanner across a car battery vaporises metal. The same reasoning explains why a bench supply with its current limit turned down is a safe way to power an unknown board while a battery is not — see Current-Limited Bring-Up: Powering a Suspect Board Without Cooking It. It is also why a protection component that survives a coin-cell fault may be blown straight through by a larger cell: protection parts are sized for an assumed fault energy, and the assumption travels with the original power source, not with the connector. Related fundamentals: Electrochemical Cell.