WHY do the batteries in a flashlight run out on their own?

The same law applies everywhere, that the accumulated charge cannot last forever, and as a consequence, batteries and accumulators have an expiration date not only when they are lying on the shelf in the store, but also when they are inserted into some device which may be a consumer.

Frequently, the current consumption is so low that the components of the circuitry warm up only slightly. However, this consumes the battery, and as a consequence, one gets a dead battery sooner than one expects.

Conductors run from the contacts of the battery to the flashlight switch. Even if the circuit is open, the distance between the switch contacts is about a millimeter. A weak current can flow through this gap and discharge the batteries.

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