Wirebook › Glossary › Internal resistance
What is an internal resistance?
In one sentence: How hard it is for electricity to get through a battery itself; it rises in the cold and as batteries age, so the voltage sags under heavy use.
A little more
Every battery fights a little against the current flowing out of it. That fight is its internal resistance, and it costs voltage: the harder a device pulls, the more the voltage dips while it pulls (dip = current x resistance). It also turns some energy into heat inside the battery.
Cold raises internal resistance a lot, and so does age. When the dip pushes the voltage below the lowest level a device is allowed to run at, it switches off to protect itself, even with plenty of charge left. That's why old phones and cold phones die early during games, photos with flash or video calls, then work again when warm.
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