why do batteries charge quicker when they’re empty.

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I’ve noticed that my phone will charge really quickly up to 50%ish and then the speed it charges to 100% is much slower. I’ve found this true for other devices too, why do batteries charge quicker when they’re empty and then the speed slows down to get it to 100%?

In: Technology
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This is because of how modern lithium battery charges.

The battery starts with a “constant current charge”. Where the charger gives the battery the amount of electrical current the battery is rated for. And the battery charges at it’s best speed.

However, as the battery fills, the voltage inside the battery goes up. And to avoid damage to the battery the charger now has to switch to “Constant voltage charging”. Here, the charger feeds the battery only with a certain amount of voltage or “pressure” and the battery simply takes as much electrical current as it can handle. And it charges signifficantly slower therefore.

Think of pouring water into a mug. When it’s empty, you can pour quickly. But unless you want spilling boiling water all over, you have to slow down when it’s getting full.

Overcharging batteries, especially the lithium kind is bad. It damages the battery and can result in the battery catching on fire if you really overdo it.

That’s why these days we have charge controllers that take care of doing the charging right, and protection circuits that will disconnect the battery if something goes wrong.

Resistance. This is true for lead acid & lithium batteries. Nimh & NiCad batteries also experience something similar, but they just get hot and can still charge at a higher rate.

Lead acid batteries will automatically use less current until it is fully charged. It’s a self regulating system.

Lithium batteries have regulators built into the battery, charging circuit, or, the charger. They charge fastest between 15 & 85%.

You can charge some lithium batteries in as little as 15 minutes or less, but it seriously effects their useful lifespan.