Why does it take longer for the last 10-20% of a (EV) battery to charge compared to the bottom ranges?

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Why does it take longer for the last 10-20% of a (EV) battery to charge compared to the bottom ranges?

In: Engineering
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So it doesn’t overcharge. Kinda like you can fill a bucket with water fast at first but need to slow down as you reach the top so you don’t overfill.

Think about filling up a bucket, but if you spill too much water the bucket explodes violently.

Sure, you can go fast at first but then at the top you better slow down.

Think of a lithium battery (used in EVs) as having a spring-like nature. When you charge it you can quickly get it up to full but if you take it off the charger it will spring back to only 80% full. Doing the last bit of charging is basically slowly letting the spring relax until the battery is 100% *and* the spring will not bounce back.

It actually has nothing to do with over charging as the other answers state and everything to do with heat.

Another clarification worth nothing is that not only is the last 10-20% slow to charge but the first 10-20%.

Due to internal resistance batteries heat up when charged and discharged. Taking the battery above a certain temperature will damage the battery permanently. The internal resistance is higher when the battery is nearly flat and nearly full. The charger thus puts in less power for these periods to stop the battery over heating.