eli5 why is it that if we leave a battery overcharging it starts losing its charge-retention capacity? For example in laptops or phones.
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It doesn’t. That quirk only applies to Ni-MH batteries. Li-ion does NOT do that. It, however, wears out with recharge cycles, so you can only get so many (~1000) 0-100-0% cycles out of it before it can’t keep charge anymore. In Ni-MH it’s apparently crystals forming inside and inhibiting the movement of ions as a result of repeated overcharging.
Overcharging isn’t really a thing anymore since modern gadgets are smart enough to stop charging when a battery is full. They then bounce between 95-100%, discharging and charging as necessary. (Maybe not exactly, but roughly)
But this still introduces wear since the battery is still providing and receiving energy. Lithium ion batteries also don’t like to stay at the extreme ends of their capacity. (fully discharged or fully charged – I don’t have an explanation to that at this time, I saw you asked in another comment)
To help preserve life you sometimes get the feature to keep the maximum charge at 80% (again, this number can vary but it’s a default) to reduce long-term wear. This is why the iPhone can learn your routine and charge up to 80%, wait, and then complete the charge when it’s closer to when you wake up to reduce time at 100%. My laptop, some Android phones also have that feature but how it’s accessed can vary widely.
there are different kinds of batteries and they require different kinds of charging and maintenance.
Normal NiMH batteries require CC (constant Current) charging with delta peak detection, meaning when they are full they get hot and their voltage starts to decrease again, which is what intelligent chargers detect and stop charging. They also have a pretty high self discharge rate which is why many chargers trickle charge them after they have finished the actual charge.
Modern LSD (low self discharge) superlattice NiMH batteries like the Sanyo eneloops have extremely low self discharge and trickle charging them will kill them within months. This is the reason why eneloops dont work well in those portable phones that get charged in their “base station”.
Lithium batteries have an electrolyte that ages much more quickly when the battery is very full or nearly empty, which is why those states should be avoided. High temperatures, like when you leave the device in a car on a hot summer day can massively speed up that degradation even further.
unlike NiMHs you cant overcharge them however because they work with constant voltage charging, that means the charger maintains a voltage of exactly 4.20V per cell and the cell can never go beyond that.
If the battery seems to lose capacity due to constantly being connected to the charger it is because it degrades much faster when kept at 100% state of charge, not because of “overcharging” which is not possible with Li-ion, especially not if the battery consists of a single cell like the one in a phone.
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