eli5 why is it that if we leave a battery overcharging it starts losing its charge-retention capacity? For example in laptops or phones.
In: Technology
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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