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As far as I understand, keeping a mobile phone plugged in would draw current from the source (i.e. an USB port) rather than from the battery. Therefore, the battery would not be used as long as the phone was plugged in, meaning that it would not experience any current draw. Given that batteries tend to "die" faster when they are used too much, this would mean that if a phone was plugged in most of the time, the battery would last longer because it would take more time to experience an amount of charge-discharge cycles such that its capacity starts to decrease.

Is this idea correct? If so, is there any evidence that supports this? By evidence I mean some kind of circuit or any other physical explanation that is not just based on common sense or intuition.

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    \$\begingroup\$ "As far as I understand, keeping a mobile phone plugged in would draw current from the source (i.e. an USB port) rather than from the battery" Actually this may not always be the case. The USB port may drive the charging circuit, but the electronics may be powered by the battery. It really depends on how the power system is designed, but I know at least one of my old phones would not power on unless the battery was plugged in, even if the charger was connected. \$\endgroup\$ – Ron Beyer Aug 8 at 1:10
  • \$\begingroup\$ @RonBeyer That makes sense. But well, I believe that the current drawn from the battery would at least be less than the one that would be drawn if the phone wasn't plugged in, right? This would also lead to a longer battery lifetime for a permanently plugged-in phone, according to the reasoning in the OP. \$\endgroup\$ – Tendero Aug 8 at 2:11
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Li-ion batteries degrade faster when fully charged. If the charger keeps the battery fully charged then it could reduce lifespan by half or worse, particularly at high temperature. A battery that loses 5% capacity per year at 25°C when half charged might lose 20% pear year when fully charged. The higher voltage at full charge may also make it more susceptible to 'puffing'.

Charging and discharging the battery wears it out, but it can last longer if not fully discharged. A battery that only gets 300 cycles from 100% to 0% might get 3000 cycles from 75% to 25%.

If you are using the phone a lot, can supply external power while using it, and can prevent it from fully charging, then battery life could be extended. But just leaving it plugged in on standby won't help much because current draw is low so it isn't wearing out very fast anyway, and if you can't prevent it from fully charging then lifespan may actually be reduced rather than extended.

How to Prolong Lithium-based Batteries

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