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In modern devices (mobile phones, tablets, laptops) the battery charging current is automatically switched off when the battery reaches 100 %. After being disconnected, the power supply cools down. It is a healthy practice for the battery.

But if (after the charge) the device is kept connected to the mains and is used, who provides the energy? The battery or the wall socket? If it is the battery, the power supply should continue to charge it, but it doesn't, because it keeps being cold (1); if it is the power supply - somehow bypassing the battery - it should again become hot as during the charge.


(1) Moreover, the battery would continuously charge and discharge around 99% - 100 % and this could damage it.

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    \$\begingroup\$ This answer to a similar question may give some insight. \$\endgroup\$ – Roger Rowland May 16 '16 at 9:04
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    \$\begingroup\$ But if (after the charge) the device is kept connected to the mains and is used, who provides the energy? The mains adapter does as long as it can provide the required power. If it cannot (for example it is a 500 mA adapter and everything on the phone is at maximum power) then the additional needed power comes from the battery. This is the most sensible way to handle this and easy to implement in the electronics so in almost every phone it works this way. \$\endgroup\$ – Bimpelrekkie May 16 '16 at 10:11
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When the battery is full and the charger's on, the charger provides the power, probably the main reason the charger doesn't get hot is that it's not working as had.

For example, my laptop charger is 65W and my battery monitoring tools tell me that it usually pumps up to 35W directly into the battery, at 35+ watts, the charger gets warm. However, the laptop it's self needs only 6W when running normally, the charger cools down as it doesn't need to work anywhere near as hard to run just the laptop. The power the laptop is drawing is coming directly from the charger and is bypassing the battery altogether.

As a side note, the life cycle of any rechargeable battery (not just Lithium) is heavily dependent on how much it is discharged each cycle. A battery that has a 1000 cycle life at 80% DoD (depth of discharge) may last for 2000 cycles at 50% DoD or tens of thousands at a few % DoD, the more the battery is discharged, the shorter the life. It's not a hard and fast rule though, just because it says it has a 1000 cycle life doesn't mean it'll die at 1001 cycles. It's a gradual loss right from day 1, e.g. losing 0.01% per cycle would leave you with ~90% of your original capacity after 1000 cycles (0.9999^1000 = 0.904 or ~90%)

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  • \$\begingroup\$ The lifetime depends more on how high it is charged rather than how low it is discharged, because the worst degradation occurs at high voltages (and high temps), which accelerate internal parasitic reactions. \$\endgroup\$ – Bill Dubuque May 16 '16 at 15:38
  • \$\begingroup\$ If the figure of merit is number of cycles, then shallow discharge is better. But if the figure of merit is operating hours, then somewhat deeper discharges are advantageous (because you get more total operating hours that way). In other words, 1000 cycles at 20% depth of discharge gives you the same number of operating hours as 500 cycles at 40% (approximately). So there is an optimal depth of discharge, somewhere. \$\endgroup\$ – mkeith May 16 '16 at 21:10

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