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Old 08-06-18, 08:31 PM
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From a physics point of view, starting to charge it from the beginning should be better.

If the battery is still fully charged (usually with the battery voltage at 4.15-4.20 V) then the charger circuit will remain idle while an external power source is plugged in. The unit will be drawing power for its current needs from the USB input, but essentially no current will be flowing into or out of the battery. There should be no internal heating of the battery, only passive heating from the rest of the circuitry in the device.

While the unit runs on its internal battery, current will be flowing and the battery will heat up. Likewise, when it's being recharged from an external power source, current will be flowing and that will heat it up. This is due to internal resistance.

You can also look at it a different way, with the same outcome: Both charging and discharging are energy conversion processes, which are not 100% efficient. So drawing say 500 mAh from the battery and then putting it back in there may cost you 600 mAh in charge from the external battery. The difference is waste heat that heats the battery and the charging circuit. It's usually more efficient to directly draw 500 mAh from the USB port. So your lipstick battery should actually last longer if you use it from the start to keep the main battery at 100% rather than draining and recharging the main battery.

In practice it also depends on the efficiency of the voltage conversion devices inside the main unit's charge circuit and USB interface as well as the counterparts in the external battery. If you have an external battery that has a slide switch for multiple output currents (e.g. 1A, 1.5A, 2A), a lower setting may be more efficient.
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