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I found the charge in my smartphone (Samsung Galaxy s2, if that matters) was insufficient to carry me through a day of constant usage, so I bought myself an external battery case (Lithium polymer, rated 2200mAh, 5V output, 750mAh output current). This case has a button on the back, such that until you push the button, it's sitting dormant; after you push it, it starts charging the phone.

I have no scientific evidence, just an "anecdotal" sense that when I start the day with the battery 100% full, and switch on the battery immediately, so that it sustains my battery at 100% until the external battery has fully discharged, it doesn't give me as long a lifespan as if I wait until the battery is down to about 40% and only then switch it on.

Is that correct? If so, what happens to the charge in the external battery while the phone is at 100%? Does it "leak" somewhere?

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    \$\begingroup\$ Does the Uncommented Downvote Ninja also operate on this site? Sheesh, if you're going to downvote a question, at least say why... \$\endgroup\$ – Shaul says I Support Monica Feb 13 '14 at 15:48
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Downvoter may be reflecting that this question is OT for the site.

What happens is complex and would need analysis for specific cases, but I'd expect to get somewhat more capacity by discharging to 40% (or less) and then recharging rather than floating at full charge. Because:

LiIon battery charges at CC (constant current) max allowed current to about 70%-80% of charge then enters a CV - constant voltage decreasing current mode.

When operating in CC at highish load the boost converter is probably more efficient than in CV mode at lower load.

In top-up from start mode the converter will be being cycled on and off (probably) and will be 0% efficient when standing by (but at low load.

As well as the converter, the battery itself charges more efficiently at lower % capacity levels. A LiIon battery is close to 100% CURRENT efficient when charging but in the top 0% or so voltage rises faster than charge % as cell impedance rises and energy efficiency drops.

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  • \$\begingroup\$ +1 Thanks! Why is the question OT? Isn't this exactly an electronics question? \$\endgroup\$ – Shaul says I Support Monica Feb 13 '14 at 16:23
  • \$\begingroup\$ A subset of the active users want to limit it to electronics design, specifically excluding "how does this consumer product work" questions. \$\endgroup\$ – pjc50 Feb 13 '14 at 17:16

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