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I was curious about this. I read that you should generally avoid completely discharging and then full charging your Li-ion battery because this can strain the battery (though some suggested doing this every so often for calibration).

When your charging your phone via a PCs USB port it will take longer to charge because the USB port does not supply as much power as a wall charger.

Because of the fact that it will take your battery longer to charge, does it strain the battery more to charge when plugged into a PC?

Say for example my battery is almost completely discharged (like 10% life left) and I want to charge it to 100%. Plugging it into a wall charger vs a PC- what will be the impact on the health of the batter?

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    No; It does not; The USB port can provide as much as the wall charger depending on its physical configuration. Charging the battery is charging the battery. – Ramhound Jan 14 '15 at 18:57
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The issue of loss of capacity with battery charging and discharging is due to crystallization of pure concentrated chemicals inside the battery. Once crystals start to form they do not want to revert back to a liquid state again, and the battery permanently loses some charge/discharge capacity.

This is why sometimes old lead-acid batteries bulge outward when they are failing... huge lead crystals have formed which apply pressure to the lead plates and case and bend them. Growing crystals can also form electrical shorts between plates and ruin the individual cells.

The goal of not discharging too far is to try to stay out of the area where crystallization becomes much easier due to high concentrations of the chemicals in the deeply discharged state.

Lead acid cells should ideally never be allowed to stay discharged very long, and rapid recharge is recommended. Though once charged they need to trickle charge to prevent damage from overcharging.

Lithium cells are generally more tolerant of full charge and discharge cycles. The construction of the cell and the surface features of the plates can help prevent crystal formation. Slow charging generally should not affect the life of lithium cells as much as it can with lead-acid cells.

It's often said that lithium cells have a fixed life of about five years, though my own experience does not bear this out. We have about 15 IBM Thinkpads with original lithium battery packs from a decade ago and they all still hold at least 2 hrs of charge.

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There are all sorts of conditions than can affect this, but as a general rule of thumb, the faster you charge/discharge a battery, the more you will 'strain' it (lowering lifetime).

Again, not always true, but as a general rule of thumb your wall charger will deliver more current than your PCs USB port, charging your battery faster.

Therefore, the computer will typically put less strain on the battery (by taking longer to charge it).

Honestly though, this would be a small effect and you'd be unlikely to notice a difference. For lifetime, there are much more important things to consider like temperature and limiting how often/much you need to re-charge your battery.

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Wall charger has adapter power of 25w compared to the usb charger where only maximum 10W available so wall charger is faster 2.5 times than USB

Another fact is that Charging via USB decrease the life of battery because it generate more amount of heat which would degrade the battery life time.

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    Why does the USB charging generation MORE heat if it outputs less watts? – user1028270 Jan 14 '15 at 20:51
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    Uhh... your numbers are wrong. USB 2.0 only specifies up to 500 mA (2.5 W) from a standard USB port, not 10 W. Also, a 25 W charger via USB would equate to 5 A current - you'd need special cables to carry that much current. A typical mains-powered phone charger tends to put out 2 A max (10 W). I do not know of any current phone that can draw a higher current than that. – Bob Jan 15 '15 at 2:21
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It's true that the USB port may deliver less power than a phone charger, or not support a specific protocol required by the phone to charge at full speed. This is possible when using USB from a desktop and almost certain if using USB from a laptop.

Generally speaking, you can "power charge" batteries, to charge them faster, but it will hurt their lifetime. The electronics in the phone and the protocol in USB make it impossible to accidentially "power charge" a phone, so the only thing that does happen is that they will charge slower than usual. Charging batteries somewhat slower (2x, 4x) will not hurt them.

However, because of the slower charge speed it's now possible to use battery faster than you are charging it, if you use the phone heavily while charging - that wouldn't be good for the lifetime of the battery, but it also wouldn't kill the battery outright.

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