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My smartphone is serving as internet hot-spot over WiFi for my home/office laptop. I’m keeping phone always plugged in charge. Most of the day I’m home and using a lot of traffic. What is the best strategy to prolong Li-ion phone battery life?

  • Keep always connected?
  • Full discharge/charge cycles?
  • Try to keep its state of charge in some range of ⅓ or ⅔?
  • Use fast or slow charge?
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Lithium-ion batteries work the best between 40 and 80 percent of charge. Correspondingly, you can use some apps which notify you when the charge level goes below (say) 35% while discharging or above (say) 85% while charging.

You should definitely not go for full charges and discharges.

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The best solution would be to charge the battery to around 50% or a little more, and take it out of the phone for storage in a cool place. If the phone supports running without a battery in it.

The next best I guess would be to try to keep its state of charge in some range of ⅓ to ⅔, using slow charge.

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To get the longest life span from a Li_Ion based battery you should:

  • charge it to a charge level of between 40% to 60 %, this makes the internal stress in the battery the lowest.
  • Not use the battery, so not charge or discharge the battery at all. Any charging/discharging causes wear.

The 2nd point will be difficult as most phones will simply charge the battery to 100% when the phone is connected to a power source. On a rooted phone there might be something that can be done to change this but that might depend on the phone model.

Most phones do not work and or start up without a battery so removing the battery (when it is at around 50% charge level) might not work for you.

If there is no "hack" to make the battery stay at around 50% charge level then I suggest just keeping the phone plugged in all the time so there is no charging or discharging.

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  • \$\begingroup\$ So from a casual consumer point of view, that means that best practice is to unplug phone unless it will discharges till 40% and then to connect it back until it will charge up to 60%? \$\endgroup\$ – misanthrope May 16 '18 at 9:31
  • \$\begingroup\$ Yes that is correct. And I'm not the only one saying this Linus did so as well: youtube.com/watch?v=AF2O4l1JprI recently. But it is a hassle to keep the charge between 40 and 60 % so I guess almost no one will do this. \$\endgroup\$ – Bimpelrekkie May 16 '18 at 13:08
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This seems to be a rhetorical question because normally you don't have any control over last three points. But the full charge-discharge cycle would be the worst strategy.

Practically every smartphone has an intelligent charger that follows the standard charging procedure for Li-Ion batteries: first it applies constant current until the voltage gets up to float voltage (4.20 V for regular Li-Ion, and 4.35 for Li-Po cells), so-called constant voltage stage. Then the charger sits at this voltage level while the battery sucks the full charging amperage for a while. Then the battery (by itself) begins to reduce its current. At this point the battery recharges about 80% of its rated capacity. The dropping current indicates the final charging stage. Chargers usually terminate (disconnect) when the current drops to 1/10 of the rated charging current.

If battery is not loaded, the charger would wait until the terminal voltage drops by 100 or 200 mV (by itself, due to self discharge or some sleep-mode consumption), and will kick off a new charging cycle. But since the battery is already charged to the top, the voltage will rapidly rise to 4.2V level, and current will be already low, near the termination threshold, so the cycle will be fairly short, few seconds maybe. Then the cycle repeats. It is sometimes called "trickle charging". Modern charger ICs do this autonomously, without SOC intervention.

For the good lifetime for a battery it would be better to terminate the charge right at the beginning of the current drop, or at something like 20% drop. It will save the battery from overcharging electro-chemical effects and prolong its life. Theoretically a SoC can adjust charger parameters, but it would be a challenge to find and alter the kernel code for this.

The fast or slow charge also can be controlled via computer interface, at least in most Texas Instruments ICs. So if you have an access to charger configuration, slow charge will also help to maintain good SOH - State of Health of the battery.

In short, keep it always connected, don't do deep discharge cycles, reduce charge current if you can, and rise the termination threshold to maximum if you can, and you will be fine.

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  • \$\begingroup\$ I’m using Samsung S9+ and found that in Android settings I can to turn off fast charge feature. But isn’t trickle charge harmful for the Li-ion batteries? Should I trust smartphone’s battery charging controller (and keep it always on) or unplug phone unless it discharges to some point then plug it back? \$\endgroup\$ – misanthrope May 16 '18 at 9:22

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