will battery casings increase the lifespan (not standby/active use) of phone's built-in battery?

sunhux
sunhux used Ask the Experts™
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As batteries are built into mobile phones nowadays & to replace them
requires it to be brought to the workshop who has the tools to change
them,  I'm thinking of getting battery casings so as to reduce the
number of charge+discharge (that is what typically reduce the lifespan
of batteries.

Specifically I'm looking at Galaxy note 9 ie batteries like those below:
https://www.amazon.com/dp/B07GB5R27M/ref=psdc_7073958011_t1_B07H3256JB
https://www.amazon.com/gp/product/B07H3256JB/ref=ox_sc_act_title_1?smid=A2QAZ8RQ4KWQ44&psc=1
https://www.amazon.com/dp/B071NWVT9M/ref=psdc_7073958011_t2_B07H3256JB 

Q1:
Am I right or mistaken?  With above battery cases, will the phone  charge
directly from the casings' batteries rather than charging from the built-in
batteries?

Q2:
Or it'll shorten the built-in battery's lifespan instead as the casings will
charge the built-in battery whenever the built-in battery goes down by
1-2%, thus maximizing the number of charge+discharge cycles?

Q3:
Any other ways to increase life span of built-in batteries?  Understand
we should let it discharge till almost 1-2% before charging it up again
& we should always charge it to the max of 100%: this is a known
good practice but often this is rather inconvenient & risk discharging
completely, resulting in phone being powered off, missing crucial
calls/messages.

What I do know is if laptops are frequently being left connected to
AC or even an external powerbank (Hyperjuice, Vinsic Warrior with
AC output ports) sources, it certainly reduces the number of charge
+discharge cycles (as measured by BatteryInfoView software) &
increases the laptop's life span.
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Q1: "With above battery cases, will the phone  charge
directly from the casings' batteries rather than charging from the built-in
batteries?"
If you're really talking about "charging", the internal battery in your phone won't know the difference between charging from an external battery or from an AC adapter.

Q2: The external battery will charge the phone the same as an AC adapter.  It doesn't wait until the internal battery gets low.

Q3: "this is a known good practice": actually, I think it is much more complicated than that.  Some of the "rules" about rechargeable batteries are based on a old study, very specific to one set of NiCads.  

"What I do know is if laptops are frequently being left connected to
AC or even an external powerbank...."  The batteries in the external case to which you've provided links are basically the same as the external powerbank.

Author

Commented:
Alternatively, question is if we leave the phone plugged into AC (or
external power supply) constantly when it's at 100%, will the built-in
battery still come down or it'll remain at 100%?

I've noticed at 100%, the charging stops.

Author

Commented:
Guess the answer is probably found below:
https://sea.pcmag.com/smartphones/19135/charging-your-phone-overnight-battery-myths-debunked

extract from above link:
"If you leave the smartphone plugged in overnight, it's going to use a bit of energy constantly trickling new juice to the battery every time it falls to 99 percent. That is eating into your phone's lifespan (see below)."

Is there any app that will stop the charging & only kick in the battery
charging when it hits about 20-30% & then stop charging when it
hits 80%?

Author

Commented:
or it makes more sense to
get those casings when
built-in battery cant hold
charge well anymore?
Buy a new battery when the old one dies down.  Lithium Batteries already last far longer and has more capacity for its size and weight than NiCad and NiMH batteries.  You used to have to swap them out far sooner than you do now.  Because they last so long, people are just forgetting that you should be able to swap out the batteries, partly because some shit companies are not making them user replaceable just to make them a millimeter thinner.  
https://www.youtube.com/watch?v=DELZttuW1hw

Those external batteries are cheaper and much easier than replacing the battery.  I wouldn't buy it until the battery charge is down to half, or down to what you need to keep the phone running all day.  Keep your phone lighter weight until you need that external casing.  However, if you're down below 20% battery capacity, you should swap out the internal battery so that they don't start bulging out from the constant charging.

Also, that battery article is incomplete.  Do you periodically, at least once per week, discharge your phone down to 40%?  If you do, you should not have to worry about the trickle charge when keeping it plugged in all night.  It does no appreciable damage to the life of your battery.  It's mainly the charge/discharge cycle that affects the battery life and keeping it plugged in reduces the life by an insignificant amount compared to that, unless you never discharge your battery.  Lithium batteries have no memory effect.
https://housetechlab.com/nicad-vs-lithium-ion-battery-which-is-better-for-cordless-tools/

Lithium batteries are also never overcharged, because the battery circuit prevents it.  Unlike NiCads they can't handle overcharge, so they have protection circuits.  Lithium batteries for power tools are only ever charged to fast charge "80%" capacity (What they mark as 100%) because they charge in an hour and are expected to "fully" discharge each use.  All early lithium batteries can fast charge to 80% in one hour, then trickle charge the remaining portion in another hour.  However, new tech has allowed faster charging up to the ~95% capacity now.  All that research into fast charging Tesla's car batteries paid off, and is trickling down to all consumer batteries.
https://www.powerstream.com/LLLF.htm

If your battery is running low on charge, then replace it or buy the external pack,  You can also attempt to recondition them and extend their life a little, like you would with old NiCads.  I don't recommend it unless you understand electricity and batteries well enough to do it safely.
http://www.batteryreconditioninglab.com/how-to-recondition-lithium-ion-batteries/
https://mods-n-hacks.gadgethacks.com/how-to/fix-dead-lithium-ion-batteries-wont-hold-charge-anymore-0147197/

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