On a Smartphone one day it is used constantly, on another day it is rarely used and on some days it is kept off unused, what tips to preserve the battery and increase its recharge cycles?
This is not a problem. There is nothing to do. Li-Ion have low self discharge so it will be find going unused, though if you really don't "want" to use it then you could power it off if you can do without immediate notification of anything happening for people trying to contact you, reducing the battery drain so taking far more time till a recharge, so more recharge cycles left in the battery.
Keep in mind that a partial recharge cycle is fine, does not count as detrimental to battery life as a whole charge cycle would. For example if the battery were rated at 1000 recharge cycles and you always recharge it at 30% drained, you would get significantly more than 1000 cycles doing this, the amount depending on the total capacity used vs recharged.
i have a Doogee N20 and when his battery dies it will be disposable because it is difficult to find the same battery for him
I'm not going to go look for this battery but the odds are that any semi-popular phone has some aftermarket battery support.
OKAY I lied, I looked and there are sellers of this battery so I don't see the problem., expect that it's a built-in battery using a ribbon cable so you'd need decent quality *small* tools to extract the old battery and insert the new one... or pay someone to do it.
Whenever I charge the smartphone for 50%? use it at 50% even in standby or off or in constant use
It is not a problem. The battery has protection circuitry. If it is more convenient to you to keep using it then do so before charging it. If it is more convenient to you to stop using it and recharge to a higher charge level so you will have more charge while you are out and about using it without a chance to recharge it then, do that instead.
It is not some problem like you're trying to make it out to be. Recharge it based on whether how much charge it has left will suffice until your next chance to recharge it and when you find that capacity drops due to the battery aging, then replace the battery.
Why are you asking this here? This is a world wide issue with everyone having mobile phones, not a new topic that needs especially electronically oriented people to solve. Any local phone dealer or repair shop can tell you this.
Granted, there are minor differences in when you recharge, how you store the phone, etc. It is more trivia than anything, not so useful information for a product you need to use rather than one you need to save because you're on a deep space mission with no chance to recharge it or replace the battery and/or phone. There is not a big difference to be gained in battery life relative to the lifespan vs cost of a different battery.
