What? Keep a battery at 0 is the fastest way to kill it, I forgot to charge an ipad once and when I tried again in 3 month, it's dead and would not charge. I also read in that case do not even try other way of charge the battery again, it's not safe anymore.
What are you confused about? I described battery longevity and how to maximize it...
If you have a phone with a battery that would last 500 charges of 0 to 100%
If you were to charge that phone to only 85% each time, you would then get >1,500 charges out of it (1,667 to be more precise).
Imagine that battery is 1000mAh in capacity. 100% would give you 1000mA for 1 hour. If you multiply that 1000mAh by the 500 charges you would get, the battery would store 500,000mAh over its lifetime being charged to 100%
If you were to charge that battery to only 85% each time and got 1,667 cycles, you would multiply 850mAh by the 1,667 cycles, and you'd get 1,416,950mAh
1,416,950 / 500,000 = 2.83
By charging to 85%, the battery will be able to store 2.83 times the energy over its lifespan, compared to being charged to 100%
Pretty great news, eh?`
Eh???!
It is! And it gets better! If you charge to only 75% (...continuing from the example above), which is 750mAh, the battery would be able to store 2,499,975 mAh over its lifetime!
This is because the damage (wear) done to a lithium battery when charged to 75%, is 0.15 that of a 100% charge.
1 / 0.15 * 500,000 gives the large number above. 1 / 0.15 can also be multiplied by the number of charges the battery is capable of when being charged to 100% (500) to give how many times it can be charged to only 75%.
The answer is 3,333 times.
Even though 3,333 / 500 is 6.67 times more charge cycles, because the capacity of each of those cycles is smaller, (75%), you only get ~ 5 times the capacity.
This is hardly a bad thing though... For charging 33% more often, you get a battery that will last you FIVE FLIPPIN TIMES LONGER!
And it doesn't stop there!!!
Lowering to 65% gives you about the same amount
again!
But past that point, the wear is about the same - basically, the best target to charge devices to is the low 60% range. Unless you need more continuous runtime for some reason...
Here's the math for 65%
1 / 0.07 * 500,000 = 7,142,857mAh is the total amount of power you would store in the battery over its lifespan by charging it to only 65% capacity.
65%: 7,142,857mAh in 10,989 cycles
75%: 4,999,975mAh in 6,667 cycles
85%: 1,416,950mAh in 1,667 cycles
100%: 500,000mAh in 500 cycles
Charging to only 65% gives you 7,142,857 / 500,000, or 14.29 times more lifetime energy storage. 10,989 cycles / 500 cycles is 21.98 times more cycles.
21.98 / 14.29 = charging 1.54 times more often for 14.29 times more energy storage...
Charge to 65%!!!!!
About your comment:
Keeping a sealed lead acid (SLA) battery empty is the fastest way to kill it. SLA batteries are car batteries, UPS batteries, boat batteries, solar storage batteries, tractor batteries etc. etc.
Pretty much all other batteries (including lithium ion), do not suffer for being left partially charged. Or even empty...