as an engineer, thats been building batteries basically my whole life, i started building them back when i was a kid, back when we still used ni-cads (nickel-cadmium). i feel like i really want to nerd out here, but ill try and keep it as simple and to the point as i can.
over the decades, between all the hundreds, possibly thousands of batteries ive built that are lithium based. for RC planes, cars, helis, ebikes, power tools, and recently with energy storage, like the batteries i built for solar storage, and even portable power systems. I keep very accurate notes on most my batteries.
lets first define what "good means". in the context that ive seen , "good" means you can still charge to 80% capacity, and still be able to pull 50% of the rated current. 50% of the current is very important because if you have a high load and the voltage drops too low, even if you still have 95% of ur capacity the battery can still trip its BMS and crap out. however generally speaking, the 80% capacity rule usually is followed closely by 50% current.... usually.
next, lets define what a charge cycles is, 1 CS is ,zero to 100 percent discharge and charge. so if u ride from 100% to 50% then charge back up to 100% u have only done a 1/2 a CS.in my experience, 500 used to be the standard AVERAGE, however as of late ( last 5-10 years), ive seen this average go much higher, 1000-2000 charge cycles. in fact with proper battery care, you can go well above 2000CS's.
from what ive seen, the two factors that will determine how long ur battery lasts is heat, and discharge state. Heat, specifically internal cell temperature can absolutely murder a battery in even as little as ONE charge cycles. for example, if you have a battery that is only able to output 30A continuous, but you use it on a 45A load and after 30 mins, the battery internal temperature is like 90C+. that battery has been permanently damaged, and after a load like that i would predict it would only hold 50% of its charge after maybe 10 CS's.
discharge state is much harder to quantify, but basically, leaving a battery below 10% state of charge for prolonged periods will permanently damage the cells. in fact, leaving a battery at 0% ( roughly 2.2v-2.75v per cell) for say a month, will basically permanently kill the battery. the battery can be forcefully " revived" but will get 1/2 of the life out of it moving forward.
as for me, ive killed batteries in the past in 1 CS........ ive also had my galaxy s4-neo cell phone for 12 years now and im still getting 1-2 days of life out of it because ive taken such good care of the battery. one of my ebikes which ive had for 7 years now has over 60,000km's on it and ive charged it approximately 1800 times and the battery still charges 99% of its original capacity. meanwhile my wife which does not follow such a strict battery care resgime, has ridden half the distance i have and but still has charged the battery about the same amount has approx 95% of her battery capacity left.
ive told many friends and family, if you want a battery to last, any battery, do these few easy things:
most important:
-never let the pack internal cell temperature get above 60C (aprox 50C external temp)
-avoid hard discharged for prolonged periods ( ties into the above point)
-never go below 20%, but if you do, make sure to charge it ASAP.
Less important but still makes a difference:
-avoid hard discharge below 0c
-do not charge past 90% state of charge
-do not leave a battery fully charged for months at a time ( 3.6-3.8v is ideal for storage)
-the slower you charge the longer it lasts
-differential heating/cooling, as in if one side of the pack gets hotter from say a heat source, try to keep the pack the same temp throughout.
hope this helps.
cheers.