^^^ this. Before I more or less retired it, I had a Luna 17.5ah 52v pack with almost 4000 cycles on it. That came from twice-daily charging (commute and charge at home and office) where I religiously charged to 80%, did not discharge below about 40% and performed monthly 100% balance charges. It was also using Samsung 25R cells which laugh at heavy loads, and since it was a single battery in a 2-battery 2wd bike, it didn't get beat on that hard. The pack is still alive and well. So, if you treat a pack right the 800 cycles you hear about are meaningless. You can get less than 400 too if you treat the pack wrong, or its made by chimps.
Insofar as being able to determine battery wear: Your battery will still charge to its full capacity as it did on Day 1. However, when it starts to wear down you will see that voltage 'flash off' until it drops to a lower level, and that is you actual voltage capacity. I can do a better job of explaining this with an example:
I have a battery that is a 52v battery. aka a '14S' pack. A 14S pack's actual peak voltage is 58.8v (just like a 48v battery is really 54.6v when full). So on this worn battery, I can charge it to 100%: 58.8v. Almost before I get down my driveway (which is about 20 feet) my voltage drops to 56.5v. From 56.5v on down, the pack slowly discharges as I'd expect it to.
To be able to spot this sort of thing, you have to have your screen display set to show you voltage and not percentage or worse.. a battery graphic with bars.
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Whats an 80% charge on a 48v battery? These charts give answers to questions like this on common ebike battery voltages: 36v, 48v, 52v, 60v and 72v.
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