I bought an Oasis in May of 2016, and had an unfortunate warranty replacement four months later when I dropped it in the road and a car ran over it. The refurbished replacement has now been running for over two and a half years with heavy use every single day and God knows how many partial charges from 40-60% or thereabouts, and its warranty is running out: the internal battery seemed to be growing much worse (and *drastically* worse after the update to 5.10.2), with charging case charges showing percentages going up 4% at a time rather than 1% as in the past, so I thought it wise to do a deep discharge to see what the battery life actually was, whether I'd really lost 75% of battery capacity, and whether it was worth claiming for a battery replacement. The deep discharge consisted of simply eschewing my usual habit of throwing it into the case when not in use, and reading a lot, with wifi off and the light at about 12%.
It turns out that the battery circuit was disastrously mistrained, probably due to slow drift across the years: after a fresh wall charge it sat at 100% for nearly two hours, plummeted at about 1% a minute until it hit 40%, then dropped to 5% and screamed that the Kindle was about to shut down. Only three hours of life from the internal battery seemed rather poor!
But then I left it on and kept reading. The battery power rose to 10%, then 25%, then fell rapidly back to 5% again, popping up another warning. It kept yo-yoing between about 2% and 30% like that for nearly *six hours*, going up as high as 40% on two occasions: it's quite possible that shaking up the battery by swapping the Kindle from hand to hand was helping with this madness, but I really don't know. It spent more than two hours at the end below "5%" and an hour at 0%, popping up briefly to 20% and then straight back down again. This provides further confirmation that the actual range of this battery is something like -20% to 120%
In the end, this little tiny internal battery lasted eight hours and 47 minutes from a fresh charge. When I bought it, it lasted about nine and a half, so this is really insignificant levels of degradation over time: Amazon advertise six. (There must be *some* capacity loss from oxidation raising internal resistance, but honestly I thought there was a lot more than this of that alone. The loss from plating must be very minor, probably because deep discharges hardly ever happen.)
So, counter to various claims on this forum over the years, I don't think we need to worry at all about capacity loss of the Oasis battery from repeated charging. The charging circuit and software, whatever their division of labour and despite their strange proclivities for sometimes charging when you plug it into the case and sometimes not, seem to do a terribly good job of preserving battery lifespan. It's more likely you'll smash the thing through clumsiness or the damn thing dropping out of its case than that it'll end up a paperweight through battery degradation.
I suspect the update to 5.10.2 simply did a massive pile of internal housekeeping and drained the battery really fast, though the mistraining was extreme before then as well. 5.10.2 clearly also adjusts the mechanism used for reporting charge levels, since in the past the charge level was always simply 'the level of the case if plugged in, otherwise the internal battery', leading to the bizarre sight of plugging the thing into the case and watching its charge level plunge as you gave it access to much more power. Whatever mechanism it uses now is more arcane, and more counterintuitive, but makes more sense: an empty internal battery and a full charging case is not reported as 100%, but 80% or 84% or something like that (probably based on an estimate of battery capacities derived from the charging circuit, and taking both internal and case batteries into account at long last).