Question on lifespan of Li-ion batteries

In general, Li-ion batteries in different chemistries are rated for certain lifespan, mostly express in number of cycles.

How do you define cycles --specifically as charge/discharge cycles ?

My question defaults to Li-ion (or Li-polymer?) cells that we are using for our lights/torches, but it is also relevant for our mobile phones and other (small) electronic devices that uses this type of batteries.

Is battery life affected on the frequency or the number of times the cell is pushed to full charge? or to full-discharge?

Most CPF'er have more lights that ordinary Joe. I do have lights that are frequently carried (EDC) but just seldom use; and some worklights that are used-better. I usually top them off after just a few or some use. Does this count as a cycle? -- maybe having a 3.8/3.9v as I load it into the charger?

In similar manner, I usually charge my mobile phone at the middle of the day, even while still having 70% of capacity; as I don't really know that I might use the device down into the evening without an access to a source or powerbank. Would it matter if I don't push it to full capacity and just pull the charger when it hits 97-98%?

For laptops -- the power management system has option that gives you 'prolonged battery life' mode and it does only allow 80-ish percent charging.

Any thoughts?

A cycle is essentially using up the full capacity of the battery one time. If a battery has 3000mah capacity and you charge it once for 1000mah, a second time for 1500mah and a third time for 500mah that combined is one cycle.
As for battery life they do have a limited number of cycles but the final 10-20% of full charge heats a battery up more than first 80% and discharging a battery to zero 8s also harder on it than down to 20-40%. I pull my phone off the charger when it hits into the mid 80 percent when I have the time to charge it that far as I often charge it in the mornings before work and at times only get it to 60 percent or so.
I don't recommend charging it till it drops into the 40 percent range unless you know that you will be stuck without power to charge it and will use it heavily. I try not to let my phone get below 30% charge and am usually find to stop charging it above 65% if I'm on a normal days schedule I can usually go 2-4 days on a 100% charge and on an 80% or so charge I can go 2-3 days. I don't like to start a day away from home less than 50% charge even though I have a car charger often I'm in and out of my car a lot and don't get much of a charge on it unless I leave it there and risk someone breaking into it and getting my phone.
There are chargers that don't charge batteries past a certain voltage as it reduces life. Many only charge them to 4v or 4.1v to increase the battery life.
Topping off counts as a partial cycle. If you top a light off by adding 10% it takes about 10% of a full cycle. I don't typically top off batteries unless I know I'm going to use them heavily and away from power as topping them off reduces more of a cycle than it gives you that final last 10-20% of charge is where the battery heats up the most usually.
I tell people using tool batteries it is ok to not totally deplete batteries before recharging them. When I work I often use my tools heavily never knowing for sure when that will happen if I use up a full battery and drop another one a bar or so I recharge them both fully so I can depend on what power I have available. I would rather top off a battery than have to get out a charger at work and find a place to plug it in and go off and work 100 feet away unable to make sure my battery and charger don't vanish for good as on occasion we have people stealing stuff.

Testing for cycling longevity requires so much standardization within just one test protocol, the results are nothing to do with real world usage.

Also, cannot compare data from one maker to the next, different protocols, relative influence between engineering and marketing "optimism".

That said, there are a half-dozen usage/care factors that can cause huge differences, so the same battery in one use case, can go 1000 - 1200 cycles before hitting EoL at 80% capacity

but only get 120-150 cycles in another use case.

C-rate discharge is a huge factor.

Charging rate less so, just because very rarely is fast charging required.

Temperatures nake a huge difference to both, even over 3C charging might be fine in hot weather, while 0.5C murders cell when approaching freezing.

Going past 4.05V charging reduces lifespan, going below 3.2-3.3V discharging also - lower the avg DoD% the better, a 10% difference can double cycles lifetime. Inter-related with C-rates though.

IOW "it's complicated".

On top of that, the lower density lower nominal voltage LI chemistries get vastly longer lifespans.

I know many well coddled LFP banks many deep cycled daily that have not hit even 95% SoH yet at well over 5000 cycles, and LTO is supposed to go well over double that, even at 100% DoD and 5C charge rates!

The nominal 3.6-3.7V chemistries would be hard pressed to hit more than a few hundred cycles in hard-use conditions, getting over 1000 requires conscientious coddling

impossible in a propulsion context, especially model racing, most especially planes.

The bigger the Ah capacity the lower the C-rate so if weight and bulk are not an issue, the packs can last much longer.

john61ct said:

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Going past 4.05V charging reduces lifespan, going below 3.2-3.3V discharging also - lower the avg DoD% the better, a 10% difference can double cycles lifetime. Inter-related with C-rates though.

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While battery life and charging management option exists on Laptops, it would be a good option that it is also made standard on mobile phones. There might be some app on Android, but I have yet to install such. Stand-alone cell(s) chargers should also have this option to prolong battery lifespan.

Li-Ion battery cost might have relatively stabilized, but (air) shipping them have become a challenge for most suppliers / users.

Android Accubattery has great educational texts built in, alarm to stop charging at 80-90%

Battery Notifier BT is good for stopping you from going too low.

Some phones can be especially hard on their batteries, to inflate their run-time claims. For example, they might charge to 4.3v (and do it fast). That gives another 10% in run-time, but only for the first few months you have your phone. After that, the hard charging starts eating away at the battery capacity. You'd be far better off to only charge to 4.1v, and do it at a modest rate. After a few months, you'd actually be seeing longer run-times compared to the abused phone.

Cycle life as a specification is a bit like clock speed being advertised for a laptop on the store shelf. It's a way to simplistically distill a complicated performance characterization into a simple one dimensional number. The reality is that life is a function of temperature, rate, depth of discharge and calendar life. For cells other than cylindrical, compression also matters.

most advertisements of cycle life assume either 100% depth of discharge or something slightly less to bump up the numbers, at room temperature, with a very low rate a.k.a. C/10. Unfortunately the specifics are often not advertised.

in the industry, cycle life is often a multi dimensional function, and often we think of lifetime Wh-throughout under different cycling conditions as the better way to represent the overall capability of a cell.

it's also important to unentangle calendar aging versus cycle aging. The former is not a major factor until the cell starts seeing ~>45C continuous exposure. At that point, calendar aging starts to increase rapidly. From an engineering validation perspective, hotbox testing of cells is often the only practical way to get aging data without having to wait months or years for it.