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Is it better to let lithium batteries stay plugged in or constant 80/20 cycle

I am doing research on best practices for my lithium batteries and lifepo4 powerstation. There's some conflicting opinions and variation for cycle numbers.

Will leaving my things plugged in at 100% hurt it more than constantly unplugging at 80% and replugging at 20%?

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  • My understanding is that it depends on the device, and most modern devices take care of what's best by themselves

    • That's not true.

      Lithium batteries have a longer useful life if not allowed to drop too low or charged too high. 20% and 80% are typical values. Ideally they would be at 50% SOC and that's why most batteries in new devices will arrive charged to around 3.6-3.8v.

      This creates a problem for device manufacturers because if they force the device to treat the battery well, users won't get as long between charges. They will sometimes give you options (most laptops will have a setting to stop it charging beyond 60% or 80%, some phones will have a setting to stop it charging to full) but they'll advertise the full battery runtime they can squeeze out while damaging the battery and that will be the default setting.

      Convenience dictates that you may need to charge above, or discharge below, the recommended levels. Which would be much less of an issue if batteries were easily replaceable. But increasingly, they're not.

      tldr; manufacturers have zero incentive to make sure their devices treat the batteries well

    • They don't outside of not doing things that cause acute damage to the battery. They can't because "best" is situational.

      If I have a phone with an empty battery and I'm going out all day starting in an hour, best is to charge as fast as possible to 100%. That's the most wear I could put on the battery out of any charge cycle, but going easy on the battery isn't my first priority in that scenario.

      On the other hand, if I have all night to charge and won't be away from charging for more than a few hours the next day, best is to spend most of the night charging to 60% and stop there. It's an order of magnitude less wear than the above, maybe more.

      For best service life, avoid fully discharging[1] the battery, charging it above 60%, storing it long-term charged over 60%, getting it hot, or charging it in less than several hours. In most devices, you don't have the ability to control any of that so the best you can do is plug it in at 20% and unplug it at 60% (or 80% if you need the extra runtime - it's still better than 100%). I'd like to see consumer devices get an "eco mode" or some such to select battery-preserving behavior manually, but that's not in the interests of device manufacturers who want you to buy something new when the battery wears out.

      [1] An actual full discharge to zero volts causes acute damage to Li-ion batteries and most devices won't let you do it

  • Afait reducing the amount of cycles is the best - my reasoning is that every cycle just slightly damages the membrane between anode/cathode.

    Also I have heard that for long storage 80% is the best but it's just something I have heard/read.

    About 10 years ago, the norm was to, from time to time, drain lithium batteries to minimum and so do a full cycle, this is something my father told me but I actually don't know the reasoning.

  • Title says plugged in and body says plugged in at 100%; these can be separate concepts if one has fine control over the charging voltage.

    Will leaving my things plugged in at 100% hurt it more than constantly unplugging at 80% and replugging at 20%?

    Plenty of academic research out there showing that pegging Li to 100% SoC reduces cycle counts to EOL (by electrolyte degradation and other processes), especially at higher voltages/temps. You didn't mention capacity reduction associated with charging at freezing temps so I assume that is a non-issue in your use case.

    It seems to me that if leaving it plugged in is an option you have shore/mains/grid power. So I'd

    • charge to middling SoC and unplug the powerstation (according to the manual); and
    • run the loads off the wall socket

    Am I missing something here?

    offgrid with LiFePO4

    I live offgrid with Li on a very limited budget, so performance and maximal cycle life is a practical matter for me. Based on my own reading and experimentation I charge my 4S LiFePO4 to 13.8v (3.45Vpc) until Absorption falls to 0.10C then quasi-float at 13.31v (3.3275Vpc). I warm them to 50F and charge at ≤0.4C.

  • If this is an Android phone, go install Accubattery and do what that says. It's designed for many different phone batteries and associated tech (e.g. overcharge circuitry).

    There are so many different models and variations in the electrochemistry, general advice is usually a miss.

    If you insist on generalizing lithium tech, keep it between 30-80% charge for good longevity. The extremes of full and empty are rougher on it.

    • If this is an Android phone, go install Accubattery

      Too bad that app has 7 trackers embedded and access to the ad ID :/

      • The permissions look fairly reasonable to me considering it needs to run at startup and monitor other apps, and the Pro upgrade is an embedded option that would need connectivity.

        I just firewall it anyway, but that requires root.

        IIRC you can change your ad ID any time but that's kinda outside the scope here.

  • Modern devices do this themselves, I guess through the EC (embedded controller). Best is to use an official or high quality charger with the exact fitting power.

    Also if you have USB C, not every charger will have PD (power delivery) and recognize what power a device needs. For example I can charge my phone with my Thinkpad charger, but not any random cheap one

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