\$\begingroup\$ Device and battery can often be separated, that's why overdischarge protection circuit on the battery is added as a last resort protection against battery damage, but not to maximize battery lifetime. Just as lithium chargers have to stop at 4.2 V before the battery's overcharge protection will kick in.
Wildly extrapolating, by allowing a starter battery to completely drain, you lose ~20% or ~50% of what is left and not much is left after good 5+ years of service in the first place. Get a new battery and send the old one for recycling with the due honors.
You must limit the maximum charging voltage to 4.2V, or else the battery will explode! When you do this the charge current will naturally taper off (once it drops below 10% of the original charging current you can consider the cell to be fully charged). At normal discharge rates the cell will be almost completely drained when it reaches 3.0V.
Connect the main plug of your LiPo battery to your NiMH charger and start charging at the lowest current possible. Typically this is 0.1A. If your charger allows you to select the voltage you should select a voltage the matches your LiPo battery’s nominal voltage. After a minute or two your battery should recover to about 3.3 volts per cell.
Once the internal lithium-ion battery hits 100% of its capacity, charging stops. Drain a phone battery to zero only when you want to recalibrate the internal sensor that displays your phone's

This is because it is made with the same battery types as mobile phones: Lithium-ion (Li-on) and Lithium-polymer. Of the two, Lithium-ion (Li-on) is the most commonly used. What Exactly Does it Mean to Drain a Power Bank? To drain a power bank means letting it get down to as low as 1 percent to 0 percent before recharging.

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lithium ion battery drain completely