There are three causes for this. Lithium ions settling into final position (biggest effect on voltage). The internal resistance of the battery (tiny effect on voltage). Battery self-discharge (affects voltage over time). Lithium Ions Settling As a battery charges the lithium ions move from the positive part of the battery (the cathode) to the negative part (the anode) where they are stored in tiny bits of carbon (graphite or graphene) until the battery is discharged later. When the battery is discharged the ions move back into the positive part of the battery. At high charging rates and as the battery gets near fully charged it gets harder and harder for the ions to find places to settle in the anode’s carbon material. This literally causes the ions to stack up in a denser concentration than what’s in other parts of the battery. This difference in the concentration of the ions in different parts of the battery causes a voltage difference that we see as an increased voltage at the battery terminals. After the charging stops those stacked up ions slowly find places in the anode’s carbon and the distribution of ions across the battery evens out. This gets rid of the extra voltage increase and the battery drops to its true voltage, it’s “resting voltage”. It gets most of the way there in a few minutes but it can take hours, or even longer, for the voltage to fully settle. The size of the drop to the battery’s resting voltage depends on the charge current level, the condition of the battery, the battery’s internal resistance, and the battery’s construction. The drop could be just from 4.20V to about 4.18V or it could be down to 4.15V or lower. For healthy cells not being fast charged I would expect the battery to not drop below 4.15V within 24 hours after being fully charged to 4.2V. If the battery does drop below this then try a slower charge current setting. If the battery still drops a lot in voltage then it could be damaged. Recycle and replace the battery. Battery Internal Resistance The same internal resistance (IR) inside a battery that causes voltage “sag” (drop) when it is being discharged causes a voltage rise when the battery is being charged. If we are charging at 1A then the charger will usually stop after reaching 4.20V and the current has dropped to 1/10th of that, 0.1A. If the battery has an IR of 0.02 ohms (typical for an average 18650) then Ohm’s Law says there will be a Voltage Rise = (Charging Current) * (IR) = 0.1A * 0.02 ohms = 0.002V. So as soon as the charging stops there will be a 0.002V drop in the battery voltage. That is a TINY voltage drop. But if the charger stops at a higher current level, or if the battery has a higher internal resistance, then this voltage drop can be larger. If we take a battery out of the charger early, before the 1A charging current starts to drop down once the battery reaches 4.20V, then there can be a bigger immediate voltage drop. Using our above example at 1A the Voltage Rise = 1A * 0.02 ohms = 0.02V. So a battery taken out when the display says 4.10V would instantly drop to 4.08V. Something to always remember is that the accuracy of the voltage displays in some chargers and devices can be pretty bad (except those with DNA or Dicodes boards). Never trust them completely until you have tested their accuracy with a decent multimeter. Battery Self-Discharge All batteries slowly lose their charge over time due to very slow internal “leakage”. The materials in a battery are not perfect insulators so a tiny amount of charge flows from the negative part of the battery (where the lithium ions are stored when the battery is charged) to the positive part of the battery (where the lithium ions are stored when the battery is empty) even when the battery is not being used. This leakage is very slow in a new battery. It’s the equivalent of a few dozen micro-amps (millionths of an amp) or less but it is going on continuously. It goes up though as the battery ages or if it gets damaged from very hard use. This self-discharge will have very little effect on a battery’s voltage unless the battery has been off the charger for a while. But it can eventually have a small effect. Badly damaged batteries can self-discharge very quickly though, in days or less, and should be replaced. Never use, charge, or try to recover a Li-Ion battery that has dropped below 2.5V. This internal leakage Is very different from what happens when using a device that still draws current from the batteries even though the device is in “sleep” mode (3 or 5 button clicks). Some devices use regulator circuits that drain the batteries a lot faster in sleep mode than others. That has nothing to do with the battery though. You might be tempted to “top off” a battery that has dropped down a little in voltage after charging is done. The next day, for example. At best that will only add a fraction of a second of vaping time to the battery and just isn’t worth it. So while a large voltage drop after charging is done is not good, down to below 4.15V or so, it’s perfectly normal for a battery to not stay at 4.20V once charging is done.