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Mooch Musing: Why does a battery’s voltage drop after charging is done?

Discussion in 'Batteries and Chargers' started by Mooch, Dec 23, 2019.

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  1. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    There are three causes for this.
    1. Lithium ions settling into final position (biggest effect on voltage).
    2. The internal resistance of the battery (tiny effect on voltage).
    3. 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.
     
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  2. puffon

    puffon Vaping Master Verified Member ECF Veteran

    Sep 18, 2014
    Florida
    Great explanation.
    Thanks!
     
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  3. Rossum

    Rossum Surly Curmudgeon Verified Member ECF Veteran

    Supporting member
    Dec 14, 2013
    NE FL
    One thing I've noticed is that brand new cells tend to stay right around 4.20V or at most drop down to 4.19V, but as they age, they drop off more, with 4.15V not being unusual after after a while.

    The one exception I've seen is my "never-say-die" VTC5 cells from 2014. Those things still sit at 4.18-4.19V every single time.

    YMMV, since I'm very easy on my cells. My typical use is with a 0.75Ω coil on a mech squonker, and I'm not using more than the top half of the capacity of the cell before it gets recharged.
     
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  4. CMD-Ky

    CMD-Ky Vaping Master Verified Member ECF Veteran

    Sep 15, 2013
    KY
    Thank you, @Mooch I charge mine as slowly as the charger allows, some times this can be nearly twenty-four hours. As I pull them out, if I put them back in or check with a multi-meter, I will always find them at 4.18. I vape at around 9 Watts and never have used them at more than 3.5 Amps. I wondered what was going on, great information.
    Thank you.
     
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  5. MacTechVpr

    MacTechVpr Vaping Master Verified Member ECF Veteran

    Supporting member
    Aug 24, 2013
    Hollywood (Beach), FL
    Great post on a not too often mentioned topic. My OPUS drains all batt's down to a resting full voltage of 4.15V. Most often for multi-batt mods I go with that. I believe the oft given advice that this helps prolong batt life. For mech's I like to top 'em off to 4.2V which the XSTAR VP2 does consistently for virtually all cells, all makers.

    The OPUS seems to do a better job when monitored to keep cells from discharging. Pull them off immediately when full. Almost invariably they'll read 4.21V on the VP2's.

    What baffles me is why some batt's accept the add charge and seem to retain it while others do not. Played around with fast charge to top them at times. Also at low 200/300mV but couldn't arrive at a consistent correlation. Also some cells keep on re-charging to that top level when topped (at least for a while). Is there a more consistent method, slow or fast?

    I guess the best advice is to try and pull cells off as soon as the charge cycle completes. This seems to keep 'em healthy longer. I'd love to see a bluetooth or similar device with a phone app that signals charge completion by channel. When things get busy that would be very helpful.

    Good luck all and Happy Holidays. :)
     
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  6. MacTechVpr

    MacTechVpr Vaping Master Verified Member ECF Veteran

    Supporting member
    Aug 24, 2013
    Hollywood (Beach), FL
    Bought loads of VTC5A's since they rolled out. Practically all charge consistently to 4.2V. Can't tell old from new in most cases except for slight internal resistance changes.

    Good luck all and Happy Holidays. :)
     
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  7. Rossum

    Rossum Surly Curmudgeon Verified Member ECF Veteran

    Supporting member
    Dec 14, 2013
    NE FL
    I have a bunch of VTC5As "for science", but none actually in service at this point.
     
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  8. AngeNZ

    AngeNZ Vaping Master ECF Veteran

    Mar 24, 2018
    New Zealand
    I love the Mooch Musing series - thank you. It's great to have such knowledge shared
     
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  9. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    The slower the charge the easier it is for the ions to find the nooks&crannies without a voltage rise being seen by the charger. This can result in a fuller charge.

    We’re only talking a mAh or two here, maybe a few. It’s not worth trying to fully top off a cell, especially since that just accelerates aging of the cell.

    Happy Holidays to you and your loved ones too!
     
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  10. MacTechVpr

    MacTechVpr Vaping Master Verified Member ECF Veteran

    Supporting member
    Aug 24, 2013
    Hollywood (Beach), FL
    Agree. It seems apparent on cells that are clearly degraded, i.e. higher internal resistance. Not so much for those exhibiting a typical normal res. Therein my quandary. Some cells with seemingly normal int res don't retain the top charge. I quickly relegate these (not fully charging) to less demanding work in lower output variables to perhaps avoid further accelerated strain…and the constant urge to max top them. Sure you're right. No help. So no mech's for them (as they go over ~75mΩ res).

    The OPUS has been useful for years there. Mostly accept the 4.15 discharged and let newer cells rest in the cue until needed. Only use a moderate charge for the batt cap, neither fast nor slo. Rotate so many, rarely see any below 4.19 if run through other chargers. But still hard to resist dropping them on a VP XSTAR before using them in a mech.

    Happy Holidays to you too mooch and thanks for everything you do.

    Good luck. :)
     
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  11. Vaping-Jeff
    This message by Vaping-Jeff has been removed from public view. Deleted by a moderator, Dec 27, 2019, Reason: unregistered supplier.
    Dec 27, 2019

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