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I changed my headphone power circuit and I noticed it was using 4.2 V to charge a 3.7 V battery. I am confused why it is using 4.2 V.

I know to charge a battery you need more voltage than its rating, so you can supply energy to separate electrons.

How do we get the exact value of 4.2 V. Why not 4.3 V or 3.9 V, why exactly 4.2 V?

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  • \$\begingroup\$ refer to the battery data sheet for recommended charging protocol \$\endgroup\$
    – jsotola
    Commented Jun 3 at 4:36
  • \$\begingroup\$ Li/Ion chemistries tend to specify a nominal usage voltage and a nominal charging voltage, the latter is a bit higher. \$\endgroup\$
    – Lundin
    Commented Jun 3 at 10:51

4 Answers 4

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A lithium ion cell may start with 4.2V across it, when it's 100% charged, but it quickly falls to about 3.7V, at about 80% charge.

Then it will stay at around 3.7V until charge falls to 20% or so, where voltage drops off sharply.

This image from Silicon Lightworks (the first google result from a search for "lithium ion battery discharge voltage curve") shows this behaviour:

enter image description here

In that graph, the flatness of the region between 80% and 20% is exaggerated. In real life voltage won't be so steady at 3.7V during discharge, but it does illustrate that 4.2V is only the initial state of a fully charged cell, and that voltage will fall precipitously below 3.7V near depletion.

The word "nominal" indicates that the value of 3.7V was chosen as being approximately representative of cell state for the most part, which is well between 100% and 0% of charge.

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    \$\begingroup\$ 0% at 2.25V? I always heard 3.2V or thereabouts. Easy to remember: half a volt from fully charged to average, then another half a volt to low voltage cutoff. \$\endgroup\$ Commented Jun 3 at 14:18
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    \$\begingroup\$ @QuittingDueToAntisemitism 100% and 0% are not strictly defined points in LiIon technologies. It's up to the manufacturer of a device to define those points. The more extreme you select those points the mor usable capacity the user gets (at least initially) but the cells will age much faster. The more conservative the manufacturer chooses the values for 100% and 0%, the smaller the initially usable capacity but the faster the cell will age and loose capacity. So yes, it can discharged to 2.25V without immediately destroying the cell, but it will let the cell age quickly! \$\endgroup\$
    – kruemi
    Commented Jun 3 at 14:29
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    \$\begingroup\$ Taking a NCR18650B cell, we can look at a datasheet and independent tests to see some actual discharge curves (which are current-dependent). It's a pretty linear drop over a 0.6-0.8V range centered around 3.7V. Contrast NiMH, which spends the bulk of its capacity in a 0.05V range centered around 1.27V with a much sharper initial dropoff. \$\endgroup\$
    – Bob
    Commented Jun 4 at 3:44
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    \$\begingroup\$ In fact, the graph displayed in this answer looks more like a NiMH curve (with shifted voltage) than a Li-ion one! Exaggerated indeed. \$\endgroup\$
    – Bob
    Commented Jun 4 at 3:45
  • \$\begingroup\$ @QuittingDueToAntisemitism - 3.2 is the save voltage that would allow you to recharge the battery. In a pinch if you really need it a lithium cell will work down to 2.25V but it will be damaged by then - think of it as the "single use" 0%. \$\endgroup\$
    – slebetman
    Commented Jun 4 at 10:35
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The nominal voltage of a battery is 3.7 V. That's just it. It's a nominal value.

Every battery chemistry has a given nominal voltage. But that's not regulated, it's just a result of the chemistry.

Other examples:

  • Lead Acid batteries (car batteries) are nominally 12 V but are charged to 14.4 V
  • 3 V lithium cells (Like the CR2032 found on many computer mainboards to power the real time clock and hold NVRAM contents) have a voltage of 3.2 V when fresh and discharge to below 3 V before the useful capacity is depleted.

Most Li-Ion and LiPo chemistries have a nominal voltage of 3.7 V but are Charged to 4.1 V or 4.2 V. Look at the battery or its datasheet for information because overcharging of Lithium based batteries can easily damage them or even lead to fires and explosions.

There is a nice article named "Confusion with voltages" on Battery University that should answer your questions.

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    \$\begingroup\$ and "nominal" literally means "what it says on the label" or "what we usually call it". The nominal voltage of a lithium battery is 3.7V because it says so. It does not mean "fully charged voltage" or anything like that. \$\endgroup\$ Commented Jun 3 at 14:19
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It must be a lithium battery then.

Different types of batteries use different chemistries and must be charged accordingly.

4.2 V is used because it is a suitable voltage for lithium battery. The battery manufacturer has likely selected 4.2 V for this manufactured battery type to be within specifications.

Less voltage would give you less capacity, but more charge cycles than specified. More voltage would start to degrade the battery and gives less charge cycles and it loses capacity faster.

The nominal voltage is 3.6 V, it's not the capacity, but the nominal voltage you calculate the nominal capacity in mAh or Ah. When battery is full, it starts from 4.2 V, and voltage drops when it discharges. At some voltage the battery is considered empty and discharge must stop or it will damage the battery. Then the nominal voltage can be calculated and for a typical lithium it is 3.6 V. The curve is not linear so it just is 3.6 V on average for the sake of calculations when converting the capacity between amp-hours and watt-hours.

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You need to push the charge into the battery by using higher voltage than its open circuit voltage. The type of battery you use is commonly charged by CCCV (constant current - constant voltage) charging where initially the current is limited by controlling voltage, and, once the maximum allowed voltage is reached, charging is completed with constant voltage and falling current.

Nothing works without losses. Consider the higher voltage needed for charging the extra energy that will be lost in the process of charging and discharging.

Here is an actual diagram of voltage over a charge-discharge cycle for a small Li-Ion cell:

X-Y-graph for voltage over time during charge and discharge

It should be fairly obvious that the medium voltage during discharge is around 3.7 V while the maximum voltage at the end of the charging process is 4.2 V. Going higher than that will damage the battery.

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  • \$\begingroup\$ I would argue that CCCV stands for constant current constant voltage, rather than constant charge ;) \$\endgroup\$ Commented Jun 4 at 15:05
  • \$\begingroup\$ @VladimirCravero Of course you are right. Sorry for the mistake, and thank you for pointing it out. \$\endgroup\$ Commented Jun 4 at 15:25

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