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I am making some experiment on the 18650 battery, I learnt that their charging voltage is 4.2v while nominal voltage is 3.7v. I made a 2ways parallel connect of 3 serial connection just like the laptop battery pack, on the laptop 11.1v is written which is 3 x 3.7v but I charge the battery with a 12.5v charger which is still with the charging range of the cell 4.20v * 3=12.6 and the battery is now charge to 12.3v and started getting warm so I disconnected it even despite that its not yet up charge to my charger range. But the problem is that I dont know if 12.3v would damage the laptop if I connect to the battery port with the circuit since they wrote 11.1v, and i would like to know what that nominal voltage mean and I would to know maybe if I increase the battery cells from 6 to maybe 9 or 12 in other to increase the amp/h would it damage my battery. If would be very greatly for your help

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    \$\begingroup\$ Wait, you connected a constant voltage supply directly to a lithium chemistry battery? Stop what you're doing right now before you hurt yourself or burn your house down. \$\endgroup\$
    – Matt Young
    Feb 3, 2017 at 2:46
  • \$\begingroup\$ Each cell in a 2P3S array must be balanced in ESR and mAh capacity within 2% for good performance. Reducing the charge voltage to 4.1 reduces heat loss which in turn extends life. Warm is OK if warm is 45'C . 60'C is excessive or limit if you dont mind shorter charge cycle life. Cut off is usually 10% to 20% of CC limit rating. \$\endgroup\$ Feb 3, 2017 at 2:49
  • \$\begingroup\$ Current MUST be limited to some Imax during charging. This is typically either Imax mA = Battery mAh or Battery mAh/2 depending on the battery manufacturer's spec, Charging 3 x LiIon cells to 12V is OK as long as you remove Vcharge when the battery reaches 12V. The "proper": way to do this is more complex by removal WHEN V = 12V will work OK. Leaving a 3S LiIon battery connected to an ~= 12V source indefinitely is liable to damage or perhaps destroy it. \$\endgroup\$
    – Russell McMahon
    Feb 3, 2017 at 6:30

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The voltage on the battery pack, really any rechargeable pack of any chemistry, is a nominal voltage. With Lipo cells this is typically the 90% charged voltage. The fully charged voltage will quickly drop over time or under high load due to its internal ESR and voltage curve (check the datasheet). A Lipo will then stay at its nominal voltage for most of the charge before quickly dropping when discharged. A 1.5V alkaline battery in comparison has a fully charged voltage of 1.6x V to nominally discharged at 1.1V, and fully discharged are 0.6V. 1.0 to 0.6 volts will be super dead and need something like a Joule thief to get out. A fully charged Lipo is much higher than its stated 3.6, 3.7 or 3.8 volt markings.

That said, there is no way to know how tolerant your random laptop is to the slightly higher voltage. When the laptop fully charges the battery, it is using a dedicated Lipo charging system, a battery monitor or gas gauge, and knows when to cut it off. You do not seem to have done the same.

That said again though, 12v isn't much higher than 11.1V. 9% higher. It will likely be fine. The laptop would use a higher voltage to initiate charging anyway. But use it at your own risk.

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    \$\begingroup\$ On the other hand it is a fairly reasonable assumption that because "11.1V" on a laptop refers to a 3S LiPo battery, the laptop should be designed to operate with those cells fully charged, or 4.2V/cell, or 12.6V. \$\endgroup\$
    – user16324
    Feb 3, 2017 at 10:43

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