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I want to make an external powerbank for my laptop, since its battery cannot be removed (MSI GP62M 7RDX). What voltage should my battery have?

18650 elements have nominal voltage of 3.7. My laptop power supply has the following parameters: 19.5 V / 6.92 A / 135 W.

Should I get to 19.5 voltage as close as possible? 19.5 / 3.7 = 5.2, so it makes 5S battery (or 6S?).

Or could I, say, make 3S battery and use a boost converter?

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  • \$\begingroup\$ I'd target something a little over 19.5V, and use an adjustable linear regulator programmed to output 19.5V. \$\endgroup\$ – Kyle B Aug 8 at 14:05
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Do you want the external battery capacity the same or much bigger than the internal battery pack. If so, then compare your choice to the internal one which has a spec of 41 Whr.

You can round the battery cell voltage nearest to 19.5V which is now a standard for laptop chargers. Consider the cell voltage as 3 to 3.7 so a 6S=18V min. is better.

But it should include a low voltage warning or cutoff switch or battery test to prevent low voltage damage.

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This is a device-specific question as different laptops will have different Li-ion cell structure. Smaller ones (13"-14") have 2S flat non-removable Li-ion cells and general 15.6" versions will have 18650 cells stacked 4S, 6S etc. Hence it totally depends on which make it is and the purpose of the machine. In your case, this MSI laptop (MSI GP62M 7RDX) has 6S configuration in which 2x18650 cell are in parallel and 3 of such arrangements are in series providing a total capacity of the battery.
If you look at the charging circuit of the laptop there will be a charge controller IC inside which can take inputs till 25V in this case (depends on the design of the laptop charge controller circuit). basic block diagram

Hence you can design a bigger battery pack and charge the pack with laptop charger with a boost converter and use a buck converter to step down and charge your battery.

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