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I understand that 5s Li-ion should be charged at 21V (4.2*5) however the only laptop chargers I can find are 19V, 20V, and 24V. Each is relatively close to the value that should be used for 4s, 5s and 6s.

Is this what they are really?

And can I use a '20V' charger to charge 5s Li-ion?

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No.

A laptop charger is typically nothing more than a constant-voltage power supply.

All the actual charging logic and protection is present inside the laptop itself, which has the necessary buck converters.

Most modern laptops use a 3s design, however some use 2s and 4s, and there may be others.

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  • \$\begingroup\$ So I'll require a buck converter from say, 20v to 21v, to then feed into my BMS for the 5s? \$\endgroup\$ – Torin Clark Dec 15 '18 at 1:32
  • \$\begingroup\$ It would have to be a buck/boost converter, as the charge voltage on the cells could be anywhere between about 15 and 21V. (3-4.2V per cell) \$\endgroup\$ – Someone Somewhere Dec 15 '18 at 1:38
  • \$\begingroup\$ It would be better to either avoid a 5s pack and use 4s or 6s instead, or use a 24V supply and buck from there. Also, check if your BMS has a charger integrated in it, which would step the voltage to whatever is necessary. \$\endgroup\$ – Someone Somewhere Dec 15 '18 at 1:39
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    \$\begingroup\$ No. You can't connect a 20V supply directly to batteries - a laptop supply will not go into constant-current mode if you try to draw more than safe current; it will shut off. You need a proper CC-CV charger. The laptop supply is only a way to get from mains to 'about right'. \$\endgroup\$ – Someone Somewhere Dec 15 '18 at 1:42
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    \$\begingroup\$ If the BMS has a buck/boost converter and balancing, that's OK. If it's missing either of those, you need a proper charger. You cannot connect 20V across the battery without a buck converter somewhere. \$\endgroup\$ – Someone Somewhere Dec 15 '18 at 1:52

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