Had I not seen that many people seem to expect to be able to charge from 5V, 9V and 12V chargers I would have expected 20V to be a requirement, allowing lower voltages for charging a 17V battery would almost definitely require additional circuitry.
we use a buck-boost charger circuit which allows charging from lower input voltages.
Lower cost designs use a buck only charger, which will only work with 20V power adapters.
This saves a little cost, as you need less switching FETS for the charger circuit on the mainboard.
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The 5v bit seems to physically work but it currently chooses not to, you can trick the laptop into doing it but that doesn’t have a lot of real world use. Hope that gets fixed at some point.
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