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I saw quite a few custom computer units and laptops that are powered by adapters/batteries totalling 18 to 24 V. What's the significance of this design choice?

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There are a few things that come into consideration with this voltage.

One item is the voltage required by LCD backlights. A higher voltage makes it much easier to power them instead of having to go into some variable voltage boosting.

Another item is the chemical make up of batteries and the number of battery cells inside. I know 6 cell batteries seem to be pretty common in laptops. I believe LiPo's usable voltage range is about 2.7-4.2v per cell. So for a 6 cell battery, you are looking at about 24 V. It makes it easier for equipment designers to have their charger voltages to be about the same as the battery voltage.

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Absolutely nothing inside the system works directly on the main DC voltage, there is always a DC/DC converter in between, so designers tend to choose the voltage high enough to get the current down to something the connectors and wires will take as long as it's not chosen so high that it becomes a risk and components get more expensive.

It's generally much easier and more efficient to do a step-down converter than a step-up converter, so that also drives up the voltage until you can get rid of most of the step-up converters, the backlight converter being the obvious exception.

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  • Low enough to make it easy to meet safety requirements (so should be <60 V)
  • High enough to make the current not much, cutting connector size
  • Common parts available.
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