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Comment by Pxtl | Hacker News Reader

2-3W? That means it's only a 0.5A, right? A lot of devices don't even charge properly off of a 0.5A USB source.

While they may not fast charge, many of the devices will trickle charge - simply because the original USB 2.0 spec didn't require high amperage. In fact, 2.0 spec allowed for a max of 5 'unit loads' with a unit load of 100 mA (5x 100 mA = 0.5A). Putting out more than that was against spec unless the port was specifically marked as a 'charging port' per the Battery Charging specification (you may have noticed USB ports with battery icons next to them).

It wasn't until USB 3.0 that the limits were increased. Additionally the Battery Charging spec was re-written to increase limits for charging ports.

If you can plug your device into any single USB jack on a computer (not just ones with a battery icon next to them) and it will charge, it will likely charge with this project.

In my experience, 0.5A chargers (and PC USB ports) are functionally useless for charging many modern smartphones and tablets, especially after a deep discharge. There has been many times I've put on a device to charge on a 0.5A source and come back to find its battery completely depleted. The charging will fail to keep up with the device's idle energy consumption.

If your device uses more than 0.5A for idle energy consumption, your phone's 1500mAh battery would only last 3 hours. Standby time on most smartphones is usually at least 12 hours, consuming between 100-200mA. Being able to turn the device off, and charge for 3 hours while cooking dinner and hanging out around the campfire is awesome.

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