# What are the downsides of using a LM7805 with a 11.1v Li Ion Laptop battery to charge a USB device?

I want to use my thinkpad battery(11.1v) hooked up to a LM7805 to charge a USB Device.

What are the downsides to such a setup?

• Other than pissing away half the power in the battery? – Ignacio Vazquez-Abrams Apr 8 '14 at 11:08
• What would be a more efficient way of stepping down the voltage? – Tejas Kale Apr 8 '14 at 11:11
• Using a switching regulator. – Ignacio Vazquez-Abrams Apr 8 '14 at 11:11
• Or a DC-DC converter. See Digikey/Mouser/Jameco – gwideman Apr 8 '14 at 11:12
• What charging current to the USB device? – Andy aka Apr 8 '14 at 12:42

• The 7805 is a linear regulator, but it is defintiely NOT a low-dropout regulator. The 78xx series needs a minimum of about 3V of overhead ($V_{in} - V_{out}$). – Dave Tweed Apr 8 '14 at 15:22