Build a USB Powered AA NiMH and NiCd Battery Charger

This project, which can charge a pair of AA Nickel Metal Hydride (NiMH) or Nickel Cadmium (NiCd) cells using a laptop`s USB port for power, arose to address part of that problem. (By the way, if you want to lighten your laptop load, take a look at the MoGo Mouse. ) Any USB port can supply 5V at up to 500mA. The USB standard specifies that a device may not use more than 100mA until it has negotiated the right to use 500mA,
Build a USB Powered AA NiMH and NiCd Battery Charger - schematic

but apparently no USB ports enforce that requirement. This makes the USB port a convenient source of power for devices such as this charger. The USBCell is a 1300mAh AA NiMH cell with a removable top that allows it to be plugged directly into a USB port. No separate charger is needed. Unfortunately, the cell capacity is very low (most NiMH AA cells are 2500mAh these days), and each cell requires its own port. There is a two cell USB powered AA charger available, sold under a variety of names, but it charges at a very low 100mA rate. The distributor calls it an overnight charger , but at 100mA, a 2500mA cell would take about 40 hours to charge (40 instead of 25 due to the inefficiencies of charging at low currents). I found a 2/4 cell charger that can be powered by a USB port, auto adapter, or wall wart, but it is as large as the wall charger I`m trying to replace. Different ones can be found here and here, but these take 10 to 12 hours to charge 2500mAh cells. [December 2007 Update: Sanyo has introduced a USB powered charger for their Eneloop batteries. This charger has none of the drawbacks listed above, and will charge a pair of 2000mAh cells in about 5 hours, or a single cell in half that time. Although designed for Eneloops (see my review ), it will work with regular NiMH cells as well. Watch for a review on this site soon. ] The charger in this project is designed to charge two AA NiMH or NiCd cells of any capacity...

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