This log deals with creating a portable USB iPod charger that should provide around three hours of video time once the lithium battery has gone flat. Working in a photograph development lab i see hundreds of AA batteries going to waste after being used for only 27 photos maximum of which only a percentage would have used much power by setting off a flash.

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Alkaline batteries will only last a short time in digital cameras before being rejected as powerless when in fact around 80-90% of the power is still in the battery! This D.I.Y log will explain how to use these batteries to recharge your portable USB devices using AA batteries. The next version will use rechargeable AA batteries or perhaps a Lithium_ion_battery.

Specs and geek info

The device has been tested to provide:

• iPod video (tested, using alkaline batteries): 3hrs more video (1 full recharge)
• iPod shuffle (not tested): 60 hours more (5 full recharges)
• iPod mini (tested w/rechargables): 26 hours more (1.5 full recharges) AA Charger final

The reason AA batteries were used makes sense once you understand that a 9 volt battery has a high internal resistance of around 2 ohms and will provide about half an amp over its life. That means it will supply you with 500mA for one hour or 100mA for five hours. The high internal resistance means alot of the usable energy is lost as heat which means you are not really getting your 500mA as usable energy.

Conversely your average AA battery is easy to find, cheap and you can also get rechargeable batteries although i recommend using three if you are going to use NiMH or NiCD rechargeable batteries as a source due to their lower output. Where a 9 volt provides 4.5Wh of power (500mA x 9) you standard AA batteries supply 3000mA each for a total of 9Wh. A USB port on a computer supplies 5 volts at 100mA (or it is meant to). USB devices require the same energy to recharge. So while it would make sense to use a 9 volt and a linear regulator to reduce it to 5 volts ... now you can see it makes better sense to use a boost converter and take the supplied 3 volts up to the required 5 volts.

AA batteries are over abundant ever since digital cameras emerged. Most users are slow to work out that alkaline batteries just won't cut it in their cameras and i am constantly asked to dispose of used batteries. Used batteries that still 90% of the power still in them! The ladyada website explains this in more detail.