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what electronics would I need to put between 3 AA batteries and a female USB to allow for proper charging [5.2v 450mA]? Answered

I bought a cheap LED flashlight that charges with a decently sized solar panel. I opened it up and found that it holds 3 AA rechargeable batteries and has a reasonable amount of free internal space, I was thinking of putting a hole just large enough for a female mini-usb port.

The 3 batteries are 1.2V Ni-Cd 500mAh.
And there's a black ... thing ... that says:

and the resister is brn blk blk gld

let me know if ya have a solution [=



Best Answer 9 years ago

Follard, I think he meant to charge usb devices, not have USB suppy power to the flash light.

In the best case, your batteries run (in series) for a 3.6v supply which cannot supply the need 5v (and usb spec is 500mA) -> 2.5Watts

Your batteries have (3.6v*500mAh) 1.8Wh of storage and thus theoretically can produce the required 2.5W for just 43min (that's before efficiency losses)

Ni-Cd's are not charged in Series, so battery pack's of Ni-Cds are charged separately in sections. I doubt that your cheap flashlight has three charging circuits, so the batteries are likely in parallel to create a 1.2v 1.5Ah battery.

So you need to convert DC 1.2v to DC 5v and its called a boost converter

A good choice is a LTC3425 (about $6) it can take in .2v-4.5v and convert it to 5v. It also has a output current limiter (required by USB spec) so you can additionally limit the output to 500mA. It does need support electronics, but the examples on the Datasheet should prove more than enough, for someone with some electronics background.

The difficulty is that 1.2v-1v usable range of your Ni-Cd cells (when not in series) is too low for most simple/cheap DC-DC converters. However IF your flashlight does use the cells in series to produce 3.6v there are lots of simple devices to take ~3.3v and make 5v.

A easy example is LTC3529 ($3) which is much simpler (its designed to run off a single lith-ion cell, but needs min 1.8v) to make a preset 5v 500mA Takes very little support parts and has a 95% efficiency rating.

Hope that helps.


10 years ago

Is your a, o, c, and w buttons broken?


10 years ago

If you install a blocking diode on the solar panel (to prevent reverse current into the panel you could hook the 5 volts in from the usb to the point where the solar panel connects in parallel. The usb port might apply too much current to the charging circuit - keep an eye on it for it overheating.