I got bored last weekend and bought a couple of cheap solar garden lights from the pound shop having taken them apart i then used some of the parts to make a small charger for my mp3 players aaa battery.
My apologies as this is an afterthought 'ible but its the thought that counts...
Step 1: Whats in the Box
In the box is very basic solar light with a very puny solar cell all sealed up in a fairly weather proof box.
I'm still thinking of ideas for the external box with just a little hot glue it would be weather proof enough. The stake and the post could also come in handy for the garden if i ever think of any thing....
Step 2: The Internals
The internals fit inside the weather proof box..
This ones actually quite good for spares and parts. Some are much less useful coming with non standard battery sizes soldered directly to the control chip.
I had a look for a data sheet for the control chip but couldn't find it on the web I'll post up the part number later maybe some one could help out.
Its job is fairly simple when there is no current from the solar cell the chip starts discharging the battery to the led.
Step 3: More of the Gubbins
In there you'll see a nimh battery, a handy AAA battery holder and a small joule thief type circuit on a pcb and a diode to stop the battery discharging back to panel in the dark.
The mini joule thief and the LED will come in handy for a torch in future.
Step 4: The Cell
The solar cell that came with this unit is fairly weedy 2v job, i'm not sure what its current rating is but i'm very sure it won't be more than 30mah since these things tend to rely on not exceeding the c/10 charging rate that nimh batteries can handle with out worrying about over charging.
In order to charge a decent sized battery in less than a month i took a few of these lights ripped them apart and run the solar cells in parallel to up the charge rate.
Step 5: The Charger
In order to charge a decent sized battery in less than a month i've put three cells in parallel which at the most will give about 90ma of charge current still under the 100ma that a 1000 mah battery can cope with.
To try and give this a modicum of water proofing i've chosen a sweets tin big enough to hold the the salvaged battery holder and three solar cells. Its a bit chunky to take out and about but it lives on my window sill charging so that doesn't really matter. Luckily my bedroom window is south facing so it gets the sun until about 5 am to about 5pm though for about an hour at mid day my irritating tree at the bottom of the garden cuts into my charging time.
I re-soldered the points on the cell with some cable out an older power supply and soldered the positive of the three cells to one of the diodes salvaged from the light then to the positive side of the battery box. Then the three negative leads were soldered to the negative point on the battery holder. Loaded the whole thing up with electrical tape to stop any shorts.
I considered a Schottky diode to decrease the voltage drop and thus increase the amount of time the device can charge as it would still work in more marginal conditions but i've got this diode from the light and i don't have any Schottky diodes to hand.
The first two cells i press out of the top of the unit but i managed to damage the cells in the proccess so for the third i just cut the top of the box plastic and all. I don't think there is too much loss of current from the damage.
A little bit of thick foamy double sided tape holds the cells well enough for this job.
Charger works well enough after a couple of days in full sunshine its enough to run my mp3 player for long enough to charge another battery.
This is more of a prototype to test if i got enough sun on my window sill on the out side of my house for a larger unit. I'll document that more thoroughly than this one.