Turning an AC/mains battery charger into a USB charger - help wanted

Audioguru

Joined Dec 20, 2007
11,248
Most "3.6V or 3.7V" Lithium rechargeable battery cells are 4.20V when fully charged, not just 3.74V.
They are 3.0V to 3.2V when dead so 3.6V to 3.7V is their average voltage.
 

CDRIVE

Joined Jul 1, 2008
2,219
If your battery is a single cell type battery the MAX1555 may be usefull. But it is sot23 type package. At sparkfun you can also order this http://www.sparkfun.com/commerce/product_info.php?products_id=726 it may do what you want to do.
Excellent find! At the start of this thread I googled 'USB Charger' but my returns were all wall warts with USB connectors. Funny thing is.. I've been to SparkFun many times but then, I wasn't looking for these. :)
 

Thread Starter

scootley

Joined Oct 27, 2009
19
I'm finding this entire thread a bit confusing. The stated goal is to charge a cellphone battery with a solar panel, right?
Not quite. It is a camera battery, and one idea is to modify an existing cheapo Asian fake Sony charger that I have. I will not be attaching this device to my camera, so the only important thing that could be damaged is the battery itself (as a receiver of power).

And I am not using any old raw "solar panel", but the Solio Rocsta/H1000, which actually seems to regulate and be able to prevent overcharge, which I didn't fully realize until just now. It has a "USB" output (5-6V, 300-800mA, 4.8W Max, USB connector)

You need a design that won't ruin your stuff, and the answer could be as simple as a blocking diode to prevent discharge thru the panel.
If you can elaborate on how to do this, that would be helpful. But with the "panel" that I am using, it may not be a concern. (since it is designed as an integrated solar charger)

The first concern is over-voltage ... Next concern is over-charge ... A final concern is charge rate
Over-voltage: Solio "panel" is 5-6V. Normally a charger for this battery gives 4.2, so it is a bit high.

Over-charge: Solio "panel" seems to have built-in over-charge prevention.

Charge-rate: Solio "panel" is 300-800mA. Normally a charger for this battery gives 0.25 A, so it could be high as well.

I don't think the original charger appears to control state-of-charge, but I'm not sure
The off-the-shelf Sony camera battery that I am starting with better stop giving power to the battery when it was charged. That is what chargers do, right? True it is not authentic Sony; I have another authentic Sony one and it certainly does not allow overcharging.

I hope the charge voltage is setup for 4.2V as this generally represents a fully charged LiIon cell.
Yes, that is what the charger I am modifying gives as a charge voltage.

I also find this confusing. From what I can see the charger shown by the OP in the first post. Is a very simple switch-mode constant voltage power supply. It is rated 4.2 volt 0.25. So all the OP has to do is to make a low dropout regulator to transform the 5 volt from the USB port into 4.2 volt.
At least this will be as safe as the fake mains based charger.
How do I make that low dropout regulator?

Milking 4.2V from my design will be a hard nut to crack but I'll try.
Would you mind just giving me a brief summary of why it's hard to get 4.2V of output from a charger circuit that takes 5V input?

In that case a single 1N4002 would put the charge voltage in range but there would be no over current protection for the USB port. Personally, I hope he decides to used solar cells and not risk PC damage. ;)
Thanks. I will either use solar cells or a AC/mains-to-USB adapter to test, but not a PC.

If your battery is a single cell type battery the MAX1555 may be usefull. But it is sot23 type package. At sparkfun you can also order this http://www.sparkfun.com/commerce/product_info.php?products_id=726 it may do what you want to do.
It looks pretty cool, but I cannot order it because I am in a place where I cannot easily receive orders like that via mail.

I have managed to find one of these "universal" USB camera battery charger clips. (manufacturer site)

It says its output is 4.25V, 500mA. Although with a multimeter I am seeing only 300mA.

It is unclear to me as to whether it has overcharge protection. But it does have 2 LEDs which are both on while it is charging. I suppose if one of them goes off after some time, then I'll know it has some sort of overcharge protection, and maybe I can just use it with the Solio solar charger's USB connection. (And not use the modified AC battery charger that we've been discussing)

Update: yes one of the LEDs on this clip charger dims as the battery charges (and is bright when it first starts charging)
 
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Thread Starter

scootley

Joined Oct 27, 2009
19
Thanks. But I doubt I will be able to find that chip here. I really only have reliable access to more basic components (resistors, capacitors, diodes, transistors, etc).
 

Audioguru

Joined Dec 20, 2007
11,248
Why duplicate everything?
The Solio Rocsta already has a "3.6V" 1000mAh Lithium battery and a Lithium battery charging circuit inside.

It also has a "3.6V" to 5V or 6V stepup converter to feed a device that has its own charging circuit.
 

Thread Starter

scootley

Joined Oct 27, 2009
19
Yes that is exactly what I am thinking. I just didn't realize that the Solio had that until today.

That is, to hook the Solio up to this USB universal clip battery charger with my camera battery. It seems to work. (2 off the shelf products with no modifications)
 
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