[jctoad]

After I did my last post I thought of something. If you have a brand new player with a battery that has no charge at all on it and it was connected to a non-powered usb hub, it might need to be charged a bit to stop the player from turning off since the player would detect a battery which is too low because the hub could not put out enough power at the start of the battery charging. However, I would like to believe that in the factory they would have had it running and charging for a while to test the player and put the demo files on it. If they didn't at least fire the player up, how would they be able to format it and put that wonderful memory hack in it? When I got my player, it was near a full charge.

For USB charging, does the 500mah limit mean you can't full charge Li-Ons with a capacity greater than 500mah via the USB? Are you for any reason better charging your player via the electrical outlet than the computer's USB?

The current draw of a charging battery starts off high and decreases as the battery becomes more charged.
You can charge a battery with a capacity larger than 500mah off the usb. Probably much larger. However, if the battery tries to draw more than 500ma during charge, it would cause the usb port to be shut down by windows or the protection circuit for the usb might just trip and then reset over and over again, limiting the current. If this were to happen, placing a current limiting resistor between the usb and the battery would fix the problem.
A charging source always has to have a higher voltage than the battery has. It's sort of like the current falls downhill from the higher voltage charger to the lower voltage battery. If the charger voltage is less than or equal to the battery voltage, the battery can't get charged (no current flow). A battery on charge will draw the most current when you first start to charge it. As it gets charged, the current demand will decrease.
The wall adapter is soooo cheap that I don't think it has any current shut down protection circuit in it other than RF1 which is explained later on. So if the battery wants more current than it can provide, it will cause the charger's output voltage to drop. As the voltage drops, so does the current demand of the battery. Once the current demand decreases to a point that the charger can supply, the voltage stabilizes. Over time, the current demand will go down and the voltage will start to climb back up to a normal state.
So yes you can charge a large battery, it just takes longer. A 1000mah battery will take 4 times as long to charge as a 250mah one. This assumes that the same charger was used to charge each battery and that the charger was able to put out as much current as the battery wanted. Otherwise it would take longer than 4 times.

People have reported the cheap chargers that come with their mp4 players have melted their batteries. If the Li-Ion batteries have built in limiters, why would this happen?

Could be urban legend, coincidence or cheap chargers.
I assume that the battery has some sort of current limiting protection. I did not verify this.
The chargers are a cheap POS. I found one like mine for $3.82 http://www.pchub.com/uph/laptop/480-...rger-Kits.html
I took it apart to see what was in there. It comes open by removing one screw under a sticker between the folded down ac prongs (mine is a US model so I don't know what other countries' look like).
Before you go taking yours apart, make sure you have it unplugged. If it is plugged in, there is enough power to knock you on you butt.

As I suspected, it contained a switch mode power supply. You could use a linear power supply (like a wall wart), but that would be more expensive and as we know, they want to make these things as cheap as they can. A linear supply would also be bigger and heavier.

I found a schematic for a supply which is fairly similar to our type.

The following is a schematic of 3.6W, 5.2vout switch mode power supply for cell phone charger or adapter application.

The topology used is flyback using the TINYSWITCH controller chip TNY254P. The input is universal AC range ( 85Vac to 265 Vac). This very low cost implementation of a charger power supply circuit can be used to charge cell phone batteries such as Li-on, NiMH, and NiCd.

The way it works is the AC power comes in on the left side. As you can see, it doesn't really matter if it is 110 or 220. No modification needed for different countries equals bonus cheap.

RF1 (called R1 on my board) is a resistor that is used as a fuse. If too much current goes through it, it will get smoked and kill the power to the rest of the charger. My resistor is 4.7 ohms. I would not say that it is in anyway a max charge limiting device. Rough calculations show that the charger would have to put out 1300ma for it to smoke. I doubt that there is any way that it could actually put out that much power. It looks like R1 would only get toasted if a "catastrophic" failure occurred within the charger.

The next part of the circuit, from RF1 up to C2, is the rectifier and filter circuit. It converts the AC to DC.

Originally Posted by website

very low cost implementation

Not low enough for our China men. The coils L1 and L2 are eliminated. They are filters. Coils are relatively expensive parts so they got left out. Note: The maker of the schematic is the supplier of L1, L2 and T1.

U1 is a small semiconductor chip which uses the DC to run as an oscillator. It produces high frequency energy which goes through T1 (a transformer), since it is high frequency and low power, T1 can be very small, light and cheap. As the power comes out the right side of T1, the voltage is now lower.

D5 converts the AC back to DC. (Yes, we have gone from AC to DC to AC to DC.)
L2 (not in mine), C5 and C6 are filters to condition the DC power.

U2 is an optocoupler semiconductor which is used to send a feedback signal to U1. This works with the oscillator and helps to regulate the charger.
Like the coils, U1 and U2 are relatively expensive. So let's not use them. Yes, you heard me. They saved money by replacing them with two really cheap transistors.
With the money they saved, they splurged and put on a red and green LED. They never told us what they mean, but they look cool.
The schematic doesn't have the LED's or the resistors that go along with them.
Even though my board has those extra 4 parts, it still has less parts and cheaper substitute parts than the schematic has.

If I made an actual schematic of my board, I could tell a lot more, but making them is a pain so I choose not to.

Is this the worst charger in the world?
Not really. It's pretty typical of this kind of charger. Your computer power supply, tv, monitor and many other things use switch mode power supply circuits. They just put out more power and have better filtering and regulation.

If I had my choice, I would rather use my quality computer power to charge the battery than a $3.82 POS.