There would be no harm in charging the battery and using the player at the same time. In fact, it would actually be better for it.
Lithium-ions have no charge memory and no scheduled cycling is required to prolong the battery's life. They also have a low self-discharge rate meaning that they can set on the shelf for a long time without losing charge. Many batteries (dry cell, lead cell, nicad, NiMH) have a cell voltage of 1.5 volts or less. This means that you have to use several cells in series to get enough working voltage. Lithium-ions have about 3.6V per cell. This means you only need 1 cell for many mobile devices including these players. They are also not as environmentally toxic as other batteries are.
However, they are not a deep cycle battery (full charge to no charge to full charge). To prevent this, a protection circuit is built into the battery pack. The protection circuit limits the maximum voltage of the cell during charge and prevents the cell voltage from dropping too low on discharge. I have not taken a player battery apart to see if it has the circuit in there or not. I just assume that it is since this should be standard practice. The circuit could be on the player's circuit board also. The maximum charge and discharge current on most packs are limited to 1C on smaller batteries (ipod, cell phone, etc.). 1C does not relate to temperature. C is the C-rate (Charge rate).
C-rate explained
The battery on mine is 250ma.
This means that it can supply 250ma for 1 hour. That is a 1C.
It could supply 125ma for 2 hour. That is a .5C
If it could supply 500ma for 1/2 hour, that would be a 2C rate.
However, if the protection circuit is designed correctly, it should limit it to only 1C. This works as a current regulator for the battery and prevents it from being discharged too quickly causing deterioration of the battery.
Once the battery can no longer supply the required amount of current, the voltage starts to drop. I don't know if the protection circuit detects low voltage or low current, but the result is the same; the battery gets shut down to prevent it from being totally discharged. This probably also protects the firmware too. Low power could cause who knows what to happen to it. The firmware must detect low power before the protection circuit does because the player will shut itself down instead of just dying when the protection circuit kicks in.
If this is the case, then the battery's protection circuit may not be needed for low power and some sort of current limiter could be on the player board to stop high current charging. I have not looked for something like that and I don't feel like taking the player apart now.
All this protection and 1C stuff also applies to charging (which was what this topic was about in the first place).
The 1C protection should not allow a charge current greater than 250ma in this case.
My wall charger says 200-300ma output, which would make sense.
The amount of current supplied by the usb gets a bit more complicated. The usb specification says that 500ma is the max output for a usb port coming from your computer. The usb supply (on the mobo) also has a protection circuit which trips around 700ma - 850ma depending on the design and will try to reset in about 100ms.
Software can also cut power to the usb. XP will shut down the port and pop up saying that the usb port is overloaded.
If you put a non-powered hub on your usb port, each device connected to it will no longer get a max of 500ma. They will only get 100ma max (I believe that is regulated by the hub). However, if your hub is plugged into an external power supply (wall wart), the hub should be able to deliver 500ma to each of the connected devices.
Just because it can supply 500ma does not mean that the battery has to use all 500ma. Your house may have 100 Amp electrical service going to it, but that does not mean that your house has to use all 100 Amps. It only draws what it needs just as your player and battery do.
With these precautions in place, the possibility of metallic lithium plating occurring to the battery (which would be a bad thing) due to overcharge is virtually eliminated.
If the player shuts down due to low power, it can be powered back up as soon as the usb connection is made. The usb can supply at least 100ma and maybe as much as 500ma, which is more than the player needs to operate, so the player runs totally off the usb with power left over to charge the battery.
We know this because we can see the charge level go up even when the player is being used, but it may charge more slowly depending on the max current that the usb can supply.
Some rough math says that if a player will run for 3 hours with a 250ma battery, then 250 / 3 = 83 or 83ma would be the current draw of a running player. Of course the number varies depending on what the player is doing (video, music, volume, display on, etc.)
The slower a battery is charged, the better it is for the battery.
This brings us back to my opening line:
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There would be no harm in charging the battery and using the player at the same time. In fact, it would actually be better for it."
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Then again, I could be totally wrong. :lol:
More about
Lithium-ion batteries.
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C-rate