Cheers for the info. It’s kinda interesting given the USB Spec states 4.5V - 5.25V, it worries me that a commercial product goes to 5.5V.
As you say, most USB ports are badly regulated and the player is likely to handle a higher voltage.'m hoping for a Sandisk Official to confirm this. It would be odd for the Li-Pol charging circuit to not have a regulator on it.
Can’t add a regulation circuit without upping the voltage to ~7V (regulators need headroom, and simple zener diode circuits need inline resistors for current limiting. at 350mA charge current (someone claims it to be so) that’s one hell of a drop on just a 1ohm resistor. I’ve considered the internal resistance of the battery pack to act as the resistor, but I have little confidence.
NiMH sit at 1.4V on full charge and go to 1.2V after about 5-10% use (and fortunately hold that voltage until damned near flat).
So, for the initial use there may well be 5.6V at the output. And, now for the complex bit:
Voltage: 4.5 - 5.25 V
On intitial attachment a client may only draw 100 mA from the host *, to draw more the client MUST negotiate this with the host first. Provide the host gives the thumbs up the client then may draw up to 500 mA. And, there-in lies the problem.
Assuming an internal resistance of 0.17 ohm ± 0.04 ohm per cell.
Therefore the internal resistance of the battery is 0.52 - 0.84 ohm (again, it’s an assumption but it does give leeway for various brands)
Time: 0 sec
Open Circuit: 5.6 V
100 mA: 5.6 (V) - ( 0.1 (A) * 0.52 (ohms) ) = 5.548 V (above both USB spec, and that of the previously mentioned mains / USB adapter)
350 mA: 5.6 (V) - ( 0.35 (A) * 0.52 (ohms) ) = 5.418 V (worst case, Fails USB, passes previously mention adapter spec)
350 mA: 5.6 (V) - ( 0.35 (A) * 0.68 (ohms) ) = 5.362 V (still fails USB spec at typical internal resistance)
Time: some point later
350 mA: 4.8 (V) - ( 0.35 (A) * 0.84 (ohms) ) = 4.506 V (Just in spec - assuming the device honours the USB Spec**)
500 mA: 4.8 (V) - ( 0.5 (A) * 0.68(ohms) ) = 4.46 V (Below the USB Spec)
There is generally very little overhead for USB powered Li-* chargers (at least according to Maxim) so under voltage is likely to cause a huge issue.
So, provided the device can accept up to 5.6V and only draws 350 mA (someone has measured it to be this apparently) then it’s all hoopy. There are certainly enough AA / USB adapters on the market that assume this.
I know, I’m being a paranoid penguin!
* The spec has been amended so that if the data lines are shorted (if I’ve remembered this correctly) the device can assume it is connected to a dumb charger and can ignore negotiations, going straight for high current.
** There are some devices which don’t like having as little as 4.5V, so technically shouldn’t hold the USB logo as they aren’t spec compliant.