Converting Nintendo 3DS Battery Voltage for Developers

[Joel16]

Hello I need a bit of some help here. I have a Nintendo 3DS and I'm trying to get my program to display its battery voltage. The thing is the voltage function that Nintendo uses returns values in an unknown format, and I don't have access to a multi-meter to get the correct readings right now, so I'm wondering if there's just some math required to convert these units to standard units. (V / mV)

The current information I have is this:

When the charger is plugged into a fully charged 3DS unit, this value will be maximum 0xD1 (209)

Unplugging the charger will make the value go down to 0xCE (206)

The voltage at 5% battery percentage just after the LEDs start flashing is 0xAF (175)

Plugging in the charger at this point will make the voltage go up to 0xB9 (185)Additional information:
The adapter has a voltage of 5.0V
The battery (LiON) has a voltage of 3.7V

Edit:
From what me and a few other developers have gathered is that this magic number may be 5.0. That is, 5.0 * (batteryVolt / 256.0), but we aren't too sure about this.

 

[Averagesupernova]

It has more to do with the a/d convertor and how the voltage is scaled ahead of it. While it is entirely possible that 255 (not 256) is the max it will read, how was what is being measured scaled down ahead of the a/d? I could measure 10000 volts with the appropriate voltage divider ahead of the a/d with poorer resolution of course than feeding the a/d directly with a 0-5 volt signal. Or, they may have scaled down lower than this for some unknown reason.

 

[Tom.G]

Check the sites below for info. They were found using: https://www.google.com/?gws_rd=ssl#q=li-ion+battery+charger

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

http://www.instructables.com/id/Li-ion-battery-charging/

 

[jim hardy]

Search also on "battery gas gage"

http://www.ti.com/lit/an/slua364/slua364.pdf