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Sansa charging

This page describes the results of experiments on getting charging to work properly on Sansa players running Rockbox.

A patch for rudimentary charging support on c200 and e200 is available in the patch tracker.

AS3514 charger (c200 and e200)

The charge status can be monitored through a set of status bits that indicate whether the charger is connected, enabled, done charging and if battery temperature is too high. The presence of a charger can also be detected through a separate GPIO pin. Various voltages related to charging can be monitored through the built-in ADC of the AS3514.

The SansaView uses an AS3517 chip, which seems to be very similar to the AS3514 w.r.t. charging.

c200 charging and monitoring

The Sansa c200 has a 3.7V Li-Polymer battery with a capacity of 530 mAh. The charging current used by the OF seems to be 300 mA (measured max. 341 mA current when charging, 41 mA when not charging). The charging voltage used by the OF is currently unknown, but most likely 4.20 or 4.25V.

When monitoring the charge process, the interesting ADC voltages seem to be BVDD (the main battery voltage), RTCSUP (voltage of RTC battery?), CHGIN (charger input voltage) and BATTEMP (readout of thermistor inside battery, value decreases for higher temperature). In the early stages of charging, there is no significant change in the voltages, with the following typical values (charging on my laptop with 200 mA to 4.1V):

• BVDD: 4.610 V
• RTCSUP: 4.540 V
• CHGIN: 4.825 V
• BATTEMP: 0.868 V

The CHGIN voltage turns out to depend on the configured charge current: for each increase in the current by 50 mA, the voltage drops by about 50 mV. Also the CHGIN voltage starts to increase near the end of a charge cycle (charging in constant-voltage mode) when the charge current is falling. When the charger finally indicates end-of-charge and the software turns off the charger, the CHGIN voltage jumps up a little further. All of this strongly suggests the presence of a resistor (1 Ohm) between the USB voltage and the CHGIN voltage. The voltage dropped across the resistor can be used to measure the charging current.

Conclusions so far:

• the charging current used by the OF is 300 mA.
• charge voltage can not be monitored during charging.
• charge current seems to be measureable and can give an indication of the charge progress (i.e. constant-current mode or constant-voltage mode).

e200 charging and monitoring

The Sansa e200 has a 3.7V LiIon battery with a capacity of 750 mAh. The charging current used by the OF seems to be 300 mA (derived from measurements made by Peter D in FS#8363.) The charging voltage used by the OF is currently unknown, but most likely 4.20 or 4.25V.

The battery voltage during charging can not be read from the BVDD voltage as expected, it is way too high (like 4.550V). As an alternative it seems that the RTCSUP voltage works. During charging it stays at realistic values and saturates at the configured maximum charging voltage near the end of the charge. When not charging, the RTCSUP voltage is very close to BVDD.

During charging, some of the other voltages (BVDD, CHG_IN) are also increasing. It is not clear yet how they relate yet.

Conclusions so far:

• the charging current used by the OF is 300 mA.
• the battery voltage during charging can be read from the RTCSUP channel of the ADC.
• charge current seems to be measureable and can give an indication of the charge progress (i.e. constant-current mode or constant-voltage mode).

AS3514 peculiarities

• when the charger cable becomes connected, the charger status shows some spurious status. The solution is to ignore the status while the charger still indicates it is unconnected.
• when the charger has never been turned off, it may fail to indicate end-of-charge (the hardware voltage/current limits are still effective, so this is probably not dangerous). The solution is to explicitly turn off the charger first before configuring it.
• the charger may indicate an end-of-charge directly after configuring it. The solution is to ignore the first charger status reading.

Links

• SansaRuntime
• SandiskE200BatteryMeasurements