Battery Voltage when it's Low

Official Hardware MKR Family MKR NB 1500
1

Hi All,

After some testing I've noticed the 3.3V/VCC is not at 3.3 volts on battery once the battery is less then 3.5 volts. The BQ24195L is supposed to keep the SYS Voltage 150 mV above the sys Min Voltage but it seems to be the same at the battery voltage (Currently 2.85V).

Is there a flaw with the design of this board or do I have one with a bad component?

Everything else seems to be functions properly.

Regards,

2

From .pdf:

8.3.1.5 Boost Mode Operation from Battery
The bq24195L, bq24195 supports boost converter operation to deliver power from the battery to other portable
devices through USB port. The boost mode output current rating meets the 1 A (bq24195L) or 2.1 A (bq24195)
charging requirements for smartphone and tablet. The boost operation can be enabled if the following conditions
are valid:

  1. BAT above BATLOWV threshold (VBATLOWV set by REG04[1])
  2. VBUS less than BAT+VSLEEP (in sleep mode)
  3. Boost mode operation is enabled (OTG pin HIGH and REG01[5:4] = 10)
  4. After 220-ms delay from boost mode enable

In battery boost mode, the bq24195L, bq24195 employs a 1.5-MHz step-up switching regulator. During boost
mode, the status register REG08[7:6] is set to 11, the PMID output voltage is 5.1 V.
For power bank applications, the boost current is supported from PMID pin as in the application diagram. It is
recommended to use the minimum PMID cap value 20 µF (bq24195L) or 60 µF (bq24195) for boost current.
Please note that there is no boost current limit setting when the boost current is sourced from PMID pin, hence it
is important not to overload the boost current under this condition.

REG04[1] is defaulted to 3.0V. This might explain why it is not working at 2.85v?

Looks like boost mode has to be enabled using OTG pin. OTG appears to be connected to PA18 which is mapped to arduino pin # 24. Although it is mislabeled in variant.cpp?...

3

Also, REG01[5:4] is set to 01(charge battery) in variant.cpp. It needs to be set to 10(OTG) to use boost. So I guess this is the third reason it is not working...

4

Ooops, I misunderstood boost mode. It is for powering 5v devices from the USB. Disregard my nonsense above.

5

There is some good info on Boost Mode there but unfortunately not what I'm after.
I've been trying to figure out how to monitor the battery voltage so I can disconnect the BatFet in the BQ24195L and power off the Arduino before I damage the battery / brown out the arduino and corrupt my data.