Expansion Board LiPo save power down function so save lipo undervoltage?

[lollisoft]

Hi,

are the expansion boards that have a Lipo connector save to operate until the lipo battery dies? Eg. does it power down at a LiPo minimum voltage level?

I just running my first LoPy without deep sleep on an expansion board with a 800mah Lipoand expect a runtime of about 8 hours. I don't like to kill the Lipo at it's first usage tests :-)

[dbrgn]

According to the Pysense pinout diagram, the controller chip is a BQ24040.

http://www.ti.com/product/BQ24040

In the datasheet, I found:

The bq2404x family is in power down mode if the IN terminal voltage is less than UVLO. The part is considered “dead” and all the terminals are high impedance. Once the IN voltage rises above the UVLO threshold the IC will enter Sleep Mode or Active mode depending on the OUT terminal (battery) voltage.

In the electrical characteristics, I found this:

UVLO Undervoltage lock-out Exit, MIN 3.15V, TYP 3.3V, MAX 3.45V

So it appears that there is some kind of undervoltage lockout. Confirmation / documentation by Pycom would be very much appreciated though.

[jmarcelino]

Yes, power to the LoPy is disconnected once battery voltage goes below 2.1v.

[dbrgn]

@jmarcelino said in Expansion Board LiPo save power down function so save lipo undervoltage?:

Yes, power to the LoPy is disconnected once battery voltage goes below 2.1v.

2.1V? Where did you get that number from?

But it would match my experience. I have one (unprotected) battery that was around 2.1V after a few days on a unoptimized script (LoPy didn't start anymore). The charger initially considered the battery dead, but managed to charge it after a few minutes. Isn't that value a bit low for LiPo / LiIon batteries?

[jmarcelino]

2.1V? Where did you get that number from?

Experimentally.

It is very low for LiPo, at the edge of catastrophic ...

I recommend using protected cells. This is included in many LiPos, they have a little circuit board like this:

or use protected 18650 cells.

[dbrgn]

@jmarcelino said in Expansion Board LiPo save power down function so save lipo undervoltage?:

It is very low for LiPo, at the edge of catastrophic ...

I see. Yep, I only used the unprotected cell for initial development and could recover it fortunately. Are you sure that it's actually a protection circuit at 2.1V, and not simply the battery failing due to its chemistry?

Unfortunately it's not really possible to know without this experimentation whether there is any protection on the expansion boards and at what voltage it kicks in without schematics and/or more detailed specifications. Makes a bit hard to develop hardware solutions that work together with the Pycom boards.

[jmarcelino]

Are you sure that it's actually a protection circuit at 2.1V, and not simply the battery failing due to its chemistry

Yes, I fed power supply voltages directly through the LiPo connector and measured the output into a 100mA load at Vin + Ground on the Py connector. When input voltage at battery terminal dropped below 2.1V, Vin + GND became 0v.

[dbrgn]

When working with unprotected cells, a circuit like this should work fine, right?

With a MAX6430EGUS+T, the ~LBOH output turns low when battery voltage is <3.3V, while ~LBOL turns low when battery voltage is <3.0V. That should trigger the left MOSFET and turn off the entire circuit.