Not seeing +CGEV: ME BATTERY LOW when battery voltage drops after enabling with AT%XPOFWARN

[JVantol]

I'm enabling external power off warnings, as specified in section 5.22 of the AT command guide.

When I drop the supply voltage I see a LTE network disconnected message from the lte_handler, but I don't see the +CGEV: ME BATTERY LOW message.

I'm using this little chunk of code to do this, called right after modem initialization is complete:

static void XPOFWARN_Handler(void *context, const char *response)
{
   LOG_ERR("XPOFWARN response: %s", response);
}

void XPOFWARN_install_handler(void)
{
	int err = at_notif_register_handler(NULL, XPOFWARN_Handler);
	if (err != 0) 
   {
		LOG_ERR("Can't register handler err=%d", err);
	}
   if (at_cmd_write("AT%XPOFWARN=1,31", NULL, 0, NULL) != 0) {
		LOG_ERR("Can't start XPOFWARN handler");
	}
}

The handler installs successfully, the AT%XPOFWARN=1,31 command gets an "OK" response, and I see lots of other modem responses being printed by my XPOFWARN_Handler, but not the "+CGEV: ME BATTERY LOW" response that I was expecting when I lowered the battery voltage. I do see the LTE handler indicate that the LTE connection drops, right as I drop the voltage, so the modem is noticing the voltage drop and going offline.

Any idea what's going on here? I'm using compatible modem firmware and a board with a nRF9160-SIxA-B1 module, so that shouldn't be the issue.

[Albrecht Markus Schellenberger]

Hello Joshua,

but not the "+CGEV: ME BATTERY LOW" response that I was expecting when I lowered the battery voltage.

how fast did you lower the battery voltage? Could it be possible that the voltage drops too fast and thus the modem doesn’t manage to send low battery level notification before it has to shut down?

The handler installs successfully, the AT%XPOFWARN=1,31 command gets an "OK" response, and I see lots of other modem responses being printed by my XPOFWARN_Handler,

Could you try the %XVBATLVL instead? Does that make any difference?

Regards,

Markus

[JVantol]

Markus,

using XVBATLVL does get me "%XVBATLOWLVL: 3382" warnings, as the voltage drops. But I never see the warning that the modem is going offline because of low voltage.

It's possible I'm reducing the voltage too fast, and the modem goes straight to shutdown, without first issuing a warning, but I would have thought you should get the warning anyway so that you can tell WHY the modem went down.

I'm going to test this further, but if you have any thoughts on what might be going on I would appreciate them.

Regards,

Josh

[Albrecht Markus Schellenberger]

Thanks for the feedback, Joshua! I would recommend you to set the power off warning to its highest level:

AT%XPOFWARN=1,33

and drop the supply voltage to a constant level of 3.2V, as this should make sure that the modem is able to run through the whole shut-down process.

As you can see in this table, which was measured at a supply voltage of 3.7V, the current consumption can vary a lot depending on the modem state.

Regards,

Markus

[JVantol]

I tried this, and even tried sneaking up on the voltage threshold 2 mv at a time, and never saw the low voltage warning. Is it possible something in the SDK is disabling this, or "eating" the response before my handler can get it? Is there a recommended project to use to test this feature in isolation?

[Albrecht Markus Schellenberger]

That is interesting… and not really as expected I would say! I think the code snippet you are using should work fine. And the fact that you are getting other AT notifications confirms that as well.

What modem firmware are you using? I might wanna report this to our modem team and check with them. You can use the +CGMR command in case you are unsure.

Regards,

Markus