Detection of low charge in battery

[DMS]

Hi,

I'm using a custom nRF51822 board powered by a coin cell and need to trigger a specific LED when the battery is low.

Using this thread as the reference, I tried to find the analog inputs on my board but it appears that the only pins available are vdd,gnd,clk and i/o.

 Assuming that I am unable to access analog inputs, is it possible to directly compare internal Vref to the Vdd input instead of Analog input?

Are there any other solutions I can try which do not involve external hardware?

Thank you.

Edit: I tried using "Power on fail" for 2.7V as the threshold, which for some reason triggers the "low charge LED" even if the battery provides >2.7V

[Abrie]

Hi. Do you have an answer on this? I need a solution to get a low battery warning without adding any external circuitry.

[Kenneth]

The answer depend on what battery you are using (e.g. max and min voltage).

[Kenneth]

Is this question for nRF51-series or nRF52-series?

For new development I recommend to check out nRF52-series, in such case you can find that you can connect VDD internal as input for the SAADC:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/saadc.html#register.CH-0-7.PSELP

In addition you can set gain to 1/6 and use 0.6V internal reference:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/saadc.html#register.CH-0-7.CONFIG

Then when you sample VDD you should get a value between for instance for a fresh battery 3V : 3V * 1/6 = 0.5V, which is less than the internal reference than 0.6V so this should work without any external components.

[Abrie]

Hi Kenneth

Yes I think this will work. I am finalising hardware design. Do you need to make a physical connection (track) from VDD to one of the analog inputs or can the connection be made with software?

I allready made a board with no physical connection that is why I am looking for a way that does not require any connections from VDD to the analog pins

Thanks for the help

[Abrie]

I am using the Ublox ANNA b112 which uses the NRF52832

[Kenneth]

There is internal path to VDD that can be used yes. No need for external connection.

Best regards,
Kenneth

[Abrie]

Thanks! Got it to work from the SAADC example just now but cannot find an example of how to set the gain? Just need the command to put in the saadc_init() function