Measuring Lithium battery voltage with nRF52

RE: Measuring Lithium battery voltage with nRF52

Harsh Bhatt — Tue, 11 Apr 2023 09:40:40 GMT

Hello In my case r1= 1M and r2= 3.16M. Output voltage need to be 3.13v when battery voltage is 4.12v, but we recived the 2.91v. Dont know why but we get the voltage drop at output side. please share the solution.

RE: Measuring Lithium battery voltage with nRF52

mudroprogramer — Sat, 21 Jan 2023 18:39:08 GMT

There is a mistake in the following equation.

Vsaadc = Vain * (Cext – Csample) / Cext

To test this equation, lets consider we choose Cext equal to Csample

--> the result is Vsaadc = 0

While intuitively, the charge should be split equally between the two equal caps and the voltage should drop by 2.

I believe the equation should be:

Vsaadc = Vain * Cext / (Cext + Csample)

RE: Measuring Lithium battery voltage with nRF52

DanaK6JQ — Tue, 15 Feb 2022 15:57:45 GMT

Minor nit: "we choose R1 to be 800 KΩ. This is the maximum source resistance we can have if we use an acquisition time of 40 us, see here."

The series resistance of a resistive voltage divider is the parallel of the two resistors; R1 alone does not determine the series resistance. In this case, series resistance is 1 / ( 1 / 2000 + 1 / 800 ) -> 571.4k. In theory, you could use a larger values for R1 and R2.

RE: Measuring Lithium battery voltage with nRF52

Radio — Mon, 31 Aug 2020 12:16:10 GMT

Hi All,

In the above voltage divider circuit, is the AIN out is connected to the saadc input pin as well as nRF52 VDDH pin?

When ADC starts to sample could it affect the overall power supply to the chip?

Thanks a lot!!

RE: Measuring Lithium battery voltage with nRF52

Nasib — Tue, 19 Nov 2019 10:22:53 GMT

Ufff I have a couple of doubts:

1) Here they say that we don't want the current to flow through the ADC. Why?

https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/measuring-lithium-battery-voltage-with-nrf51

2) In my case (NRF51822) the internal resistance of the ADC is 360K, so it is below R2 ... Does this fact affect the measurement?

Thank you !!

RE: Measuring Lithium battery voltage with nRF52

Shashikant — Tue, 07 May 2019 08:50:20 GMT

Hi Ole Bauck,

As mentioned in datasheet input impedance of the ADC pin is >1M ohm.

It means if we use a voltage divider circuit then the impedance of voltage divider should not exceed the input impedance of the ADC pin. please correct me if I'm wrong.

Does higher value of external impedance at ADC pin affect the performance?

If I'm selecting this value R1 = 4MOhm, R2 = 10MOhm, Cext = 10nF, it is fine for >1M ohm impedance?

Thank you

Shashikant

RE: Measuring Lithium battery voltage with nRF52

Geom — Wed, 10 Apr 2019 09:15:18 GMT

What can you say about using new feature on nRF52840 chip -
check battery voltage on input SAADC_CH_PSELP_PSELP_VDDHDIV5?

RE: Measuring Lithium battery voltage with nRF52

PY — Fri, 29 Mar 2019 11:03:13 GMT

Hello Ole,

Thank you for your great work!

I still have one question about the capacitor part, how do you calculate the new resistor value of 4MOhm for R1?
According to the documentation, with a 3us acquisition time, the source resistor has to be 10KOhm.

Thank you for your answer

RE: Measuring Lithium battery voltage with nRF52

jackiemoon98 — Wed, 20 Mar 2019 19:56:34 GMT

Hi,

I noticed this example from a while back and am trying to reproduce the example but with nrf52840 and sdk 15.2.0. Is there any way to use this technique with the examples/peripheral/saadc example provided in the sdk? Is the same thing possible in the examples/ble_peripheral/ble_app_template? If so, could you please provide some insight on how to do this, specifically using nrf log to output the battery level to terminal/debug terminal?

Thanks,

Jack

RE: Measuring Lithium battery voltage with nRF52

ProfHindsight — Thu, 14 Feb 2019 20:29:03 GMT

Does the input impedance of the SAADC not play a role in the measurement?

RE: Measuring Lithium battery voltage with nRF52

awneil — Thu, 14 Feb 2019 10:52:21 GMT

Note that battery terminal voltage is a very poor indicator of remaining capacity for lithium batteries.

This comes up very frequently in the forum; eg,

https://devzone.nordicsemi.com/f/nordic-q-a/20389/battery-level-check/79486#79486

https://devzone.nordicsemi.com/f/nordic-q-a/6914/battery-level-discharge-curve-how-can-i-read

For accurate battery capacity estimation, you need a so-called "Coulomb Counter" or "Gas Gauge".

RE: Measuring Lithium battery voltage with nRF52

devops — Mon, 24 Sep 2018 11:51:18 GMT

What does the "gain of 1/5" mean? From where is that number (1/5) taken from? Is there a sample code somewhere?

RE: Measuring Lithium battery voltage with nRF52

awneil — Fri, 27 Jul 2018 07:39:36 GMT

The link explaining the R1 800k maximum is broken

RE: Measuring Lithium battery voltage with nRF52

Moe — Tue, 08 May 2018 09:43:47 GMT

I was looking exactly for this kind of information. i just found the link to this blog through the discussion forum. if possible please highlight it in a more obvious and clear way to other beginners! thanks a lot

RE: Measuring Lithium battery voltage with nRF52

Ole Bauck — Wed, 17 Aug 2016 12:57:37 GMT

Post updated with capacitor value calculations