Battery Operated Device: Appropriate time for getting a voltage reading?

[raccoon]

Hello to all EE jedi masters outhere!

I am trying to build a battery operated device, for opening and closing my bathroom door, via some sort of Bluetooth Low Energy (BLE) proximity sensing.
My board will basically contain a BLE SoC (e.g. nRF 52840) and a motor controller (e.g. DRV 8830).

Let us assume that the device will run on 4 x AA alkaline batteries, connected in series.

Most of the time, the system will be in a deep sleep mode, waking up to advertise its presence as a BLE peripheral.
On average, let us assume that in this state, my system draws around 1 mA.

For 80 seconds daily, the motor will be operating, consuming around 200 mA.

Now the question comes, when is the appropriate time for reading the battery supply voltage, in a way that it actually reflects the energy left in the batteries.
Is it before a motor operation, during a motor operation or after?
If after, shall I take it immediately, or wait for some time?

Being a noob (yes, I am a software guy), the only thing I understand is that when my battery supply's voltage reaches a certain level (4x1.1?) then more or less the batteries have been depleted.

I have observed that immediately after a motor operation the voltage goes down, but then it slowly recovers. I have also observed that this fluctuation becomes greater as times goes by.

Any reference would be greatly appreciated!

[BravoV]

Not an jedi though, heck, not even qualified yet as a jedi apprentice "candidate". 

As I'm aware of, reading alkaline cell's voltage while unloaded or lightly loaded like at sleep mode (monitoring voltage) to determine the capacity can be misleading.

As your own observation already telling you, the voltage can bounce back again, even its depleted.

You can only get a approximation with voltage "while loaded", but that again also is a very crude.

Ideally, using coulomb counter, this is the only reference I can suggest, but again, its prolly beyond my Jedi's pay grade either, sorry. 

[Benta]

You can make all kinds of checks, but alkaline batteries are relatively easy to monitor. Just measure the "idle" voltage, if it drops below 1.35 V per cell it's time to change.
For your application (bathroom door), you don't even need to think about temperature related issues.

[Audioguru]

The used batteries in a flashlight have an idle voltage almost like new but when it is turned on the voltage and brightness drop very quickly.
If you want your project to do its job only partly then measure the voltage at idle. If you want it to do the job properly then measure the voltage while it is working hard and decide at what voltage you want to replace the battery cells.

Some batteries work fine until they suddenly don't have any power anymore. Rechargeable batteries are like that.
Maybe you need a counter instead of a voltage measurement. Replace the batteries after so many times of work.