My circuit using batteries to power the 555 timer. I measure the voltage across the batteries before pressing the button is 11V and after pressing the button, when the LED is on is 10.5V. I want to know what value should I look at to know when the batteries get close to be fully discharged: before the button is pressed or after, when the system is running.

 

You'll need to tell us what size and chemistry of the battery. For example, size AA chemistry alkaline. Also, you can easily find discharge charts in the internet for almost any battery. If I were going to answer your question, that's the first thing I would do.

 

It's 8 D size battery, alkaline. The discharge chart I found shows one battery discharge rate. But because I'm using 8 of them, I don't know if that chart can be applied. Also, my system has sleep mode that still drain the batteries (although very little). The load takes about 250mA.

 

Well if you found that chart for 1 battery and you have them connected in series then you just multiply by 8 . So if they are dead at 0.9 volts then 8 in series would be dead at 7.2

 

Well if you found that chart for 1 battery and you have them connected in series then you just multiply by 8 . So if they are dead at 0.9 volts then 8 in series would be dead at 7.2

I agree, if all the batteries started out new.

 

Thank you very much for all the help

 

I agree, if all the batteries started out new.

Well they certainly all end up old.