This project uses very few components to build a clever little analog blinker.
And you might find it interesting to see the circuit simulation:
The oscilloscope output clearly shows the ramp of the base of one of the transistor and the square wave that results on one of the LEDs:
Bill of Materials
https://www.findchips.com/org/10-open-hardware/list/35514-astable-multivibrator
I had never given thought to re-using the contacts from a dead batt but will give it a try someday. I have always admired the form and circuit of a Astable, it just seems like like a art form in itself. It even works with a pair of Light Logic switches used instead of the transistors :-)