Running a battery completely flat can permanently damage it, lead-acid and lithium-polymer types especially. I put together this circuit to protect a small lead-acid UPS battery. It's cheap, has a high current capacity, and use only very common parts. Did I mention that it's cheap?
Today's challenge is to drive a cheap 8-digit LED display. Months ago I bought a cheap module from Aliexpress. Too cheap. Instead of a nice smart LED driver chip as I had expected, it features instead the jewel of 1980's technology: a pair of 74HC596 shift registers. Let's explore this puzzle!