I am looking for something even simpler than this, with low power consumption (so no relay):
I need a circuit which can be built from basic components which can cut the battery completely off in case of the voltage dropping below 3.5 V, let's say.
I got a solar panel plus a charging circuit to charge a small 2000 mAh battery. At night there is another circuit which turns on and blinks some LEDs. The problem is that this circuit has a too high a standby current even during daytime, so it never lets the battery charge.
Any suggestions are welcome. I'm sure I can order many off-the-shelf circuits from Ebay to do this, but I would like to do it on the weekend.
Screen capture schematic from linked video: