I'm trying to build a schematic which will turn on a relay in case motorbike battery voltage will reach some level. I've ended up with the following schematic.
Input voltage can vary from 12 to 14.5 V. So the plan was to compare half of the input voltage from the 1:1 divider to the Zener voltage, which is always 6.8V. So when input voltage exceeds 13.6 volts, the transistor should energize relay coil. However there is a hysteresis in relay switching, from 10.1 to 12.8 volts. It switches on at 12.8, and then releases at 10.1 volts. What is the reason of that, and how can I adjust hysteresis loop to my needs?