I found this circuit in an electrified fork lift. The battery was flat and I thought the charging circuit was damaged, as it would’t output any voltage when plugged into mains. After reverse-engineering it results that the circuit runs of the battery itself, but only if the battery is above some 8.5 volts. Also, if you forget to turn the kill switch off (not shown in the schematic) but leave it unplugged, it will drain the battery. The LED also only lights up when mains is plugged in AND the battery voltage is above 8.5V. I found it peculiar, because it uses a PUT (D5), a [programmable unijunction transistor](https://en.wikipedia.org/wiki/Programmable_unijunction_transistor) which was a first for me.
I found this circuit in an electrified fork lift. The battery was flat and I thought the charging circuit was damaged, as it would’t output any voltage when plugged into mains. After reverse-engineering it results that the circuit runs of the battery itself, but only if the battery is above some 8.5 volts. Also, if you forget to turn the kill switch off (not shown in the schematic) but leave it unplugged, it will drain the battery. The LED also only lights up when mains is plugged in AND the battery voltage is above 8.5V. I found it peculiar, because it uses a PUT (D5), a [programmable unijunction transistor](https://en.wikipedia.org/wiki/Programmable_unijunction_transistor) which was a first for me.