I've had this posted at another forum as well, but i'd wanna run this by some experts on the topic here.
As the title sugguests, i tried making a simple UPS for a 12 Volts application. The specs are:
nominal current: 750mA
output voltage: 11~13.7V
The UPS should switch off when the battery voltage drops below 11V to prevent to battery from dying, and automaticly switch back on once the AC input is restored.
I came up with this schematic:
EXT_12V is an external 12 Volts line (actual voltage is more like 16V and should be followed by a 7812 for example). It is only present when AC power is plugged.
I would like to know if i made any mistakes.
I've created a schmitt trigger using an op-amp to prevent the UPS to start oscillating once the battery is almost empty, and the MOSFET is switched off.
I've calculated that the power supply switches off at 10.95 Volts, and stays off until the voltage over the battery returns to 12.95V or higher. This should be enough to compensate when the .75 Amp application is disconnected and battery voltage will rise.
The output of the LM317 is set at 14.4V to have the battery being charged at roughly 13.7 Volts (0.65V drop for the 1N5400)