Electronic – Designing 12v DC UPS

battery-chargingpower supplyups

My requirements are:
I need a UPS system which supplies power to a 9 volt modem. UPS should Charge 12v battery (SLA , charging curent 800ma), and it should have overcharge/discharge protection system [optional: it should show the battery capacity with four leds(25% interval)]. I want to achieve this without using ADC.
I can make 230V AC to 12v DC converter with step down transformer. Switching from mains to battery can also be achieved with two diodes. But i want to make overcharge/discharge with led indicator which i cant figure out without ADC. Is there any way to achieve this without microcontroller and ADC. Any suggestion would be appreciated. Thanks in advance.

I am thinking about something like this. I have designed this circuit on my own. But i have no idea how to link these two switches to charge level indicator.

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Best Answer

If you are concerned about trying to use one switch to change from (Charge battery, power device from AC) to (power device from battery,) then what you need is a single pole double throw (SPDT) switch.

It looks like this electrically:


simulate this circuit – Schematic created using CircuitLab

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