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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.

enter image description here

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  • \$\begingroup\$ What is the purpose of D1 and D2? D1 is useless, and D2 will prevent the battery from being charged at all. \$\endgroup\$ – Dmitry Grigoryev Jun 10 '15 at 17:21
  • \$\begingroup\$ This picture is just a reference to switching. This circuit does not explain charging mechanism. Thank you. \$\endgroup\$ – Arjun Jun 10 '15 at 17:24
  • \$\begingroup\$ So what is your question then? The simplest way to have a LED charge indicator is to buy one, the same is true for the charging circuit and DC/DC converter \$\endgroup\$ – Dmitry Grigoryev Jun 10 '15 at 17:48
  • \$\begingroup\$ If you are just trying to solve a practical problem, I suggest that you should just buy a commercial battery charger and connect it to your battery. Then run a DC-DC converter from the battery to supply 9V to your modem. The DC-DC converter should be rated to work from 16v down to around 11V. You can buy or design your own DC-DC. Instead of LED's to indicate battery level, just connect a low-cost panel meter to monitor battery voltage. \$\endgroup\$ – mkeith Jun 10 '15 at 18:21
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    \$\begingroup\$ You already have this question. \$\endgroup\$ – R Drast Jun 11 '15 at 11:42
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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:

schematic

simulate this circuit – Schematic created using CircuitLab

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