The following explained circuit of a DC UPS can be used for providing back up power to modems or routers during mains failures, so that the  broadband/WiFi connection never gets interrupted. The idea was requested by Mr. Galive.

Technical Specifications

I need a circuit like,
I have two 12v dc adapter(600mA and 2A).
When input Mains is present, with the 600ma adapter i want to charge the battery(7.5AH) and with the 2A adapter i want to use my wifi router.
when the AC mains fails the battery will backup my wifi router without UPS.
MY modem is rated as 12V 2.0A. That is why i want to use two 12v dc adapter.

The Design

Two adapters actually are not required for the proposed application. A single adapter, probably the one which is being used for charging the laptop battery may be used for charging the external battery also.

Looking at the given DC modem UPS circuit diagram we can see a simple yet interesting configuration involving a couple of diodes D1, D2, and resistor R1.

Normally a laptop charger is specified with 18V, so for charging a 12V battery this needs to be lowered to 14V. This is easily done using a transistor zener stage.

When mains is present, the voltage at D1 cathode is more positive than D2, which keeps D2 reverse biassed. This allows only D1 to conduct, supplying the voltage from the adapter to the modem.

D2 being switched OFF, the connected battery starts receiving the required charging voltage via R1 and begins getting charged in the process.

In an event AC mains fails, D1 gets switched OFF, and therefore allows D2 to conduct, enabling the battery voltage to instantly reach the modem without causing any interruptions to the network.

R1 must be selected depending upon the charging current rate of the attached battery.

A much better and improved version of the above is shown in the following diagram:

router modem DC UPS circuit

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