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I need to create a interruption-proof power for a project of mine, very similar to the design of this link.

My problem is that I need to adjust the current delivered of the solar panel (21V, 1.2A) to 5V and simultaneously charge a battery 12V, 18Ah. If the solar panel does not supply power, the battery takes its place.

Does anyone know any circuit to do something? Or can someone give me a light as do something?

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    \$\begingroup\$ What kind of battery? It could matter. I mean, is it a lead acid battery? \$\endgroup\$
    – user57037
    Feb 11, 2015 at 19:55

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There are many different ways to make an AC UPS/battery-backup. One of them is to always use the battery/inverter to power the load, regardless of the input power, and make the charging circuit capable of charging the battery faster than the UPS's rated load. This eliminates most concerns about switchover behavior, at the expense of some efficiency. You might do something similar with your solar rig.

Surely there's a 12V fast charger that can run from a solar panel...

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There are many different solar charge controllers for 12V lead acid batteries. The easiest thing would be to buy one of those, and connect it between your battery and your solar panel. Some of them also have a "load" output connection.

You can design a 12V to 5V buck converter to connect to the "load" terminal. Or you can connect it directly to the battery. If you connect directly to the battery, you should have some way to avoid over-discharging the battery. For example, design the buck so it switches itself off when the input voltage falls below 11V or 11.5V or something.

You can also design a solar charge controller based on various single-chip solar charge controller IC's. Take a look at sparkfun, Linear Technologies, Texas Instruments, and, I am sure, many others.

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