DIY 12V powerwall

[Faisal A]

So, I want to make a device that takes both solar 12V and mains 12V in an uses solar 12V to power my router. When the solar battery is nearly finished, I want it to use my mains 12V. Does anybody know how I would go about doing this without an arduino or microcontroller.

[Levent]

What you are trying to do is a lot more complex than you actually think. I assume your solar panel is 12V DC, mains is AC. You will need a charge controller and a rectifier and another controller for switching between solar and mains. Electricity is no joke, especially mains.

[Faisal A]

I have 12V mains DC @Levent (I use an adaptor)

[mariushm]

What router do you have?

 

Your router works internally with 3.6v or less.  It has a switching power supply inside which converts 7.5v ...12v to lower voltages like 3.3v for the processor , 2.5v for Flash memory, 1.8v for RAM (if any).

12v is used simply because it's cheaper for the router manufacturer to buy thousands of standard 12v wallwart adapters and reuse same wallwart adapter on lots of models and devices. Also, it's cheaper and easier to use 12v 1A (12w) instead of 5v 2A (10w) ... there's lower losses on the wire between the adapter and the router and parts used in the 12v wallwart can be a few pennies cheaper.

 

Anyway....

 

You can use diodes to auto switch between sources - whatever voltage is higher will win.

 

12v in ----> diode --->  device

panel -----> diode --/

 

If panel voltage + voltage drop on diode is above 12v from psu, it wins and powers device.

If voltage goes below 12v - voltage drop on  diode, then the 12v input will win.

 

A schottky diode will have ~ 0.15v drop, and that's low enough your router will not care... your router will most likely work with 10v ..14v, will probably work with as little as 6-7v

 

There are "ideal diodes' which have a voltage drop in the mV range, but they're really not needed. Here's an example.

 

I have a problem understanding what you actually have... do you have a solar panel with a built in circuit and some battery that makes the panel always output 12v, or do you have a plain solar panel which outputs some random voltage?

 

If it's the first, the panel will have a battery and a regulator inside and it will output 12v up to some current amount all the time, until the internal battery is discharged (if the panel can't produce enough power)

If it's the later, you don't have 12v, you'll have something that will fluctuate between some low voltage and some high voltage and depending on how cloudy it is outside the maximum current the panels provide can vary a lot.

You would need to regulate that, either by using a battery or by using some capacitors at the very least.

 

I don't see how you would do without a microcontroller or some smarts.

 

My advice would be to open up the router and figure out (using a multimeter for example) what's the output voltage of the voltage regulator that converts 12v into something else, and then you can disconnect that voltage regulator and use your own regulator to produce that voltage from a lithium battery (your energy reservoir) or from mains.

 

You could use a cheap lithium battery charger like MCP73831 to charge a battery and then use 3v..4.2v to produce 3.3v or whatever the router needs.

A cheap microcontroller can monitor the battery voltage and when it drops below some threshold, like 3.2v for example, it can switch to DC In and lets the battery charge up from solar