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Quick question, I am not very good with my electrical circuit terminology so I'm not sure what to call what I'm trying to do. When I try to Google "How to switch power supplies" I got inundated with results on switching power supplies, go figure.

I have a RPi that is powered via a car charger. (I found a car charger cable thing, the "female end if you will" at a garage sale, so I have that hooked to my power source!) The female car port... thing... is powered with 12 DC volts via 8 double A batteries in a battery pack. The car charger then powers the RPi. Runs quite a long time for what I do with it! However, I'd like to extend the running life of this.

My idea was to somehow have two alternative power supplies to the RPi charger, a battery pack, and solar panels! I picked up the panels to provide what I need. What I would like to achieve, is the solar panels and battery pack somehow hooked up through some sort of "switcher" if you will. I feel like this could be done with a voltage regulator and a transistor, just not sure how!

Anyways, I would like the solar panel to provide the power to the RPi when it is producing the required 12 volts or so, and then when there isn't good light and it stop producing the need voltage, the "switch" flips the charger over to the battery pack for power, until the panels can produce the 12 volts again, in which case it flips back over to the panel for the supply!

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    \$\begingroup\$ There are better people on stackexchange than me for solar stuff but two diodes can be a simple way of connecting two supplies to one input. Whichever voltage is higher is the supply. If you want to do something more interesting like use solar to charge the battery, that's more tricky \$\endgroup\$
    – carveone
    Jul 16, 2014 at 16:43
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    \$\begingroup\$ If you Google "ideal diode" you will find some IC products that are designed specifically for this sort of application. \$\endgroup\$
    – Dave Tweed
    Jul 16, 2014 at 17:03
  • \$\begingroup\$ Can your solar panel provide upwards of 500mA? \$\endgroup\$
    – Passerby
    Jul 16, 2014 at 23:01
  • \$\begingroup\$ @Passerby if memory serves, I bought 6 panels, 6 volts each at .5 amps each. Run three pairs in series, in parallel, should get me the amperage I need, if I run into issues i'll just grab another pair. \$\endgroup\$
    – 0xhughes
    Jul 17, 2014 at 2:38

3 Answers 3

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Connect a relay to the solar panel. When there is enough voltage, the relay will flip and power will be provided by the panel.

As said above, regulate the voltage to 5V before it enters RPi but it seems that you are already doing that.

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  • \$\begingroup\$ The only relays I worked with in the past were pretty bulky. Do they make tiny relays for smaller applications? \$\endgroup\$
    – 0xhughes
    Jul 17, 2014 at 2:44
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    \$\begingroup\$ Of course, use search engines. Look for latch-on and latch-off voltages. Look for coil current needed for latch-on. Look for how much current they can carry. And then compare those to what you need. \$\endgroup\$
    – nvd
    Jul 17, 2014 at 12:17
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You can't just connect a solar panel to the rPi. You will need a controller that regulates the solar panel voltage. Otherwise in too much sun your panel could spike up well above 12V.

I think you can parallel the following. 12V car charger you already have and a 12V regulator. The input to the regulator will be a solar panel. You may or may not need a diode depending on the controller. If it is sunny enough and your panel is sized appropriately it should power your device. You need to understand your current requirements though to choose the correct panel.

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    \$\begingroup\$ Most car usb chargers are based on common switching ICs (MC34063 for one), which have a VERY wide range of input voltages (up to 40V). And automotive power systems range from 12~16V when on, 14V nominal, plus spike when turning on. The same car charger can work with the solar panel. \$\endgroup\$
    – Passerby
    Jul 16, 2014 at 22:58
  • \$\begingroup\$ Mirroring what passerby said, the car charger can handle wild variances in input. Now granted the one I have doesn't like to go over 20 (may or may not of fried one already). Might be a good idea though to put a 12-14 volt regulator inline if the solar panels are really wild in hot sun. \$\endgroup\$
    – 0xhughes
    Jul 17, 2014 at 2:42
  • \$\begingroup\$ Make sure to go for a switching regulator or a linear one with good heat sinking so that in the worst case, maximum voltage input and maximum current draw, things do not fry up. \$\endgroup\$
    – nvd
    Jul 17, 2014 at 12:19
  • \$\begingroup\$ I like to err on the side of caution. Not knowing the ratings of the charger IC nor the solar panel is just risky unless you know the solar panel won't put out more than 20V or the IC can take much more than that. Especially with all these cheap 12-5V car chargers coming from china. \$\endgroup\$
    – ACD
    Jul 17, 2014 at 13:04
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This is about as simple I would care to get, the Ideal Diode.

Solar Panel Charging a Battery

Or you could just have the battery pull down the solar panel output (typ. 18V) down to 12-14V. NOT RECOMMENDED

enter image description here

IF YOU WANT TO DO IT RIGHT
Use a Peak Power Tracking Buck Solar Charger to Maximize Efficiency

enter image description here

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  • \$\begingroup\$ The Peak Power Tracking Buck Solar Charger looks intense :) \$\endgroup\$
    – 0xhughes
    Apr 1, 2017 at 15:18

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