Electronic – Power-Supply “Combination”: Solar for voltage + Battery for current

batteriespower supplyraspberry pisolar cell

I'm rather new to circuits & DIY electronics, but am thoroughly enjoying tinkering with my new RaspberryPi and assorted components. I'm trying to power the device using a few simple solar panels I stripped from cheap LED lawn lights from Menards.

I've combined three 2V solar units in series to output 6V total. The RasPi (B+ model) device needs 5V @ 800mA (including some components I've added). While I expect steady, consistent lighting (will be inside under relatively bright fluorescent lights, but I haven't gotten to measure their luminosity or anything yet), I believe the solar panels can only put out a couple hundred mA.

I have a couple old Samsung Galaxy batteries, which put out 3.7V, 1650mAh. My thought is that a charged battery (or perhaps two in parallel) could provide sufficient current, while the solar panel could keep the voltage in the circuit high enough. But my understanding of the intricacies of circuits (especially many elements in combination) is really limited, so I'm not sure this is possible.

My hope is this: Fully charged, two Samsung batteries in parallel could provide 3300mAh at 3.7V, which could supply the RasPi's needed 800mA for just over 4 hours. But the voltage is too low, so I hope I could "combine" the battery's current with the solar panel's 6V voltage supply "upstream" of the RaspberryPi.

I also know that using the solar to charge the battery is a whole other question, given the RasPi's power needs. Even without any load, at 1650mAh, each battery would take 11 hours to charge, or 22 for both! So I'm not too concerned about that just yet. But I'm curious for now to see what's possible with the pieces I've got.

Thanks in advance for any help or advice! Let me know if I can clear up or expound on anything I said.

Linking two posts I found that I think are related:
1) Multiple Power Sources Each For a Certain Purpose
2) Arduino Project Powered by Solar or Battery

I'm not entirely certain whether they address my questions, but I'm working on it.

Best Answer

Don't even think about creating the setup you just described. It is bloody dangerous.

If you wire the "solar cell pack" and the two battery packs in parallel without connecting the Raspberry Pi, you'll get a loop. Kirchhoff's second law explains that the sum of voltages around a loop must be zero. In this case, if you start going around the loop in one direction, you'll encounter the two power sources with opposite directions, so now their difference must be zero - so they must be at an equal voltage. Will this be 6V (dictated by the solar panels) or 3.7V (dictated by the battery packs)? The following will happen:

  • Initially, without sunlight, the common voltage will be 3.7V. No current flows, since the solar panel does not let current flow backwards (its resistance goes near infinity). All is well for now.
  • Then you apply sunlight. The solar panels try to increase the voltage to 6V, but at this voltage the batteries would allow through much more current than they can supply. So the panels drop their voltage to 3.7V, but still begin to charge the batteries with the couple hundred milliamps they can supply, until the voltage in the battery packs reaches 6V. And there's a pretty good reason the batteries are rated at 3.7V.
  • If I learned anything about Li-ion and Li-polymer batteries is that they are very easy to upset. And they especially don't like being overcharged. If they are indoors, they will blow up your desk and burn your house down. If they are oudoors, they'll happily ignite the grass around them. Then burn your house down. Li-ion and Li-polymer batteries are not toys. Don't even think about putting them in a circuit where there's even a slight chance they'll get overcharged.

Connecting the Raspberry Pi before the detonation wouldn't work out well either. The 3.7V combined power supply is not enough for the Pi, which will then do one of the following (I'm not familiar with the Pi's power supply circuit):

  • Pull a lot of current overheating the battery, the solar panel and maybe even its own on-board voltage regulator. It will not boot, or even if it does, it will frequently crash and reboot because of the inadequate voltage. This goes on until one of the components fail: if it's the solar panel, you're safe. If it's the Pi, it's the time bomb scenario all over again. The battery packs also don't like being over-discharged, but as far as I know, they don't burst out in flames then. They just don't work anymore.
  • Don't pull any current at all. Then it's like you didn't connect it at all. Time bomb again.

Creating a circuit which safely combines solar and battery power requires advanced electronics skills and dedicated circuitry. In your case, I would follow S.J. Becker's advice (+1) and buy a (solar powered) power bank from eBay. The circuitry is there, pre-made for you and it can power your Raspberry Pi longer than your setup would have even if it worked. I know they are not as cheap as using things from your parts bin, but they are definitely cheaper than replacing your burnt furniture.

Additionally, does the thing have to be solar powered?

Edit: Some battery packs have built-in protection circuitry that shuts the power off if the battery is overcharged, so there's a chance your setup won't actually ignite but just not work at all.

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