I've pulled together a few spare parts that i've had laying about for ages now and I finally have a bit of time to try do something with them.

I've got a solar panel which puts out ~22vDC @ ~2Amps when its in decent sunlight, and i've got a 12vDC lead-acid car battery which i pulled out of my old car.

My plan is to hook up the solar panel to charge the battery and to have the system power a small Raspberry Pi computer 24/7 (assuming the battery lasts) which should draw no more than approx 10W.

I've seen some videos which make it look like all you need is a simple inverter and to plug the solar panel straight onto the battery, but I was under the impression you need a charging circuit to make sure you don't damage the battery?

Are there any detailed guides to doing this available or does anyone have any suggestions on how to go about doing this?

  • This question appears to be off topic, because this is a hobby project not a home improvement project.
    – Tester101
    Aug 30, 2013 at 11:15
  • Perhaps you could share a home automation project that the computer would be used in, to avoid the dreaded 'Hobby" label
    – HerrBag
    Aug 30, 2013 at 20:02
  • Bottom line is that you will need a charge controller. You can get one from the usual sources. Then you need a 12-to-5V converter. The cheapest way is to hook up a cigarette lighter socket and use one of those plug-in USB converters. Old batteries that are not in use are probably that way for a reason. Walmart has the nice little MS24 marine battery for a very good price. Please edit this question to explain how this will be used in your home so it can continue to generate answers. Even an RV can be considered part of your home for this purpose. Your home-away-from-home, as it were.
    – SDsolar
    Apr 1, 2017 at 6:14

3 Answers 3


Since you have an old car battery, the cost of damaging it by overcharging isn't much ($40?) and that would only happen if it got a lot of sun over weeks. A small home solar system might have thousands of dollars of batteries, so standard practice has long been for those to always have charge controllers.

For small systems the cost of a charge controller used to be significant so it was common to forego it in favor of simplicity. However, there are now very - inexpensive - controllers which no longer as easily justify the old practice.

If you can afford an additional $25, you might as well do it right. Also, it would come with instructions on what to do.

  • Those controllers look great (and idiot proof)! Thanks :)
    – NULLZ
    Aug 30, 2013 at 7:33
  • @NULLZ I use top shelf controllers because quality matters, and morningstar's sunguard is $30. Can't beat it. Oct 19, 2017 at 22:26

Get a small charge controller such as a Morningstar SunGuard (quality + low-end) to feed the solar panel into the battery. Competent charge controllers such as this top-shelf manufacturer unit are so cheap there is no reason to irresponsibly destroy a battery. Do not use garf off of eBay/Amazon/Alibaba.

Then feed the Raspberry Pi off the battery using an appropriate DC-DC power supply, such as automotive USB sockets sold at any gas station.

Do not under any circumstances use an inverter to make AC to run a wall-wart power supply to power the Pi. That is a double conversion loss you simply do not need, and will consume the lion's share of energy.

Your "10 watt" armwave should be nailed down to a more accurate number so that you can sensibly size your battery. A lead-acid battery should not regularly be dipped below 25% of capacity (i.e. It should bottom out at 75% full). This is a very old battery, so let's armwave 100AH and 10%. Assuming you can hold this Pi to 6 watt actual, that is 20 hours of runtime - that may be doable since your solar panel is I'm guessing 30W aimed well. The battery still seems undersized. A new 100AH batttery would suffice. Or a nickel or lithium pack that can be deeper discharged.


Of course I also recommend you a cheap, off-the-shelf charge controller, but for completeness: there are alternatives.

Charge controllers do not have to be of the PWM or MPPT type (basically the only ones commercially available), there are simpler designs – see Energypedia on Charge Controller Designs. If you wanted, you could certainly build one from your Raspberry Pi and a few electronics components.

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