I've a 5v 2200mAH battery and a 12v solar panel and charge controller.

Is there any danger in charger my battery from the 12v panel/controller? The batter will then power my raspberry pi zero micro-controller.

What would be the recommended/ safest way to do this?

Many thanks,


  • \$\begingroup\$ How large is the solar panel? That is - how many watts? \$\endgroup\$ Apr 23, 2016 at 15:29
  • \$\begingroup\$ And what kind of battery? At 5 volts it's not a lead-acid, so direct charging is probably a bad idea. \$\endgroup\$ Apr 23, 2016 at 16:25
  • \$\begingroup\$ hi and thanks for the reply. The solar panel is 12v & 20w, the battery is here; trust.com/en/product/… \$\endgroup\$
    – Tullio_IRL
    Apr 23, 2016 at 17:37

2 Answers 2


What you refer to as a "battery" is actually a USB power bank, and you must not attempt to charge it with 12V!

The only safe method of charging that USB power bank, is a power source electrically equivalent to a USB port i.e. well-regulated 5V (I will see if I can find the minimum current requirement - check the USB power bank user guide for that information).

Edited to add: Based on the user manual for that USB power bank, it takes up to 5hrs to recharge, and has a 2200mAh capacity internal battery. Therefore it is unlikely to attempt to draw more than the expected 500mA from the external power source (which could be a standard USB port) when recharging its internal battery.

You mentioned "4200mAh" in your question - a typo of 2200mAh perhaps, as it doesn't agree with the webpage you linked?

Also note that not all (in fact I suspect a minority of) USB power banks can be recharged at the same time as also "charging" (i.e. powering) their output device. I did not find any mention of that feature on the web page which you linked - if simultaneously being charged and powering the Raspberry Pi Zero, was actually your intention.

  • \$\begingroup\$ Thanks for the advice, so I need a good 5v regulator between the charge controller and battery bank? Are these easy to find? \$\endgroup\$
    – Tullio_IRL
    Apr 23, 2016 at 17:49
  • \$\begingroup\$ Might this suffice? ebay.com/itm/… \$\endgroup\$
    – Tullio_IRL
    Apr 23, 2016 at 17:52
  • \$\begingroup\$ Yes, from its basic specification, the item you linked is indeed what you need - a "buck regulator" to convert the varying (but >5V) input, into 5V output. (I can't promised that the specific Ebay item works as advertised, of course!) I'm assuming that your solar panel charge controller doesn't have a "USB power output" itself? I see an increasing number which do... One final point: That Ebay item claims to "automatically identify iphone" - that may, or may not, work well when your USB power bank is attached. You may need to reverse-engineer the board to disable that unnecessary "feature". \$\endgroup\$
    – SamGibson
    Apr 23, 2016 at 17:59
  • \$\begingroup\$ @Tullio_IRL - Just FYI, I have updated my answer with a couple more details. It wasn't clear to me whether you intended to try charging that USB power bank at the same time as it powering your Raspberry Pi Zero, but that may not work with this specific USB power bank. \$\endgroup\$
    – SamGibson
    Apr 23, 2016 at 18:17
  • \$\begingroup\$ Thanks Sam, yes it was a typo, corrected it to 2200mAH. I'll get the regulator and see how it works out, thanks for your help :) \$\endgroup\$
    – Tullio_IRL
    Apr 23, 2016 at 19:08

Take a slightly different approach and you can use readily available products.

Start with a common 12 volt lead acid battery (e.g. gell cell, AGM, whatever is the right combination of price, size and availability).

Almost all solar charge controllers are made for 12V lead-acid batteries. I'm a fan of this $33 one for small jobs - not the cheapest but it's a top shelf manufacturer and the units hold up well.

From there, a common 12V-to-USB adapter... literally available at any gas station, drugstore or nicknack shop in the US.

This approach wastes the USB power bank you already have, but that is a packaged consumer product definitely not intended for this purpose, and I think you'll have serious problems trying to adapt it. Not least, lithium batteries are nothing to fool around with. Also you'd need it to supply while being supplied and it probably can't.


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