Other designs

Using a solar panel for USB charging.

Solar panel to USB cable

This circuit, well, thingy, is intended for charging devices or batteries with a solar panel through a USB port. If there is no sun, you can use this emergency charger.

Rather than designing something myself like this SLA solar charger, I decided to reuse the innards of a standard USB car charger.


I wanted to put something together that would go between my 10W 12V solar panel, which has a jack plug attached to its cable, and a device that can be charged using USB. For this, I needed the following ingredients:

ISY Car charger GP Car charger Neutrik Cable jack

So, I went out and bought two cheap car chargers for testing, the ISY® IPO 1200 Car Charger, made, or at least distributed in Europe, by Imtron, and the GP® AutoPower 2.1A Car Adapter.

I also got a sturdy cable jack with a locking mechanism, but if you're not afraid of wear and tear, or stepping on it, you can use a cheapo one.

Comparing the car chargers.

After opening and removing the housing of the ISY car charger (in a totally non-destructive way, really, it can be done), I found a small PCB which, to my delight, had an ACT4065SH IC on it. This means they use a proper DC/DC converter.
The GP car charger uses an IC marked 309B1 P3453 or 30981 P3453, I couldn't tell which. I can't seem to find what IC this is (anyone?), but the PCB holds a coil, a diode and an elco in a typical buck converter configuration, so this one also uses a DC/DC converter.

The GP car charger has two USB outputs and can deliver up to 2.1A, the ISY car charger has one USB output and delivers 1.2A. For my intended use, this was not an important difference.

I noticed that the ISY car charger doesn't start up when the sun comes up, no matter how bright the sun starts to shine. It seems to need a more sudden transition to 12V on its input to start. This is perfectly OK for its intended use, but not very convenient for use with a solar panel; getting it to work would require disconnecting and reconnecting it every morning.

Also, just after sundown but under a bright sky, there was a period of at least 90 minutes when the GP car charger would cheerfully put 100mA into my battery, while the ISY could not even get its status LED to turn on. So, either the GP car charger is far more efficient, or the ISY charger has a "power good" circuit that prevents it from starting up when a car battery's voltage is too low. That would be a nice feature to have in a car charger, but for use with a solar panel, not so much.

I didn't bother measuring the output currents of both car chargers to establish their relative efficiencies, as the above disqualifies the ISY car charger for my purpose.

Putting it together; some disassembly required.

I took out the PCB of the GP car charger and cut off the bits I did not need (a small spring on the +12V input and two bits of metal on the sides connected to ground). You can make it slightly more efficient by also cutting the LED off, but I decided to keep it. I like blue LEDs.

I then connected the soldering pads on the PCB to the cable jack with some wire and wrapped the whole thing in heat-shrink tubing for protection and waterproofing.


Solar Panel Characteristic

Note that the input impedance of the car charger's DC/DC converter is not controlled. This means it will present an input impedance to the solar panel that pulls the solar panel well away from its Maximum Power Point. So, don't expect a 10W at 12V panel to deliver anything near 10W/5V = 2A on the output side of the car charger. This has nothing to do with the efficiency of the DC/DC converter used, but with the characteristics of solar cells.

Measurements show that the voltage on the input of the car charger drops to a little over 5V under load because of this impedance mismatch. To the right you can see how much power this set-up isn't using: the blue area is the maximum power the solar panel I use can deliver under ideal circumstances (0.57A at 17V is about 10W), in red the power that is actually finding its way into the car charger. Under full sun I get 0.61A at 5V on the output of the car charger, just over 3W.

Adding a MPPT or using a lower-voltage solar panel would greatly increase efficiency.

Adding a buffer.

GP Power Pack

I decided to add a battery pack to the set-up, and bought a GP 10.4Ah portable PowerBank, as they call it. It also has blue LEDs.

It charges well with the solar panel set-up as described above, and it will charge my cell phone and camera, and provide light when I need it.

In the manual, it says you shouldn't charge the battery pack while it is supplying current to your devices, i.e. do not charge with a load connected. Interestingly, this warning was included in some language versions of the manual, like English and Dutch, but not in others, like German and French. I decided immediately to try what I wasn't allowed to do in Dutch but could feel free to do in French, and I found no problems with charging the pack while it was charging my cell phone. I did not measure the currents, and I don't know if the presence of a load throws off the charging algorithm, so try this at your own risk if you're unfortunate enough to have the battery pack's functionality limited by your mother tongue.

Kidding aside, I contacted GP about this, and they said the warning should be there in all languages, and gave the reason for its inclusion. I will not repeat the answer here, but it didn't have me worried about the magic smoke escaping. It is not often a company answers technical questions about consumer products this quickly, or at all. Kudos to GP.


I do not speak for Imtron or GP, nor do I have an interest, financial or otherwise, in any of these companies.

You build everything at your own risk, no functionality is implied, the resulting apparatus might not work, be unfit for drying pet dogs, and contain small parts that could suffocate children when swallowed or inhaled. Also, you will most certainly be voiding warranties when you build this.

This page was created by Oscar den Uijl, oscar@den-uijl.nl