Solar panel connected to usb socket

lightseeker2009 · Sep 10, 2017

I've got a 10 Watt solar panel that I want to connect to a 12V adapter set, the kind you put in your cars lighter socket. The panel is gathering dust for more than 10 years so I want to make use of it. My concern is the following.
It gives up to 20 volts of power, way more than the 12V its rated for. Will giving the 12 volt cigarette adapter this higher voltage cause any damage at all?
Will the max volts at the usb output not go over the limit for a USB outputs? Will it be safe to charge ipads etc this way directly from the usb output when the cigarette socket adapter receives up to 20 volts from the solar panel?

hiuintahs · Sep 10, 2017

lightseeker2009 said:
I've got a 10 Watt solar panel that I want to connect to a 12V adapter set, the kind you put in your cars lighter socket. The panel is gathering dust for more than 10 years so I want to make use of it. My concern is the following.
It gives up to 20 volts of power, way more than the 12V its rated for. Will giving the 12 volt cigarette adapter this higher voltage cause any damage at all?

A 10 watt solar panel will barely raise the voltage of your car battery. At the maximum power point of the 10w panel which is around 18v, well that amounts to about 0.55 amps (but only at ideal, pointed straight at the sun conditions). 0.55 amps is the max that panel can put out which isn't a lot of current for a car size battery. And that won't raise the voltage on your battery very much to begin with. If left there for days, you could end up over charging the battery but it would take awhile. At first what will happen is your battery will drag the voltage of the panel down to a level just above the sitting voltage of your battery that represents the needed difference over and above the existing battery voltage that will make the solar panel current flow into the battery. It's an Ohms law thing where voltage divided by resistance = current. The current is fixed due to the max ability of the panel and the resistance is fixed. Since the current is fairly small and the resistance (in the battery and the wires) is fairly small.........well that equates to a small delta voltage over and beyond the present battery voltage. But if left unchecked you could over charge the battery eventually. That is why charge controllers are needed with solar setups.

lightseeker2009 said:
Will the max volts at the usb output not go over the limit for a USB outputs? Will it be safe to charge ipads etc this way directly from the usb output when the cigarette socket adapter receives up to 20 volts from the solar panel?

Not sure what you mean here. A USB output that is derived from a 12v source has its own control electronics that regulates the output to 5v regardless of the variance of the input. Keep in mind when the engine is running with an alternator on a car, the battery is sitting at around 14.5v. So any automotive USB sources derived off of that 12v source (which can vary) are regulated to hold a constant 5v output. Hope that answers the question.

lightseeker2009 · Sep 10, 2017

hiuintahs said:
A 10 watt solar panel will barely raise the voltage of your car battery. At the maximum power point of the 10w panel which is around 18v, well that amounts to about 0.55 amps (but only at ideal, pointed straight at the sun conditions). 0.55 amps is the max that panel can put out which isn't a lot of current for a car size battery. And that won't raise the voltage on your battery very much to begin with. If left there for days, you could end up over charging the battery but it would take awhile. At first what will happen is your battery will drag the voltage of the panel down to a level just above the sitting voltage of your battery that represents the needed difference over and above the existing battery voltage that will make the solar panel current flow into the battery. It's an Ohms law thing where voltage divided by resistance = current. The current is fixed due to the max ability of the panel and the resistance is fixed. Since the current is fairly small and the resistance (in the battery and the wires) is fairly small.........well that equates to a small delta voltage over and beyond the present battery voltage. But if left unchecked you could over charge the battery eventually. That is why charge controllers are needed with solar setups.

Not sure what you mean here. A USB output that is derived from a 12v source has its own control electronics that regulates the output to 5v regardless of the variance of the input. Keep in mind when the engine is running with an alternator on a car, the battery is sitting at around 14.5v. So any automotive USB sources derived off of that 12v source (which can vary) are regulated to hold a constant 5v output. Hope that answers the question.

Thank you sir. I agree with your point when its connected to a car battery. But in this case I will not connect it to my car battery, it will function on its own without any assistance from a battery.

hiuintahs · Sep 10, 2017

Oh, I reread and I think what you want to do is put a cigarette lighter adapter (ie: female receptacle) on your panel but not insert it into the car but connect that to a car charger input. The voltage that is then going to the USB car charger will be somewhere below that the panels max voltage (ie: 20v) because the car charger will draw the panels voltage down depending on the load that the car charger is dishing out. If you know what the max rating of the car charger input is, then you can do it with assurance that nothing will happen to the car charger. The only time the car charger will see the highest voltage from the panel is when you have no load on the USB output. You just have to make sure it can handle that input level. But the USB output should be regulated to 5v regardless of what the input voltage is. That is unless something burns out and then somehow there isn't isolation between input to output.

lightseeker2009 · Sep 10, 2017

hiuintahs said:
Oh, I reread and I think what you want to do is put a cigarette lighter adapter (ie: female receptacle) on your panel but not insert it into the car but connect that to a car charger input. The voltage that is then going to the USB car charger will be somewhere below that the panels max voltage (ie: 20v) because the car charger will draw the panels voltage down depending on the load that the car charger is dishing out. If you know what the max rating of the car charger input is, then you can do it with assurance that nothing will happen to the car charger. The only time the car charger will see the highest voltage from the panel is when you have no load on the USB output. You just have to make sure it can handle that input level. But the USB output should be regulated to 5v regardless of what the input voltage is. That is unless something burns out and then somehow there isn't isolation between input to output.

:thumbsup:

hiuintahs · Sep 10, 2017

Well I sure answered that wrong in post #2. I'll leave it alone in case someone ever thinks of doing that with a small solar panel.........they might learn something from it.

BattBoi · Sep 10, 2017

Hi,

I'd suggest you buy a special buck converter that not only steps the output voltage down, but is also able to control the input voltage so you can manually control the maxium power ponit of your panel. This allows you to fully untilize the power of your solar panel. I bought this unit https://www.ebay.com/itm/MPPT-Buck-...099737?hash=item2a6ca0bc59:g:dpgAAOSwdzVXjfYx It works really well with 10w panels. Only the led display on my model has a loose connection, which makes the current reading inaccurate. But you can take the display off or even buy a unit without the display.

terjee · Sep 10, 2017

A lot of car cigarette to USB support either 12V or 24V systems. If you get one of those, you're pretty much done.

Quality of such things vary a great deal. Anker generally has good quality, but I haven't checked them for 24V.

LEDAdd1ct · Sep 10, 2017

See if this thread here helps.

lightseeker2009 · Sep 10, 2017

Thanks everyone, I got some really good advice:twothumbs

terjee · Sep 10, 2017

Quick addition... there will probably be some differences in efficiency of different USB chargers, and how they behave if the panel has less than ideal conditions. Might be worth testing a bit.

terjee · Sep 10, 2017

Sorry for commenting multiple times...

If it's rather for 12V, but giving 20V, that's probably because it was measured with no load, and it would probably be closer to 12V if connected to anything.

If this is for somewhat stationary use, I'd consider adding a small or even tiny SLA, then connect both charger and panel to that. It'd give you a "pool" of energy to keep the charger running smoothly, would serve as a buffer that the panel can dump energy into, and generally make everything run much more smoothly, and allow you to capture energy all the time. Just be slightly careful with regards to overcharge.

lightseeker2009 · Sep 10, 2017

terjee said:
Sorry for commenting multiple times...

If it's rather for 12V, but giving 20V, that's probably because it was measured with no load, and it would probably be closer to 12V if connected to anything.

If this is for somewhat stationary use, I'd consider adding a small or even tiny SLA, then connect both charger and panel to that. It'd give you a "pool" of energy to keep the charger running smoothly, would serve as a buffer that the panel can dump energy into, and generally make everything run much more smoothly, and allow you to capture energy all the time. Just be slightly careful with regards to overcharge.

I agree. Its not a bad idea to get a small lead acid to complement this. But don't you get a powerbank that can connect to a solar panel? Not the one's with a built-in solar panel, but one that can connect to a stand alone solar panel.

degarb · Sep 10, 2017

I took a toolbox, threw in a 12 or so ah agm, glue to inside, charging using a mere 5 watt plastic protected solar panel charger, a smart ac charger, and a buck 5v usb, with at least dual usb. I am guessing the buck 5v better efficiency than the car 5v you find in stores. https://www.amazon.com/gp/your-acco...ageId=1&returnSummaryId=&shipmentId=DpYnzPZr4

Over charging my agm is a concern, but doubt if will hit that threshold in my area. Also, using deep cycle. Got an 18 amp hour deep for $36 and a 12 ah for $25,as i recall.

Only need a motion detection pir that works on Bluetooth to best complete my project.

Lifespan of a cheap solar panel chip is dubious.

terjee · Sep 10, 2017

lightseeker2009 said:
I agree. Its not a bad idea to get a small lead acid to complement this. But don't you get a powerbank that can connect to a solar panel? Not the one's with a built-in solar panel, but one that can connect to a stand alone solar panel.

Virtually all 12V/SLA setups support charging and using at the same time. Virtually no power banks support it, at least officially. Some even block it mechanically or electronically, others discourage it in the manual, and so on.

Then there's also the sensitivity to low voltage with cloud cover for example.

A straight connection from a solar panel to SLA should "just work" most of the time, the voltage of the battery will keep the panel in a good range, and the current from the panel feeds into the battery. 5V-systems are much more complicated and prone to hiccups etc.

It's a good point though, feeding directly to a power bank would be much more efficient (in theory) than feeding an SLA (you'd get out a higher percentage of what you put in), but it's much harder to find the right parts and verify everything. I'm still looking for the perfect power bank for such a setup, still haven't found anything (if we're limited to "normal" power banks).

degarb · Sep 10, 2017

I did buy a few $20 10 ah BlackWeb power banks from Walmart--which could supply and charge at the same time, unlike the Anker supplies. I put my 12 to 5v buck to charge the Blackweb, which has 2 usb outs. The key spec was to be able to charge via solar. (The power banks have a button, which can be accidentally pressed and turn off. Which contributes to the power banks adding random failure in my system.)

I did buy some generic 8ah usb phone charger with built in solar charging. So, apparently, someone is doing it. I just didn't feel I had enough knowledge currently to build a safe lithium solar charged setup. Also, deep cycle advanced glass mat is much cheaper, with massive watt hours (18x12=216 watt hours + 50 watt hour Blackweb (probably, lying so 20 wh) x %90 buck efficiency to 5 volt, then internal losses to electronic from 5 v).

terjee said:
Virtually all 12V/SLA setups support charging and using at the same time. Virtually no power banks support it, at least officially. Some even block it mechanically or electronically, others discourage it in the manual, and so on.

Then there's also the sensitivity to low voltage with cloud cover for example.

A straight connection from a solar panel to SLA should "just work" most of the time, the voltage of the battery will keep the panel in a good range, and the current from the panel feeds into the battery. 5V-systems are much more complicated and prone to hiccups etc.

It's a good point though, feeding directly to a power bank would be much more efficient (in theory) than feeding an SLA (you'd get out a higher percentage of what you put in), but it's much harder to find the right parts and verify everything. I'm still looking for the perfect power bank for such a setup, still haven't found anything (if we're limited to "normal" power banks).

degarb · Sep 10, 2017

I figure 200wh is just about enough juice to power my smart phone for most of the work day, at least. Probably, wont be true with next generation of phone, with their glued in batteries.

Timothybil · Sep 10, 2017

First, a 12v to USB car adapter can routinely see and handle up to 14v when the car battery is charging after a serious discharge like starting the vehicle, so the output of your panel should not pose a problem there.

Off the wall idea. I am assuming that your panel does not have the associated circuitry to output USB level power. If that is true, you could always purchase a second panel that does have such circuitry, and parallel the output of your first panel into the circuitry of the second. At least, it would be interesting to try it and see what happens.

Solar panel connected to usb socket

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