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Relationship between volts and amps

I'm working on a project found in this thread
https://www.instructables.com/community/Solar-powered-cell-phone-charger-case-hack/

I think I'm almost ready to buy parts and start building but I have a small issue with getting the exact voltage and amperage output I need. I've been researching for hours and I can't find a simple answer to this problem.

As I understand it, as voltage decreases, amperage increases to compensate and maintain wattage. Working on this basic understanding I've come up with this model.

If I have a solar array consisting of 6 PV cells (1.5V 50mA ea) wired together I get 9V and 300mA. This is too much voltage and is on the low end of amps I want. If my math (and understanding of the subject) is correct, then if I use a 5V voltage regulator I will have a final max output of 5V and 433.3 mA which is right about perfect.

Am I doing this right or is there more to it, like how I'm wiring the cells together? Am I correct in the idea that using a voltage regulator will increase my amp output?

I'm not schooled in electricity in any way so I would appreciate any help I can get on this in as simple of a form as possible. Everything here I've put together from several hours of studying sites on solar DYI and electricity. Thanks.

For reference, these are the solar cells I was referring to: http://cgi.ebay.com/High-Efficiency-Solar-Cell-32-22mm-1-5V-50mA-0-075W-/320541888508?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item4aa1c90ffc#ht_2428wt_1148

*EDIT* Below is a diagram of what I suspect may give me a 5V/490mAh output which would be absolutely perfect. I need to know if I can wire two different sizes of solar cells together in this manner and how it will effect the output.

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kelseymh6 years ago

Note to other people commenting: The author here is not talking about Ohm's law.

Photovoltaics have a complex non-linear relationship between their intrinsic voltage, the load to which they're connected, and their current output. See, for example, the Wikipedia article on solar cells and their theory of operation.

If you've some practical guidance on how to build a good PV circuit, and can summarize the I-V behaviour under more-or-less constant conditions, that would be of great benefit.

The BEST way to extract energy from a solar panel is called a "Maximum Power Point tracker" or MPPT. Its essentially a switch mode regulator which changes its INPUT impedance to be the best load for the panel at the current light level, but it boosts its output voltage to a constant level
HittingSmoke (author)  kelseymh6 years ago
I read the article and it's quite outside of the scope of my electrical understanding. For reference this is what I was reading to understand the volts/amps relationship for solar cells
http://www.freesunpower.com/watts_power.php

"Well, for one thing it becomes clear that an appliance (load) that draws 1 amp (ampere) of current at 120 volts will draw 10 times as much current at 12 volts (1/10 the voltage) or 10 amps."

I think what I'm going to have to do is pick up four cells and a voltage regulator tonight and try wiring them together in different ways then measuring the output. Even if I don't understand the science behind it I can get a working model to give me an idea of how to attain what I need.
gmoon6 years ago
I'm no expert on solar cells, but a (linear) voltage regulator alone will not increase the available current (potential amperage). The regulator itself is a load, and simply converts excess voltage to heat.

BTW, six cells in series don't give you 300mA @ 9V. You'd get 50mA @9V. Six parallel cells output 300mA @ 1.5V. A combination of  four cells in series can output approx 6V, with more cells parallel, for more available current.

A 5V LDO regulator can work with as little as 5.5V. There are other, more complex IC / power supply circuits that more or less output a fixed 5V, even if the supply drops below 5V...

IFAIK, solar cells still have a rated voltage / amperage (in specific light conditions), which interacts via Ohms law, similar to other power supplies. Of course, there's lots of other considerations with solar technology... For instance: in series, cells which are shaded can become reverse biased and usually have a shunt diode connected to bypass that cell.
HittingSmoke (author)  gmoon6 years ago
Thanks a lot for the reply. I just learned about the difference between wiring in series and parallel tonight. I bought some cheap solar lights and broke the panels out of them. I could get the voltage right pretty easily but I couldn't measure amps. I'm not sure if I was using the meter wrong or if I just needed it to get a working charge to draw amps before testing.

I think I may have found a solution assuming 140mA would be enough to keep a charge in my phone. http://cgi.ebay.com/High-Efficiency-Solar-Cell-54-54mm-5-5V-70mA-0-385W-/220615140528?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item335dafe4b0#ht_2380wt_1148

Two of these wired in parallel would provide 5.5 volts and 140mA. Since 100mA is the minimum spec for bus provided power over USB I'm guessing that should be at least enough to hold a charge while using the phone, and enough to charge it while idle.

http://www.radioshack.com/product/index.jsp?productId=2062599
I will don't quite understand the regulator issue. I bought the above 5V regulator from Radio Shack. If I wired two of those cells together and put the 5V regulator on it, would that be safe or would 5.5 volts be a tolerable USB voltage?
That regulator is just what you need. Unless you can find the charging specs for your phone....hmmm....Google...nothing! Wait, instead of "EVO charging spec", what about "USB charging spec"? Ah, ha!

http://www.usb.org/developers/devclass_docs/batt_charging_1_1.zip.

Open the ZIP file, and you'll find both the charger spec PDF and a license agreement.

Look at page 19 (section 3.5), and you'll see that in fact a USB charger can supply up to 1.5A to a device for charging. Page 33 (table 5.1) shows all the limits, and the charger voltage is only allowed to be 4.75 to 5.25 V. You'll need the 7805 regulator in your circuit.

You will need to check your phone's battery (or search Google) to see what current it really needs in order to charge. The 140 mA your PVs supply might not be enough.

HittingSmoke (author)  kelseymh6 years ago
The articles I found about USB listed a bit different specs about the USB amperage allowances. 500mA for a powered hub, and a minimum of 100mA straight from the PC.

I searched for hours for required charging amps but came up only with USB specs. The battery is an 1800mAh battery. Doing a search for anything related just comes up with batteries for sale. Is there some formula for how many amps are needed to charge a battery if you know the mAh?
This article doesn't address the question about input current for your battery, but I think you may find it useful in discussing issues of "how to charge." 

As long as the input voltage is sufficiently higher than the battery's output voltage, you'll get charging.  The current you supply, combined with the battery's total charge capacity (1 mAh = 3.6 coulombs) just tells you how long it will take to charge.
HittingSmoke (author)  kelseymh6 years ago
For a little more clarification on my last reply, I've made a diagram of what I'm talking about. How would the 1.5V cells effect this? Would this give me a 5V output with 490mA or is running the cells parallel in this manner even possible?
HittingSmoke (author)  kelseymh6 years ago
Hmm ok, so I don't *need* to hit a certain amperage to charge the battery, I only need to be supplying more amps than is being drawn from the battery to give it a positive charge is what I'm hearing there. That was where the confusion was, I was under the impression I needed a certain level of amps to get power to flow into the phone.

I can only fit two of the 70mAh cells onto a case unfortunately. Would it be possible to throw another size cell into the circuit for a boost in amps?

Example: If I put two of the 54mm 70mAh cells on a case, then one row of 32x22mm 50mAh cells along the bottom, would there be a way to wire those in parallel to give an amp boost without a voltage boost or do I need to keep them all the same spec?

BTW thanks for sticking around with me on this. I really appreciate it. This is my first real DYI project that's not a PC case mod of some sort. Never touched a solar panel in my life. :)