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I have the following components:

I'm thinking of connecting them like this: Solar Cell -> Converter -> Charger -> Li-Ion Battery. The reason being is that the converter will keep a constant 5V into the charger, which will charge the battery. My questions:

  • Why can't I remove the converter if I'm willing to accept inconsistent charging? It will also avoid any power usage that existed across the converter.
  • If I take another solar cell and connect in parallel to the first, will I get 5V, 400mA?

[EDIT] My constraints:

  • Using it indoors, will get some sunlight, varies between 3-5V (maybe more on a really sunny day), but a lot of the time will be crappy fluorescent light.
  • Will trade consistency with efficiency i.e. It doesn't have to be on all the time, but when it's on, I'd like it to power the charging module properly, and charge the li-ion battery.
  • The converter seems to boost to 5V for inputs less than 5V, but if input is over 5V, then output of converter will NOT clamp to 5V and actually rise with input.
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Parallelling panels: This can be done to increase the maximum producible current, but generally good practise to put a diode in series with each panel's output to deal with the situation where the panels are not equally illuminated. One panel that is in bright sunlight, and the other that is in shade, the panel in sunlight will be driving current back down into the panel in shade and potentially could damage it, so put some diodes in to ensure current can't flow back into the panels. Some panels come with built in diodes for this, and some don't. Chose a diode with a low forward voltage drop (you don't want to waste too much energy as the panels are naturally inefficient anyway).

DC-DC converters are not necessarily the device you want to use. You need to understand the difference between DC-DC converters, voltage regulators.

Often what you really want is a low drop out (LDO) voltage regulator to keep the output at a nice steady level, regardless of what voltage is coming out of the PV panel. Of course, if the sunlight intensity drops too much, then the output voltage of the panel will drop so low that the voltage falls below the minimum input voltage for the regulator and the regulator will not be able to maintain its output at the specified regulated output voltage.

It's difficult to maximise the power output from a solar panel because the input voltage can vary so much, you might want a 5 volt steady output but find the panel voltage can vary from say 3V to 12V. When the sun is bright enough to produce 6 or 7 volts or more from the panel, then you'll get a nice steady 5V on the output of the regulator, but when the sunlight intensity drops and the panel only produces 5.0V volts or less, then the regulator can't function and the output drops, but the panel is still producing power but you can't use it. This is where you need to start thinking about Maximum Power Point Tracking (MPPT) solar charge controllers. (I haven't yet seen a decent accurate explanation of how they work on the internet).

To answer your specific question, why can't you remove the DC-DC converters? As I indicated earlier, I'm not convinced they are really the type of device you want to use. If you remove them (or remove a voltage regulator), you may find your panel output voltage goes too high and causes damage to the device it is powering.

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  • \$\begingroup\$ I originally wanted to use the converter because I was getting less than 5V output from the solar panel. I planned to use the panel indoors and was getting about 3V, so wanted to boosted it to 5V. But under direct sunlight (not super bright) it can actually output 5-5.5V. I would still like to use it indoors though. This was why I couldn't use something like the LM317T, which has something like a 0.7V drop. Do you think I can use something like a zener diode to clamp the voltage? I don't really care if I waste power, I'd just like my charger module to be safe (4.5-5.5V Vin). \$\endgroup\$ – PGT May 14 '15 at 19:16
  • \$\begingroup\$ You might be able to use a zener to clamp it, but zener's require a minimum operating current in order to break down at the voltage specified. If the zener doesn't get enough current then it tends to break down at a higher voltage than specified. As the solar panel voltage drops so will the the current. You might make it work, build it and test it. There do exist micropower (low current) zeners which require a much lower current to break down at the specified voltage. \$\endgroup\$ – Dean May 15 '15 at 9:24
  • \$\begingroup\$ From what I recall of my electronics education, solar panels are diodes (pn junctions) and so their VI characteristics should mimic that of a diode, (exponential), voltage changes little, but current can vary greatly. So the theory is, if the voltage of the panel is a long way below maximum that can be produced, then the current output will be tiny. I've played around with a few panels but I haven't attempted to plot the response to varying light levels of illumination. I'd be interested to hear how you get on. \$\endgroup\$ – Dean May 15 '15 at 9:30
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A few things

  • There isn't a big issue with under-voltage, it looks like the charger will just cut out. The issue is over voltage. That 5V panel can easily put out 10V in the right conditions, and I don't know what it would do to that charger.
  • I'll let others chime in re: paralleling the panels. I think the issue there is that even if they're identical, they may see different light levels (uneven shading, whatever), and will need blocking diodes. The typical scenario is putting them in series.

There are several off-the-shelf components that do this. Adafruit has one that works well.

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  • \$\begingroup\$ A step-up converter set to 5V out would not need more than 5V in; such is the point of a step-up converter. \$\endgroup\$ – sherrellbc May 14 '15 at 4:33
  • \$\begingroup\$ sigh. yes, of course. \$\endgroup\$ – kolosy May 14 '15 at 4:44

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