If you're next project uses a solar panel, this may give you some idea of what you could expect.

Solar panel is a Banggood 5v, 200ma panel (99mm x 69mm). You should be able to scale this to approximate any other size monocrystalline panel.
* Panel laying flat.
* Time of year: April 11
* Full sun all day, no clouds, average humidity.
* Location: North Carolina
A little over 1.14 AHr charging current was obtained for the full day. Solar panel load was 5 diodes in series + a .612ohm resistor. I'll do this graph again soon using a 3.7v 2.6AHr lithium battery. I started this test about an hour before sunrise, but when I checked on it at 8AM it had stopped working. My data logger circuit board was soaked from the dew! I hit it with a torch for a few and dried it out and restarted. Not sure why the panel had such an irregular output from 8-9AM, possibly from the shadows from tree limbs but I thought I was completely in the clear. I also wonder about the appearance of noise from 11AM to 4PM. Samples were taken at 1 minute intervals, with 128 readings averaged to produce 1 sample.

Anyway, this should give you something to go on if you're planning on using a solar panel for battery charging.

 

A handy reference. Thanks.

 

 

Is the battery voltage drop-off after ~ 5pm due to it discharging into the panel?

 

No, not discharging. The battery still has a positive charge current all the way to 7pm.

But... now that you mention it, it does seem to be excessive. I see this when charging any battery - if you turn up or down the charge current the battery voltage moves with it, right away. It's due to the internal resistance of the battery. But in this case I failed to subtract out the voltage drop across the current sensing resistor, a 0.6 ohm between the battery negative and the negative end of the charging source. This had the effect of increasing the internal resistance by .6 ohms. In the graph below the drop across the sense resistor has been subtracted out, shown in the purple line. (The blue line is the same as in the previous graph.) It still shows a slight drop off as the charge current decreases below about 50ma.


Your comment made me take another look at what I'm doing to prevent the battery from discharging back through the solar panel at night. It's a different subject so I'll be posting that in another thread soon. Sometimes I'm ok with what I'm doing and sometimes I'm not, so I'll be asking for suggestions.

 

Hello,

Do you have a schematic on how the solar panel and battery are connected?
Also would it be nice to see the real intensity of the sunshine.
Perhaps a LDR or other light depended circuit might help in this.

Bertus

 

Hello,

Do you have a schematic on how the solar panel and battery are connected?
Also would it be nice to see the real intensity of the sunshine.
Perhaps a LDR or other light depended circuit might help in this.

Bertus

See my post "Solar Charge Switch".
And unfortunately I have no instrument to measure actual solar power.

 

Hello,

If you want to measure the sun intensity, you might want to have a look at the TLS250 or TLS251.
Those chips contain a photodiode and opamp to give a voltage out depending on the amount of light on the sensor diode.

Bertus