Capacitor for tiny solar charger

[RocketScienceSmurf]

I am setting up a very small off-the-grid installation with a 5W solar panel and a PWM charger

http://www.epsolarpv.com/en/index.php/Product/pro_content/id/773/am_id/136

Since my panel is VERY small I thought I would install a small film capacitor between the panel and the charger, but how should I think when choosing the correct capacitor? I know I want a MKP film cap, but do I need to measure the frequency of the charger before I can properly decide on capacitance value? I guess I want something like 2-5uF.

[petespaco]

What is the purpose of the capacitor?
I don't see why you would want to use one.


 

[RocketScienceSmurf]

The reason for the capacitor is the switched load of the charger. In large installations the DC link capacitor is a crucial part for optimizing performance. Photovoltaic panels normally don't provide high current fast enough so the voltage dips when the charging circuit starts to load it.

Here are some links that has information about DC link capacitors in solar panel arrays.

https://www.researchgate.net/post/Why_is_a_DC_link_capacitor_required_in_Renewable_Energy_System

https://www.eetimes.com/document.asp?doc_id=1278778

https://www.eetimes.com/document.asp?doc_id=1278778

Since I am trying to use a 5W panel to keep a SLA (sealed lead acid battery) I need to squeze every tiny bit of power I can from the system.

[petespaco]

I read the lengthy comments in the first link. They are ONLY talking about grid tied systems, which you are not implementing.
MY opinion:
  Skip the "capacitor" and hook it up.

Pete Stanaitis
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[RocketScienceSmurf]

Already tried that. I get no charge so I need to do something.

[petespaco]

If you are not getting any charging, then you have some other kind of problem.
   I have a bit of experience with small solar collection.  You might look at:
https://spaco.org/PV/PVSolar.htm
to get some hints on what to do.

A 5 watt solar panel is considered to be a "trickle charger".  Your controller may not be able deal with this small amount of power.

[Mads Barnkob]

I agree with Pete, the panel itself is simply too small to deal with the losses in regular/China chargers. The experiments I did myself with a 10 Watt home made panel and some of the cheapest China chargers was also very disappoiting, the only thing that worked for sure was draining the battery, some chargers did not even work correctly at such low charge power, so that power flow was uncontrolled between panel, battery and load. I am not sure if its due to the low power of the panel or the charger was just a linear regulator burning off the excess power.

I wrote a little about it, but no details on the chargers yet: http://kaizerpowerelectronics.dk/general-electronics/10w-scrap-solar-cell-panel/

[RocketScienceSmurf]

Made some tests yesterday with a 300W halogen directed at the solar panel and it actually worked

First I hooked up my oscilloscope parallel over the panel when connected to the charger and I could see that I had about 13V with a small 100Hz sinusoidal curve on the signal which came from my fluorescent lamp so I hooked up a 300W halogen lamp instead. My setup made it impossible for me to take measurements and screen shots simultaneously as holding the lamp but I will try and make a better setup so I can show you my oscilloscope readings.

I probably should have two panels in series to get a better voltage supply but I want to try with the panel I had that was only collecting dust anyway.

[RocketScienceSmurf]

I did some more testing with my setup this weekend. Fortunately the sun was shining and I got some charge to the battery. When directing the panel as directly as possible towards the sun I got 1,9W peak power which is about what I need so I am happy.

I tried to connect a film capacitor in parallel with the PV but I didn't do much good from what I could see so I think I will use the setup without one.

(I took a few screenshots from my oscilloscope that I will upload later)

[Mads Barnkob]

I would guess that adding a additional capacitor would not do much difference if we look at a basic schematic of a solar inverter, when it is most likely to supply directly to the intermediate inverter's DC bus capacitance.

[RocketScienceSmurf]

A little update to my solar project.

I received two 30W panels the other day and hooked them up in series to my charger/regulator. These deliver a LOT more power than the initial 5W Velleman panel.

I connected my oscilloscope with the clamp probe on the negative lead between the charger and the panel to have a look at the current draw and to see if I could figure out the PWM frequency. The PWM frequency is 100Hz.

When I connected a motor run cap I had laying around the peak current increased from around 3-4A to close to 20A. Still not sure what this proves. I guess I need to take measurements over time and see if the overall battery input increases with a cap installed