Solar bike trailer

[Mike_Kelly]

I am building a solar bike trailer from Chris Nafis design. I am using two RIch 80w panels which I plan to isolate using blocking diodes. I am using Renogy boost charger controller and it just dawned on me that I don't know what will happen paralleling two panels into the one charge controller. When the panels are blocked so they can't affect one another then what happens when one is shaded and the voltage drops? When paralleling two battteries the higher voltage battery will "charge" the lower voltage until they equalize and the combined output is the equalized voltage but what happens with two protected sources that don't equalize?
Thanks for any thoughts

 

[amberwolf]

Electrically, regardless of source type, if you have any isolated voltage sources that can't feed back into each other (diodes in series with them anywhere in their current path prior to connecting to the other source(s)), then:
--lower voltage sources are effectively "disconnected" because they can't cause current flow, and can't receive current flow back into them from a higher voltage source past the diode.
--highest voltage source will provide all of the current flow.

So...if you have isolated voltage source panels, then only the highest voltage panel will contribute anything.

If you're dealing with current sources that change resistance but not voltage, always generating the same voltage, but changing current because of changing resistance, then they could both contribute current.

So if your panels are current sources, they could both contribute whatever current they can generate (may be dependent on the load or converter).

You can test what happens by putting ammeters in series with each panel, and voltmeters across each panel, pre-diode. If you get current from both at the same time, regardless of panel voltages, then they are current sources. If you get current from only one, you'll see that it has a higher voltage than the other, and they are voltage sources.


If you had diodes on those hypothetical batteries, they wouldn't feed back into each other either, and the lower one would not contribute; etc, because they are voltage sources.

If you had unequal LED PSUs (or other constant-current sources), then they could both contribute even if using diodes.

More info on the concepts:

allaboutcircuits.com voltage source - Google Search

allaboutcircuits.com current source - Google Search

If you can instead series those panels, then you'll get output from both of them regardless, if your converter can handle the max series input voltage at max insolation of the panels.

Disadvantage is that shading either panel decreases current flow from both of them.

With them paralleled with diodes, I also don't know what your specific charge controller will do for tracking (if it's an MPPT or other tracking type), as it tries to adjust to the different and changing conditions on each panel, or if it will react to them as if they were a single panel.

FWIW, BuildItSolar is a good resource for specific panel/converter/etc information (more of it there than here).

 

[Mike_Kelly]

Thanks for your thoughts. Will read your references. The charge controller is MPPT. I can't wire them in series because the total output voltage would be higher than the battery voltage. Also not sure it is a good choice because the trailer is always moving being shaded. So I think parallel is better. If the panels act as voltage sources you would think parallel panels would never be an option because one will always be at a different voltage. My only concern was how the MPPT controller would deal with the composite voltage.

 

[amberwolf]

Thanks for your thoughts. Will read your references. The charge controller is MPPT. I can't wire them in series because the total output voltage would be higher than the battery voltage.

Unless you are directly connecting the panels to the battery with nothing between them it shouldn't matter what voltage they are vs battery, as whatever converts the solar output to battery charge will take care of that, since it has to regulate the voltage and current to charge a battery anyway.

You'll have to look at it's specs, but the MPPT you have should be a complete current-limiting DC-DC converter that can buck or boost (some can do either as needed) the input to generate the output, so the only voltage limitation is what the MPPT itself can take at it's input.

If the panels act as voltage sources you would think parallel panels would never be an option because one will always be at a different voltage. My only concern was how the MPPT controller would deal with the composite voltage.

You'll have to look at the MPPT's info / manual to see if it discusses this--it might suggest or forbid certain use cases, like this.

I don't know whether the panels will be voltage or current sources in your setup, it partly depends on how the MPPT works, and partly on the panels.

 

[Mike_Kelly]

Like most things these days there is no information because nobody makes anythng anymore so manufacturers can get away with no documentation. I know there are boost only.
Cheers

 

[amberwolf]

Boost only does create a problem if the series voltage of panels at max insolation is higher than the battery voltage (or what the MPPT can take).

But if it's lower than the battery voltage, the MPPT can still boost it.

 

[Mike_Kelly]

Sure but that is why I can't use a series connection. Max Out V is 19.8v and the batteries to be charged are 36v. I have not been able to find out, yet, how a MPPT boost controller works so I don't know the input impedance and how it changes with input voltage.

 

[amberwolf]

Just a note: Typical 36v ebike batteries are 42v fully charged (10s Li-Ion types; other chemistries use different numbers of series cells and final voltages, but all would be close to that).

So if your batteries are that type, then your series panels are still not reaching the full charge voltage of the pack, and a boost-only MPPT would still work (as long as that specific MPPT can do it).

 

[Mike_Kelly]

Yes that is true but I suspect the efficiency is going to be lower in a series configuration with the MPPT swinging wildly as one of the panels gets shaded. The input voltage to the MPPT would be much more stable in parallel.