Mitsubishi blocking diodes

System · April 2007

Can anyone tell me if Mitsubishi PV-MF165EB3 panels have a builtin blocking diode?

(if not):
when running multiple parallel connected panels in likely shading scenarios (as on a RV roof, where location in 100% sun may not be possible), should I put a blocking diode in series with each panel?

In 100% sun this would reduce my power out up to 4%. But this could save the bacon where there is no choice but to park partially under trees, dragging down the "lit" panels.

Comments? Is my logic correct? Recommendations?

Thanks for the help.

System2 · April 2007

Re: Mitsubishi blocking diodes

No, if the panels are all in parallel, you would not have to put a diode in series with each panel for any shading issues. The "lit" panels would not be "dragged down", they would just not be getting any help from the panels under shade. Any panel with even a wee area shaded, will have basically no output to add to battery charging. The shaded panels would appear to just not exist, as far as adding to the charging current is concerned. Only the lit panels would output any useful current to charge the batteries. No harm done, except for taking longer to charge the batteries.
Wayne

System2 · April 2007

Re: Mitsubishi blocking diodes

Thank you for your reply.

I have read all sorts of statements on the web. Including that shaded panels in a parallel configuration would draw current from the 'lit' panels, thus a loss of power more than simply loss of contribution by unlit panels, hence the blocking diode.

Apparently this is not true?

Since RVs tend to end up in partial shade a lot, I am using 2x Bluesky 30 amp controllers instead of a single controller, with each regulating 2 of a total of 4 panels. Therefore there will only be 2 panels on each controller in parallel, with controller outputs in parallel. The reason I did this is because of suggestion of the engineer at Bluesky, in lieu of a single larger controller.

I am confused!

Roderick · April 2007

Re: Mitsubishi blocking diodes

That's exactly the panels we have on our roof. In full sun, they seem to put out about 22 volts under load. I'm not sure if that suits what you plan to do on your RV. My perception is that these panels are made with grid-tie in mind, being large, and with a rating that supports putting many of them in series.

As far as I know, they do not have a blocking diode.

If you planned to put more than two panels in parallel, it could be worth putting in diodes, to insure that the lit panels don't gang up on the unlit one and force too much current through it the wrong way. It it's just two panels, or two strings of panels in parallel, this wouldn't be a concern.

I don't know what your charger setup is. I don't suppose it's possible to put all your panels in series, and avoid the diode question completely?

As far as whether one dark panel will drag down all the rest in parallel, I don't have personal experience, so have no authority, but the common wisdom is yes, it will. Diodes only cost a few dollars, so you could try an experiment, and if they didn't help, you could always remove them later, I guess.

System2 · April 2007

Re: Mitsubishi blocking diodes

Regarding all panels in series, that becomes a controller peak voltage issue - read $$$.

Regarding grid-tie, I am using MPPT controllers, so voltage should not matter. 2x Bluesky 3024 controllers in lieu of the 1x Outback - mistake? I do not know.

I am tempted to try the diodes. RVs do not get to pick the spot very often. The main drawback is that when I have 4 of these panels on the motorhome roof, there will be sparse area to add or remove anything - even simple diodes! My original is to parallel into 2 pairs, and bring down 2 cables. Ordered cabling to support that. Now beginning to wish I had brought down each panel cable individually from the RV roof.

By the way, isn't there a Schotky diode that has much less V drop than the silicon variety's 0.6 volt?

mike95490 · April 2007

Re: Mitsubishi blocking diodes

garryp wrote:

By the way, isn't there a Schotky diode that has much less V drop than the silicon variety's 0.6 volt?

Yes. Schottky diodes have lower Vf, [voltage forward] but they cost a bit more. Try to get at least 3x the amp rating of the current you will run thru them ie: If you have 5 amps, go for a 15 A rated diode. As you approach max current rating, the Vf goes up. Expect to get about .4V or .5V drop across one. Also, you may need heat sink: 5A @ .4 v = 2 watts dissipated. As temp goes up, so does Vf loss. Silicone diodes in same ckt will likely be .7 or .8 Vf, .6V is a no-load spec.

System2 · April 2007

Re: Mitsubishi blocking diodes

Roderick wrote:

If you planned to put more than two panels in parallel, it could be worth putting in diodes, to insure that the lit panels don't gang up on the unlit one and force too much current through it the wrong way.

Adding identically rated panels IN PARALLEL does NOT increase the output voltage, only series connections will do that.
In parallel, only the available current will be increased, not the max voltage. If the panels max out at 21 volts for example, then it won't matter if only one, two, or one thousand of those panels are connected in PARALLEL, the voltage will never exceed 21 volts, unless you use mirrors or some other means to concentrate the sunlight falling on some or all of the panels. Diodes in series with any power source will however drop voltage and produce heat, which is power that could have been used to help charge batteries. Basic electricity.
The only reason for using a blocking diode with such a sett-up, would be to prevent a wee trickle of current from flowing backwards through the panels at night, taking power from the batteries. This would only be a concern at night, but any quality controller would already have that issue taken care of.
All the best
Cheers
Wayne

crewzer · April 2007

Re: Mitsubishi blocking diodes

Garry,

Your Mitsubishi modules probably contain bypass diodes, but not blocking diodes. See: http://global.mitsubishielectric.com/bu/solar/index.html Bypass diodes will help with most shading issues, and they typically do not negatively affect overall modules performance.

Blocking diodes used to be popular when used with “older” controllers to prevent night time battery discharge through the PV array. Modern controllers (i.e., Blue Sky, Morningstar, Outback) either disconnect the array via a relay or FET.

Shaded modules may not supply much current, but they still develop a voltage, even when poorly illuminated. You’re probably OK without blocking diodes unless your arrays string voltage is at 48 V or higher.

FWIW, my array consists of two “36 V strings”. I added bypass diodes to my modules, but not blocking diodes. Despite severe early morning- and late afternoon shading issues, my array works just fine.

HTH,
Jim / crewzer

Mitsubishi blocking diodes

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