bridging diodes

System · July 2008

I am setting up a solar system with 4 panels (parrallel). Each panel can put out 7 amps, I want to install bridging diodes. The question is... Since the diode goes across each panel I would think the other 3 panels which can produce about 21 could back feed into one panel if shadded. Should I install blocking diodes at each panel equal to the maximum amps produced by the other panels and a bridging diode equal to the one panel behind the blocking diode? what about voltage loss with all the diodes? I am using a charge controller so I am not woried about night time loss.

danny

BB. · July 2008

Re: bridging diodes

Danny,

I am not quite sure what you are trying to "bridge" with the diodes...

What it sounds like is you want to use "blocking diodes" to prevent reverse current in the solar panels...

Normally, those are only needed in the cases where the solar panels are connected directly to a battery without a charge controller that has blocking/reverse current prevention built in.

Solar panels go pretty high resistance when there is no sun--so you are probably loosing little power by current leakage.

And, in fact, you would probably loose more power by installing blocking diodes because of the 0.2-1.0 volt drop caused by current flow through each diode.

What you probably need for safety is a series fuse for each panel... Look up the series fuse rating of the panel in put 1 fuse series with each panel.

This prevents a panel from over heating if it gets shorted, and the other panels feed the short, and/or the solar charger gets shorted and back feeds into the solar panels (and one panel has failed shorted).

If there is no series fuse rating for the panel, you can use a fuse rated around 50% greater than the Isc (short circuit current) of the panel... In your 7 amp panel case, around 10-12 amps.

What people sometimes do with panel installed in series is to use "Bypass Diodes"--For example, if you have 4 panels in series, and one is shaded, then a bypass diode would "shunt" the current from the other three panels around the shaded panel. You would loose a little more than 1/4 of your output voltage, and therefore 1/4 of your output power--but at least the string output is not zero amps because of one panel in the dark.

But this only works with MPPT (Maximum Power Point Tracking) solar charge controllers and Grid Tied inverters where the solar panel voltage is much higher than the battery or minimum voltage required by the Grit Tie inverter.

Does this make sense?

-Bill

System2 · July 2008

Re: bridging diodes

Bill, Thanks for getting back to me on this. What you are saying somewhat makes sence to me.
What does not make sence to me is the fact that the panels are putting out about 20v during the most of the day, and they seem to work even if partially shaded and late in the day. unless the amps are greatly affected the 1/2 volt lost through diodes seem rather minor.
I am putting together my "combiner" box and as soon as I figure out how to photograph it and upload it I will post it on this forum. The box has fuses and a place to "plug in" a diode if needed, if not a jumper wire is/has been installed.

Either way, I am having a lot of fun setting up the system. I am setting up a hybrid house. primary source on most of the 120v demands are to be off of the solar panels inverting up as needed. I am using 12v led lights and 12v accessories where possible. I am going to back up to the grid automatically before I get to a low volt disconect. The big demands such a AC will stay on the grid.

The idea is we save $, remove demands placed on the local power company and we always have power even if the power goes out. It's like a built in generator.

Is there a spell check on these post??

Windsun · July 2008

Re: bridging diodes

That 20-21 volts you are measuring is the open circuit voltage of the panels with no load. Under use it will be much lower.

Less sun = large drop in current, small drop in voltage.

BB. · July 2008

Re: bridging diodes

Danny,

A 1/2 volt drop * 7 amps = 3.5 watt drop per diode... If you don't need them, then why put them in and have the power loss. Amperage will not be affected.

Shading on solar pv panels can greatly affect power generation--especially for parallel connected panels. Just a little shading can drop the voltage in one panel below the others in parallel--and it will not be able to output any current at all vs the rest of the panels in the system. Think of 4x 20 volt batteries in parallel--and one of the batteries is 18 volts while the others are at 20--that 18 volt battery will simply not add any current. Also, remember that a solar cell goes high resistance in "dark"--so it also blocks any current flow through that cell.

You are certainly heading down the correct road for solar--working hard at conservation first. If you don't use the power, you don't have to spend the money/energy to generate it in the first place.

You can posts images two ways... One is to use the "insert image" (yellow icon with gray mountains" to point to your files hosted elsewhere (like Flicker)... Or, use the "paper clip" and you can upload smaller pictures directly.

For spell checking--the major browsers have that built in now-a-days. I use FireFox and it spell checks any website. Sometimes you have to tell it where you want spell checking performed--Right Click on this posting area and click on "Spell Check This Field"...

-Bill

System2 · July 2008

Re: bridging diodes

I like what you are saying. I would rather save the power for the house and the batteries. If my panels are not in danger of "frying" each other hooked up in parallel -- Great!! OK, my next question is if I decide to add another couple of panels, lets say 2, I will have a total of 6 on one array. Is there a optimum number of panels per array?

Also I am assuming it would be a good idea to keep all the panels the same type. I have mono-crystalline 110 watt 12v. Seems like a bad idea to change to polycrystalline and what about going up to a higher watt rating?

Got a tell you I am having more fun than a barrel of monkeys and REALLY appreciate your input and this forum-- really GREAT. Found the spell check, won't help with my bad grammer, but, it may make me look a little smarter.

Danny

mike95490 · July 2008

Re: bridging diodes

Now you need to look at the detailed specs for your panel. What's the Voltage Max Power? That needs to be about 2V above your battery recharge voltage. The charge controller eats 2 V, so if you have a 15V panel, and run it thru the worlds best controller, you will NEVER fully charge your battery .
A MPPT style controller can take a 30V panel, and convert the voltage to extra amps, so when you get 16V at the battery for charging, you have nearly twice the amps the panel was rated at.
Post your thoughts, and we'll respond

BB. · July 2008

Re: bridging diodes

Danny,

Remember, generally, when you have more than two strings of panels in parallel need a fuse (or breaker) in each string that is equal to the "series fuse" rating of the panel (should be a bit above Isc).

When putting panels together in strings--you need to know the capabilities of the solar charge controller... There are maximum voltage (max Vmp, Voc--the prevent controller damage from over voltage, and a Vmin required so the controller can move current into the battery).

Typically, cold weather causes the panels to run at high(er) voltages--so that is where you check for Voc--example 150 VDC, and Vmp not to exceed 145 VDC--both for an MX 60 controller).

And for hot weather, you check that the Vmp of the solar panels (corrected for hot weather) is greater than the maximum charge voltage of the battery plus ~2 volts (example ~15.5 volts to equalize a 12 volt battery + 2 volts for controller drop=>Vmp>17.5 volts)... The numbers are not exact--some controllers may work with 1 volt drop, also the battery temperature affects the charging voltages (cold batteries require higher voltages).

There is also a maximum current into and out of the controller that you need to worry about). For the MX 60--60 amps (maybe 72 amps max if you adjust the settings)...

And since the controller can charge a battery bank rated from 12 volts to 48 volts (say 15 volts to 60 volts)--the maximum useful panel wattages would be:

15vx60amp= 900 watts max
30vx60amp=1,800 watts max
60vx60amp=3,600 watts max

So, in the case of a solar charge controller--the same controller can handle 4x as much wattage on a 48 volt battery bank vs a 12 volt battery bank...

So, before you can really mix and match panels+strings, you need an idea of how much power and what kind of charge controller you are planning on using.

Lastly, mixing and matching panels... Think of batteries... You can put like capacity batteries in series, and like voltage batteries in parallel.

You can do the same thing with solar panels... You can put two 4 amp panels (with the same Vmp) in parallel, and place them in series with one 8 amp panel. etc....

Generally, mixing panels that are close, but not exactly matched will just will work if you just assume the lower rating (15+17 volt panels, assume both are 15 volts; 6 amp in series with 5 amp, assume the string will generate around 5 amps)....

-Bill

bridging diodes

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