Are Diodes Essential in Solar Panels?

Yes, it keeps the battery from "backfeeding" the solar panel when the panel output is low.

Normally a solar panel should not be connected straight to a battery. There is a need for a voltage regulator, that takes care of the battery's charge. This device keeps the situation clean and makes a diode expendable. In most cases, the solar panel is connected to an inverter, operating with battery bank or grid tied that also eliminates the use of diodes. I have 2 systems: 2 panels in parallel (same type) for 12Volts and more (17.2V) and 18 panels in series, output chopped and converted to 240 Volts AC/60 Hz (2,500 Watts). I have no diodes installed.

Is a solar panel accepting power from a battery in the workeable range? If talking about "dark" current, in what order does it happen?

Is there a different behavior with mono/ multi- crystalline cells?

Intriguing.

Good post.

Does the MOSFET version have the "silicon" drop of 0.6 volts, or is it different (lower?)....

I may have put my foot in my mouth here.

The MOSFET family I was thinking of have a Schottky diode included in the package that does the reverse current blocking. So even though this is one package, it contains a MOSFET and a diode. Being a Schottky diode the diode drop will be smaller (0.4) than the nominal 0.6V of a PN silicon junction and the Vds of the MOSFET can easily be less than the 0.2V difference. The body diode of the MOSFET will be forward biased without a different diode in series, so at least one additional diode will be necessary. This still can be precisely what the OP needs to prevent battery overcharge by being able to turn OFF the current flow to the battery.

The MOSFET is used as a reversed schematics - applying a positive voltage (+10V) between GATE and SOURCE (GATE more positive than SOURCE), while the SOURSE is connected to the PLUS of the photovoltaic, and DRAIN is connected to the battery (or to the inverter).

in this mode, the current is passing trough embedded diode and in parallel through the MOSFET channel with very low resistance (0.02 - 0.05 Ohm).

When an opposite current flow is detected - the GATE voltage has to be changed to 0V.

The connection is known as a SYNCHRONISED RECTIFIER.

Isn't the Schottky diode applied over panels that are in series, to compensate the panels that are shaded? Voltage is not a real problem with solar panels, but the shade of light exposure determines the current (Amp) output. In a series array, the current is limited by the panel with the least exposure. (shaded by a tree e.g.)

Makes it 2 different types of diodes, depending on the set up. The OP probably means an isolation diode, what is the direction this post evolves in.

I did here some tests with a Siemens 36 Watts panel and a battery. In full sun it provides 2.6 Amp to the battery. When I cover the panel with black plastic bags, the reverse current is 0.012 Amps. So we talk about a nocturnal discharge of less than 0.15 Ah, considered the batteries are full.

The diode with 0.3 to 0.6 Volts keeps a lot more Ah out of the battery during the charging cycle over a similar time span.

I only reply here as general input, not to criticize anyone.

"Stacking" solar panels is something that happens: or when they are to be delivered like in a carton box, as like piling the boxes up vertically.

To work, panels are put in parallel, or in series or in a combination of both.

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"Stacking" however is also a technique to put junction wafers with different (PN dopes) composition on top of each other to cover a broader or more specific light spectrum. This however is a fabrication technique before the solar cell is finished.

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My personal opinion is that with similar panels not a lot is necessary to protect the panels, but the batteries. Please comment.

All the best, D.

Before starting on a DIY job take a look at these links - some of the better information available about DIY solar PV I have seen

http://fieldlines.com/board/index.php/topic,144982.0.html
http://fieldlines.com/board/index.php/topic,144995.0.html
http://fieldlines.com/board/index.php/topic,145004.0.html

Interesting articles, thanks Russ. D

Hi D,

Some of my saved URLs - all the time at solar panel talk we get people referring to a youtube video about DIY panel construction and about 99% of those are simplistic with few facts in them.

If a person is good at DIY it is possible with certain limitations on use of the panel - such as forget drid tie. I am positive that 95%+ of the population really has no business trying DIY except as a science project.

No easier way to start a fire than with a poor electrical connection and not many do that correct or well.

Mike & I try to have this type of link available for many things from DIY panels to batteries to calculators for cable size etc.

It matters if you are using moré than one panel. If only one panel your regulator will act as a diode, but if you aré stacking panels(which is moré efficient) then diodes aré needed.

By stacking, do you mean in series?

It is highly recommended and will increase performance

Short answer is yes and yes

_{I've only skimmed through and not google the links above - so if I'm repeating - apologies.}

"Reverse currents waste power and can also lead to overheating of shaded cells" from wiki

Generally a diode is installed at 12 volt increments, as light variations can heat shaded cells, which reduces their life and performance.

Visual example of "why fit diodes in panels"

So which diode are you talking about? How the overheating of the shaded panels can occur? A dark panel limits the output current if in series with others. Here is space for a Scottky over the panel to bypass the current from the other parts of the series array. The OT has a diode in mind (in series) and I don't know who wrote this in Wiki.

It misses expertise. Who fills this in? Are here solar experts?

Even though MOSFETs and voltage regulators might not be strictly defined as diodes, they seem to include the functionality of diodes in preventing reverse current flow. If a solar panel is connected only to a load, a diode might not be necessary. But among several paralleled panels and/or batteries, prevention of reverse current would be essential. With this amendment, I stand by my original answer.

What learns this us?

I took a panel from the shelf

I took the battery out of my neighbor's car and measured the voltage

I measured the short current of the panel

The current in the shade

The voltage in the dark

and voltage with a 3/4 dark panel

panel connected in full sun and charging well

panel in shade and connected to the battery (charging 130 mA)

Here the darkened panel is connected with the battery

look at the reverse current - just 0.01 A - exactly 13 mA on a lower scale.

and with the battery charged to over 13 Volts

In absolute dark the panel gets served with 13 mA from a full battery.

If the night is 12 hours we are talking about rougly 0.150 Ah loss from

the battery.

On the other hand how much does a diode eat while charging?

How can 13 mA heat up a solar panel that can deliver 36 Watts?

Gentlemen, please convince me. Me very confused. Help. D.

A different solar panel:

KINGSUN SOLAR 125 WATTS

Here with diodes already in the wiring box: to prevent reverse current between the two branches?

doesn't look a parallel issue to me.

The diode "element" is a 15SQ.......