Blocking Diodes in Parallel Solar Panels?

[noelex 77]

The loss of blocking diode is quite easy to calculate at least roughly with a MPPT regulator.

If a Schottky diode is used the average forward voltage loss will be around 0.35V.

If a conventional diode is used the voltage loss will be around 0.55v

If we divide this by the average Vmp we can calculate the loss.

For example a nominal 12v panel might have a Vmp of around 18.5v under STC, but this will drop to around 16V in practice. Therefore the loss during production with a Schottky diode is 0.35/16 = 2.2 %. For a conventional diode it will be around 3.4%. Higher voltage panels will produce less loss.

The above losses need to to be offset against the reverse current that will occur under low illumination before the regulator shuts down (if one is fitted).

The loss of inserting a blocking diode with a PWM regulator is more complex. It will be slightly less, but still in the same ball park.

If you measure the reverse current with your system you can estimate the net gain/loss in average conditions. Generally it will be in order of a 1-2% loss for a nominal 12v panel if a diode is fitted.

Not much, but the diode is not generally helpful.

[Training Wheels]

Thank you all for your expertise!


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[roland stockham]

If using an MPPT controller you want to go for high voltage so wire panels in series. One set each side. This is much more efficient and will give more charge at lower sun angles morning and evening. Don't fit blocking diodes as you already have one in the controller but if you don't fit Bypass then any shading on one panel will knock out all the panels n that side. Shading a cell in the chain is like closing a gate, no power can get through to the next section so shading one cell closes the complete panel. Without a bypass know power can get from any other panels upstream so you loose the whole side.
If you wire in parallel you avoid this problem but with small 12v panels you need a stronger sun to get enough voltage to start charging. You also add all the panel currents together instead of adding voltages so you need much heavier and more expensive controllers and wiring and get lots of problems with voltage drop. 50A at 15v is much harder to handle than 10A at 75v, that's why MPPT controller go right next to the batteries.

[knockabout]

they run off street lights/auto disconnect from house when the dc load kick's in/wire up and test/if its not broken don't fix it/most panels charge in overcast conditions

[kudukuguam]

Originally Posted by sailorchic34

If your panels are wired in parallel, then blocking diodes are not required. They are required for series wiring as is common on land. In Series one panel without blocking diodes would shut the whole string down. That is not an issue in Parallel.

Why only 50 watt panels. Seems you could get one or two 300 watt panels and call it good with MPPT.

On the plus side for 50 watt panels, a controller is not an absolute necessity. The max voltage (if memory serves, about 17 vdc) is not enough to "cook" your batteries. The controller acts as an impedance matcher so you get maximum power transfer. There isn't a LOT of difference but desirable.

[mitiempo]

Originally Posted by kudukuguam

On the plus side for 50 watt panels, a controller is not an absolute necessity. The max voltage (if memory serves, about 17 vdc) is not enough to "cook" your batteries. The controller acts as an impedance matcher so you get maximum power transfer. There isn't a LOT of difference but desirable.

With eight 50 watt panels the op had better have a controller or the batteries will be permanently damaged in short order.

Even with one panel it takes a very small amount of current to keep the batteries fully charged. Any voltage above that tiny amount for a while will raise the battery voltage to damaging levels.

Do I Need A Solar Charge Controller ?? Photo Gallery by Compass Marine How To at pbase.com

[noelex 77]

Originally Posted by roland stockham

If using an MPPT controller you want to go for high voltage so wire panels in series. One set each side. This is much more efficient and will give more charge at lower sun angles morning and evening. Don't fit blocking diodes as you already have one in the controller but if you don't fit Bypass then any shading on one panel will knock out all the panels n that side. Shading a cell in the chain is like closing a gate, no power can get through to the next section so shading one cell closes the complete panel. Without a bypass know power can get from any other panels upstream so you loose the whole side.
If you wire in parallel you avoid this problem but with small 12v panels you need a stronger sun to get enough voltage to start charging. You also add all the panel currents together instead of adding voltages so you need much heavier and more expensive controllers and wiring and get lots of problems with voltage drop. 50A at 15v is much harder to handle than 10A at 75v, that's why MPPT controller go right next to the batteries.

There is a lot of debate about which is the most productive way to connect solar panels on a boat. I still have a slightly open mind, but the majority of evidence favours parallel rather than series connection. Solar cells produce near normal voltage before they can produce any usable current, so series connection does not significantly help the panels produce power earlier in the morning and later in the day. There are some gains, but they are very small.

Series connection does significantly reduce wire size, as you point out. Marine cable is both expensive, difficult to feed, and heavy in the larger diameters so this is a helpful advantage. However, it does not generally reduce the cost and size of controllers, the opposite is true.

The output current will be the same in series and parallel connection and the controller needs to have sufficient capacity to handle this current. In terms of voltage, the input voltage under series will be higher and generally the cost is higher for controllers capable of handling high input voltages. In most cases the same MPPT controller is equally capable of managing both series or parallel connection so it is not a big factor, but if you want to string a lot of panels in series a more expensive, rather than cheaper controller may be needed.

If you do want to wire the panels in series, fitting extra bypass diodes is good advice.