Solar panel conundrum

[kev99]

I have a cheap portable suitcase style folding solar panel which I use to supplement my fixed installation only when at anchor (it actually provides a lot more current into the batteries than my fixed installation....but that's another story!). I'm looking at how I wire my system and thinking of having a single charge controller and paralleling the panels instead of two separate systems. However, I have a dilemma about the panel specifications. Hopefully someone will be able to advise.....
My portable panel is a cheap Chinese EcoWorthy 120W foldable panel with 4*30W units presumably series wired internally. The so-called "User manual" states that the panel is monocrystalline with an open circuit voltage of 16-18V, which doesn't sound right to me.
So I measured it when in full sun, and the voltage on my meter shows 22.7V, which is would think is more consistent with equivalent "real" panels I see advertised online.
I realise that to parallel my panels I should ideally match their open circuit voltages, so my question is which Voltage number should I rely on: the one I measured at 22.7V, or what is specified in the user manual of 16V to 18V?

 

[noelex]

If you are planning to wire two solar panels in parallel using one MPPT controller, ideally the Vmp (maximum power point voltage) and the voltage temperature coefficients should be the same. The panels should be positioned such that they are experiencing similar conditions.

Matching Voc (open circuit voltage) is not important, but normally if the Voc is similar between two panels the more important Vmp will be close as well.

With a PWM controller, the above does not apply.

For a 12V panel, an Voc of 16-18V would be very low, as you point out, and your measurement of 22.7V is more typical.

As the specifications for the folding panel are obviously wrong, I would measure the Voc for both panels in identical conditions, and if this is within 0.5V, the results using one MPPT controller will be reasonable.

 

[William_H]

The Voc of a solar panel is a function of the number of cells in series. A bit like a battery each cell something like .7 volt. More cells will give more voltage in lower light situations so potentially giving some more charge into a battery under lower light conditions. An MPPT controller usually needs some volts in excess of battery volts to put in charge.
I might suggest if you only use the portable panel when on board and using power that you connect it directly to battery. On the assumption that you will not be able to over charge batteries. Or just connect in parallel with existing panels in to controller and observe an increase in charge current. ol'will

 

[PaulRainbow]

Connecting a 120W solar panel directly to a 12V battery doesn't sound like a good idea to me.

 

[NormanS]

Like the OP I have fixed panels, and also a couple of smaller 40W panels which I only rig at anchor when required. The fixed panels have their own regulator, and so do the temporary ones. I haven't found having the two regulators to be any disadvantage.

 

[noelex]

I haven't found having the two regulators to be any disadvantage.

Multiple MPPT regulators are an advantage. The better tracking results in a higher solar yield, and there is added redundancy.

The main drawback is more complex wiring. The cost can also be higher, but not in all cases. Multiple smaller controllers are sometimes less expensive than a single larger controller, so it is an option worth considering even if you are on a tight budget.

 

[kev99]

As the specifications for the folding panel are obviously wrong, I would measure the Voc for both panels in identical conditions, and if this is within 0.5V, the results using one MPPT controller will be reasonable.

Thanks very much to you all for your advice. The above does sound like a good pragmatic practical engineering approach to resolving my dilemma, so I'll do some more investigations.

I should add that my main reason for wanting to change is primarily because my booster panel puts around 4.5A at 14.4V into the batteries, and the connector gets rather hot, so exploring a better alternative.

The other reason is that I have a suspicion (although I'm not entirely sure of this) that the booster panel lifting the voltage to 14.4V is causing my main system (Victron MPPT 75/15) to drop back from Absorption to Float mode and back off the amount of power it dumps into the batteries. Hence not currently getting the most amount of charge input that would be available to me from the two systems working together.

 

[noelex]

Thanks very much to you all for your advice. The above does sound like a good pragmatic practical engineering approach to resolving my dilemma, so I'll do some more investigations.

I should add that my main reason for wanting to change is primarily because my booster panel puts around 4.5A at 14.4V into the batteries, and the connector gets rather hot, so exploring a better alternative.

That does not sound good. Some of the portable units do not have great components.

The other reason is that I have a suspicion (although I'm not entirely sure of this) that the booster panel lifting the voltage to 14.4V is causing my main system (Victron MPPT 75/15) to drop back from Absorption to Float mode and back off the amount of power it dumps into the batteries. Hence not currently getting the most amount of charge input that would be available to me from the two systems working together.

Two controllers should not cause any significant charging problems.

If the absorption voltage for the Victron is set to 14.4V, the current supplied by this controller will reduce if the battery is kept at 14.4V from another source such as flexible panels.

This is appropriate. If the battery requires 14.4V during the adsorption phase, it does not matter which charge source, or combination of charge sources, is supplying the current to raise the battery to this voltage.

The Victron controller will only drop down to float when the battery has been kept at the adsorption voltage for the adsorption time. The controller will not discriminate, and the battery will not mind which charge source has reached these set points.

If the battery is not "fully" charged at the end of the absorption time and is dropping to float prematurely, the absorption time should be increased.