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Solar - parallel or series?


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1 minute ago, Loddon said:

Even when it reads 100% it might only be 90%. However if it reaches 100 at midday then there's a good chance it will be fully charged by the time the sun goes in.

Ah, yes. Good point.

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18 minutes ago, Loddon said:

Don't forget a Smartgauge is useless unless it's dark outside😱

When charging* it is only within about 10% or so  of the actual value.

 

*Reading any voltage that is not the actual battery voltage. 

 

Yes the Smartgauge is just a guide while any charging is happening but it's pretty accurate in measuring discharge and during the day I'm also looking at tail current on my shunt monitor. I have both types.

1 hour ago, Francis Herne said:

Putting two panels in parallel rather than series halves the power threshold for tracking to start, which makes a huge difference on cloudy days.

 

Series for low-light output is a red herring in any case with modern panels and 12V batteries. With 30-50V panels, open-circuit voltage will be well over 15V before there's any power that's not a rounding error.

 

I'm not sure that's correct. What many people don't understand is that it’s the rate of rise in voltage, not just the ultimate voltage, that determines when the panels switch on.

 

Series will always switch on earlier as the voltage has to rise to a larger value ultimately. This means it’s rate of rise (_dv/dt_ mathematically) is higher.

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21 minutes ago, blackrose said:

 

Yes the Smartgauge is just a guide while any charging is happening but it's pretty accurate in measuring discharge and during the day I'm also looking at tail current on my shunt monitor. I have both types.

 

I'm not sure that's correct. What many people don't understand is that it’s the rate of rise in voltage, not just the ultimate voltage, that determines when the panels switch on.

 

Series will always switch on earlier as the voltage has to rise to a larger value ultimately. This means it’s rate of rise (_dv/dt_ mathematically) is higher.

That last paragraph makes no sense -- if you put two panels in series all the voltages at the MPPT input double, both absolute V and dV/dt. The panel doesn't see any difference.

 

The output of a solar panel is basically a current source which drops off if there's too much voltage across it, so for each current there's a voltage at the MPPT input which extracts the maximum power from the panel. Tracking MPPT controllers wiggle this voltage up and down to find the maximum power point -- which might be 30V for one panel (ot two in parallel), or 60V for 2 in series.

Edited by IanD
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2 hours ago, blackrose said:

I wasn't referring to any difference between the voltage of the two panels!

Panels don't output a voltage, they output a current, the voltage across them is defined by the MPPT controller to maximise power. If you put two panels in parallel their currents add together but they see the same voltage. If you put two panels in series the midpoint voltage will move until they both have the same current.

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On 17/05/2023 at 23:23, IanD said:

Panels don't output a voltage, they output a current, the voltage across them is defined by the MPPT controller to maximise power. If you put two panels in parallel their currents add together but they see the same voltage. If you put two panels in series the midpoint voltage will move until they both have the same current.

 

Yes I understand all of that and never said or implied anything to the contrary. If my previous post made no sense to you perhaps that's because you misunderstood it.

 

This is what I've been advised: Solar panel input voltage must be at least 5v above the battery voltage to start pumping charging current into the battery bank. So, around 18v or so. 

 

Once the MPPT switches on, it then only requires the input to remain at least 1v above the battery voltage. 

 

Therefore a *series* panel connection (which will have a higher total voltage than a parallel connection) will naturally reach 18v as the sun comes up sooner than a parallel connection. And vice versa in the evening. Hence, the higher overall solar harvest from a series connection.

 

Also I've no idea where the idea comes from that Tracer MPPT units won't start charging until panel voltage reaches 1A? It's nonsense.

IMG_20230521_173337.jpg

Edited by blackrose
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17 hours ago, blackrose said:

 

Yes I understand all of that and never said or implied anything to the contrary. If my previous post made no sense to you perhaps that's because you misunderstood it.

 

This is what I've been advised: Solar panel input voltage must be at least 5v above the battery voltage to start pumping charging current into the battery bank. So, around 18v or so. 

 

Once the MPPT switches on, it then only requires the input to remain at least 1v above the battery voltage. 

 

Therefore a *series* panel connection (which will have a higher total voltage than a parallel connection) will naturally reach 18v as the sun comes up sooner than a parallel connection. And vice versa in the evening. Hence, the higher overall solar harvest from a series connection.

 

Also I've no idea where the idea comes from that Tracer MPPT units won't start charging until panel voltage reaches 1A? It's nonsense.

IMG_20230521_173337.jpg

 

So long as the panel maximum power voltage is significantly above the maximum battery voltage -- and 5V may or may not be enough, depending on the MPPT controller -- there's little power yield advantage having much higher voltage than this, though it does mean thinner cables can be used.

 

A solar panel is -- as I said -- a big silicon photodiode, and I work with these on a daily basis. They generate a current which is proportional to light level, but the voltage (and MPPT maximum power voltage voltage) doesn't vary much with this, only the current. For example, if you have a 100W panel rated at 20V 10A for maximum power, as soon as any light hits it the voltage will be close to this but the current will be small -- see attached plot.

MPPT.jpg

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