Solar Panel Voltage and Minimum Charge Requirements

[chemist308]

I'm preparing for a solar install using an array of 6 210 watt and 6 100 watt solar panels (1860 watt array) with fixed mounting brackets. I wanted a 1200 watt system to support a 600 AH LiFePO4 battery bank, but since I'm going with a fixed configuring and not movable tilt mounts, I wanted the extra capacity to make up for the projected loss from not tilting.

All of that said, I'm debating between using a series parallel (3 series of 210 to 210 and 3 series of 100 to 100) or a straight up parallel design. My concern with a simple parallel design is that the MPPT can reduce voltage but can never increase it. So as the sun leaves its peak charging position couldn't my voltage drop to where it's not ideal for charging while still daylight? Is this a valid concern? If so, would my plan of doing a series parallel array of 2 panels in each series be enough to offset it?

[Second Chance]

@SolarPoweredRV can probably give you some good advice on this.

Rob

[SolarPoweredRV]

I'm preparing for a solar install using an array of 6 210 watt and 6 100 watt solar panels (1860 watt array) with fixed mounting brackets. I wanted a 1200 watt system to support a 600 AH LiFePO4 battery bank, but since I'm going with a fixed configuring and not movable tilt mounts, I wanted the extra capacity to make up for the projected loss from not tilting.

All of that said, I'm debating between using a series parallel (3 series of 210 to 210 and 3 series of 100 to 100) or a straight up parallel design. My concern with a simple parallel design is that the MPPT can reduce voltage but can never increase it. So as the sun leaves its peak charging position couldn't my voltage drop to where it's not ideal for charging while still daylight? Is this a valid concern? If so, would my plan of doing a series parallel array of 2 panels in each series be enough to offset it?

Your thinking is correct, you want to have increased voltage on your Solar array.

Here is an example: The typical 12 volt Solar panel actually operates at ~ 18 volts. It takes 13.6 volts to charge a typical LA battery (a Delta of 4.4 volts). So, the panel is capable of generating 18 volts in good Sun, if the Sun is not optimal (much of the time) the Solar panel will produce less than 18v, once the panel produces less than 13.6v your LA batteries will stop charging (with Lithium the voltages are even higher ~14.4v). Now, if you were to string those panels in series your voltage would be additive, 36 volts (a Delta of 22.4 volts), now if you have sub optimal Sun, your batteries will still be able to charge down to 13.6v and your panels will be able to charge your batteries better in sub optimal Sunshine.

In your case it would be beneficial to configure your Solar array with two panels in series. Since you are upgrading to Lithium for your battery bank, you might also consider configuring your panels in a 3 series config.

I am curious as to why your panels are two different Wattages?

Are you planning for two separate Solar Charge Controllers, one for each string of Solar panels? I mention this because I think your performance will be negatively effected by stringing together two different Wattage panels, which may have different voltages or different amperages. I realize having two Solar Controllers will require two combiner boxes on the roof and two sets of wires coming down from the roof, but, I think having a Solar Charge Controller for each string of different Wattages will result in the best performance for your system.

[chemist308]

Your thinking is correct, you want to have increased voltage on your Solar array.

Here is an example: The typical 12 volt Solar panel actually operates at ~ 18 volts. It takes 13.6 volts to charge a typical LA battery (a Delta of 4.4 volts). So, the panel is capable of generating 18 volts in good Sun, if the Sun is not optimal (much of the time) the Solar panel will produce less than 18v, once the panel produces less than 13.6v your LA batteries will stop charging (with Lithium the voltages are even higher ~14.4v). Now, if you were to string those panels in series your voltage would be additive, 36 volts (a Delta of 22.4 volts), now if you have sub optimal Sun, your batteries will still be able to charge down to 13.6v and your panels will be able to charge your batteries better in sub optimal Sunshine.

In your case it would be beneficial to configure your Solar array with two panels in series. Since you are upgrading to Lithium for your battery bank, you might also consider configuring your panels in a 3 series config.

I am curious as to why your panels are two different Wattages?

Are you planning for two separate Solar Charge Controllers, one for each string of Solar panels? I mention this because I think your performance will be negatively effected by stringing together two different Wattage panels, which may have different voltages or different amperages. I realize having two Solar Controllers will require two combiner boxes on the roof and two sets of wires coming down from the roof, but, I think having a Solar Charge Controller for each string of different Wattages will result in the best performance for your system.

I’m using two different wattage panels to get that much wattage to fit on the roof. They’re the same voltage, but different amperage. So what I was going to do a string a 210 with a 210 and a 100 with a 100. I have to use two different solar controllers anyway, because the wattage is so large. I believe it would be too large for a 100 amp controller to handle, so I’m going to use 2 Victron 150/70‘s.

That said, are you thinking that I might drop below the required voltage even with two panel linked together?

[SolarPoweredRV]

I’m using two different wattage panels to get that much wattage to fit on the roof. They’re the same voltage, but different amperage. So what I was going to do a string a 210 with a 210 and a 100 with a 100. I have to use two different solar controllers anyway, because the wattage is so large. I believe it would be too large for a 100 amp controller to handle, so I’m going to use 2 Victron 150/70‘s.

That said, are you thinking that I might drop below the required voltage even with two panel linked together?

Good choice on the Solar Controllers.

No, I just thought you should consider a 3s - 2p configuration, depending on the voltages. My original thought was for you to get higher voltages, especially if your panels are "12 volt" panels.

You could also consider configuring your 210 Watt panels in a 2s - 3p config while using a 3s - 2p for your 100 Watt panels.

When you are done, you are going to have a very nice system.

[Butcher]

Let's not forget the draw back with shading. As with all methods, not everything is positive. Shade a small portion of one panel can disrupt the entire series circuit. It would suck to loose most, if not all, of your output because of that. Shading is not as big of an issue with a parallel circuit. But of course, there are limitations with that method too.

Research is the only way you can make a good decision. Asking people what you should do will get really good ideas what worked for them, but maybe not the best for you.

[chemist308]

Good choice on the Solar Controllers.

No, I just thought you should consider a 3s - 2p configuration, depending on the voltages. My original thought was for you to get higher voltages, especially if your panels are "12 volt" panels.

You could also consider configuring your 210 Watt panels in a 2s - 3p config while using a 3s - 2p for your 100 Watt panels.

When you are done, you are going to have a very nice system.

@SolarPoweredRV That actually brings to mind another thing I wanted to ask. Do you think the two Victron 150/70 solar controllers will be enough to handle this array?

[SolarPoweredRV]

@SolarPoweredRV That actually brings to mind another thing I wanted to ask. Do you think the two Victron 150/70 solar controllers will be enough to handle this array?

Here is the Spec sheet on your Solar Charge Controller:

https://shop.pkys.com/Victron-Energy...gaAourEALw_wcB

Beyond the 1000 Watts you are limited to 150 volts and 70 amps.

Depending on how you decide to configure your two arrays it sounds like you might be able to chose a smaller Controller, at least for your 100 Watt panels. Mixing the Solar controllers will not be an issue as they will both output the 12 + volts needed to charge the battery.

PS: Choosing to parallel three panels together might also allow you to step down to a smaller Controller allowing you to save some money.

PPS: PKYS is where I bought all my Victron equipment, I really liked doing business with them and their online information is top notch.

[Dale B]

Very good points & the reason I stayed parallel with my smaller system.

I'm in the mountains of BC, July & August are not the problem.

Then the condition of heavy smoke form forest fires, maybe series would be better in this situation.