MPPT controller panel configuration, does it matter?

[slimbo]

Afternoon!

I'm currently in the process of building a Sprinter 170 to be a mobile home for my wife and I. I'm planning out my solar and have 4x100w panels (18.9v/5.29a each), with a 40a MPPT controller. I'm trying to better understand which panel configuration might be better for the controller.

Series - This would give me most volts and least amperage, but with the downfall that if one panel is shaded, the entire system voltage drops.
Parallel - This would give me the most amperage and least voltage which means that wiring would need to be substantial, but least impact from shading.
Series-Parallel - Kind of benefits of both worlds...medium amps, medium voltage, and limited impact from shading (compared to a full Series deployment).

Assuming shading, wire gauge considerations and any advantages of the config (ie Higher voltage will start charging sooner with less sun)
My question is this...from the MPPT perspective, would the output charging amps from a full series install (75.6v/5.29a), or 2S2P install (37.8v/10.58) or or a full parallel install (18.9v/21.16a) be best. I have a feeling that I'm overthinking this and volts in = amps out and amps in = amps out. Meaning that whether the power comes in as amps and that is sent to the battery bank, or it comes in as high voltage and is converted to amps, it will net me the same amperage either way (no magic way to create power, right).

IN an effort to make my question clearer, maybe I'l pose it this way.

Assuming full sun on a full series system would give me 75.6v/5.29a, and the MPPT converts the extra voltage to amperage - what would my amperage be? Forgetting loss in efficiency for now, would it ultimately work out to the same amperage as if I had full sun on a full parallel install (21.16a)? IE: is it simple Ohms Law and nothing more involved than that?

 

[sunshine_eggo]

Series - This would give me most volts and least amperage, but with the downfall that if one panel is shaded, the entire system voltage drops.

True, but it's not the death sentence one would fear provided the panels have decent bypass diodes. While system voltage drops, it only drops by the portion of the shaded panel(s), so the penalty isn't catastrophic. If you have 4 in series and one is fully shaded, you will still get the better part of the 3 unshaded panels' output.

Parallel - This would give me the most amperage and least voltage which means that wiring would need to be substantial, but least impact from shading.

Not as much benefit as most think. Marginally better than 4S. This also adds another burden - all 4 panels must have their own fuse/breaker. This is required for 3 or more strings in parallel.

Series-Parallel - Kind of benefits of both worlds...medium amps, medium voltage, and limited impact from shading (compared to a full Series deployment).

Yep. I prefer this.

Assuming shading, wire gauge considerations and any advantages of the config (ie Higher voltage will start charging sooner with less sun)
My question is this...from the MPPT perspective, would the output charging amps from a full series install (75.6v/5.29a), or 2S2P install (37.8v/10.58) or or a full parallel install (18.9v/21.16a) be best.

It depends...

MPPTs operate most efficiently at about 150% of battery nominal voltage. The progressively larger converter from PV to battery voltage is progressively less efficient, so 2S, 3S, 4S, etc., are less efficient.

But...

This is usually offset by improved wiring losses.

I have a feeling that I'm overthinking this

Yes, but it's a good mental exercise.

and volts in = amps out and amps in = amps out.

Nope, but you have the concept correct in the quote below. Volts_in * Amps_in = Volts_out * Amps_Out.

Meaning that whether the power comes in as amps and that is sent to the battery bank, or it comes in as high voltage and is converted to amps, it will net me the same amperage either way (no magic way to create power, right).

Correct.

IN an effort to make my question clearer, maybe I'l pose it this way.

Assuming full sun on a full series system would give me 75.6v/5.29a, and the MPPT converts the extra voltage to amperage - what would my amperage be? Forgetting loss in efficiency for now, would it ultimately work out to the same amperage as if I had full sun on a full parallel install (21.16a)? IE: is it simple Ohms Law and nothing more involved than that?

Amperage out = ((75.6V * 5.29A) / (battery voltage)) * inefficiencies

In reality, you'd be hard pressed to calculate or measure ±1% difference in any of the above cases UNLESS you have long PV wiring runs between the PV and the MPPT. In that case, more in series is almost always the winner.

Go 2S2P with appropriately sized wire and be confident you're system is well designed/deployed.

 

[slimbo]

True, but it's not the death sentence one would fear provided the panels have decent bypass diodes. While system voltage drops, it only drops by the portion of the shaded panel(s), so the penalty isn't catastrophic. If you have 4 in series and one is fully shaded, you will still get the better part of the 3 unshaded panels' output.



Not as much benefit as most think. Marginally better than 4S. This also adds another burden - all 4 panels must have their own fuse/breaker. This is required for 3 or more strings in parallel.



Yep. I prefer this.



It depends...

MPPTs operate most efficiently at about 150% of battery nominal voltage. The progressively larger converter from PV to battery voltage is progressively less efficient, so 2S, 3S, 4S, etc., are less efficient.

But...

This is usually offset by improved wiring losses.



Yes, but it's a good mental exercise.



Nope, but you have the concept correct in the quote below. Volts_in * Amps_in = Volts_out * Amps_Out.



Correct.



Amperage out = ((75.6V * 5.29A) / (battery voltage)) * inefficiencies

In reality, you'd be hard pressed to calculate or measure ±1% difference in any of the above cases UNLESS you have long PV wiring runs between the PV and the MPPT. In that case, more in series is almost always the winner.

Go 2S2P with appropriately sized wire and be confident you're system is well designed/deployed.

Awesome, thanks for confirming everything in great detail. I tend to overthink things and I think I got myself wrapped up in the "what if's" when I think I already had my answer. Thank you again for confirming!

 

[sunshine_eggo]

Awesome, thanks for confirming everything in great detail. I tend to overthink things and I think I got myself wrapped up in the "what if's" when I think I already had my answer. Thank you again for confirming!

Better to over-think now than replace stuff that didn't work like you though it would.

That's what I did.

EDIT: to be clear... I replaced stuff... like twice.

 

[mikefitz]

With a MPPT controller and 400 watts of panels under ideal conditions the controller output amps will be,
panel total watts / charge voltage.
Typical charge volts of 13.5 gives 29.6 amps.
You need a 30 amp MPPT controller, I recommend the Victron Smart 100/30.

With 4 panels in series under cold conditions you may exceed the 100 volt limit, the wire the panels as two strings each consisting of two panels in series and the two strings in parallel.

Regard the MPPT controller as an energy converter, it converts input power to a suitable voltage and current to charge the battery.

Note its the open circuit voltage that determine the maximum input to the controller, for a 100 watt panelthis is around 22 volts at 25 degrees C. It increases with lower temps , say 4% per each 10 C below 25C.

Mike