[SOLVED] Voltage drop: Acceptable when panel voltage is 2x battery?

[cdevidal]

Looks like I may need to accept a 4.75% voltage drop on my run of two Renogy 175W flexible panels in series. (23.9Voc/19.5Vmp/9.5A Ioc/8.98A Imp) Single string, both in series. But is this acceptable when connected to a Renogy Rover 40A MPPT controller and Sok 12V 100Ah LiFePo4 battery? I won't get the entire capacity of the system but I should get at least 90% right? Because panel voltage is about 2x what the battery is, I should have overhead.

I intend to cycle the system to 20% SOC daily, my daily insolation is 4.5 hours, peak efficiency of the controller is 98%, so if all is perfect I should be able to recharge 80% every day, so this might mean I'm down to something like 75% right? I can tolerate that.

 

[DThames]

5% is not too bad with the voltage levels you have. No option to run larger gauge wire?

With the drop over your PV run being V=I*R, the drop will be less most of the time because you only see max amps a couple hours in a day (fixed panels).

 

[RCinFLA]

I intend to cycle the system to 20% SOC daily, my daily insolation is 4.5 hours, peak efficiency of the controller is 98%, so if all is perfect I should be able to recharge 80% every day, so this might mean I'm down to something like 75% right? I can tolerate that.

This statement makes me think your expectations are going to result in disappointment.

You will not get 175 watts out of panels, your efficiency will not be 98%, and expecting to have 80% of battery capacity every day is not realistic.

 

[MichaelK]

I would very much agree with RC's statement. Since your panels are flexible, I'm assuming they will be laying flat out in the sun? Laying flat, there's no way you'd ever get 90% out of them. Even held directly pointing perpendicular to the sun, I'd never expect more than 80-85%. For real-world operation, expect not to ever get more than 60-75% of amps out of a flat panel.

Secondly, 4.5 sunhours is what, a summer measure? Are you located within the tropics? If not, your winter sunhours will be lower. Maybe your best option would be to get one more of these panels and wire three in series instead of two. Your Voc will only be about 70V, which will still be safe for your controller.

 

[cdevidal]

No option to run larger gauge wire?

No problem running larger gauge and in fact I just found there are 8 gauge 100 foot wires, which puts me at 3.12%.

 

[cdevidal]

I would very much agree with RC's statement. Since your panels are flexible, I'm assuming they will be laying flat out in the sun? Laying flat, there's no way you'd ever get 90% out of them. Even held directly pointing perpendicular to the sun, I'd never expect more than 80-85%. For real-world operation, expect not to ever get more than 60-75% of amps out of a flat panel.

Secondly, 4.5 sunhours is what, a summer measure? Are you located within the tropics? If not, your winter sunhours will be lower. Maybe your best option would be to get one more of these panels and wire three in series instead of two. Your Voc will only be about 70V, which will still be safe for your controller.

It's an annual average of 4.5 hours as shown by this map. Yes I can add more panels, no problem. A string of 3 will indeed do just fine.

 

[MichaelK]

It's an annual average of 4.5 hours as shown by this map. Yes I can add more panels, no problem. A string of 3 will indeed do just fine.

That number might be a bit deceiving. I think you'll be better off planning with your winter low rather than the yearly average. That way you'll know you have a working system in December (or June for southerners). I always plan for the worst conditions, not the best.

You might consider getting an additional two panels instead of just one, and wire them in a 2S2P configuration. Though if pointed directly at the sun, they would max the amp limit of your controller, while laying flat, I'd expect them to produce {(175W X 4 panels)/13V charging} X 0.75 (75%) = 40.4A. That might work well for you. A bit of overpaneling will also help better collect a few watts on cloudy days.