Charging a 24v Bank with "20v" Panel?

[Wiznaz]

How well will an EpEver Tracer MPPT C.C. charge a 24v Bank with a single "20v" 60 Cell panel?

A while ago I bought the Tracer 4210A 40 amp MPPT to upfit my van. At the time, all I planned on using was a 12v bank. Another Florida summer later, I decided to add an small AC unit to the van and I think going 24v would be a better fit. This also allows me to run smaller power cables under the door's weather strip. I got a great deal on a 310W 60 cell Silfab SLA-M from my local solar company. The van is not long enough to fit a 72 cell on the roof.

Depending on the controller, they say 60 cell panels (Voc<38v) either can't charge 24v batteries efficiently or even at all. My panel has a Voc of 40.25v which seems to be entering 72 cell/ 24v range. The local solar company also has a 60 cell Solaria PowerXT 355W with a Voc of 47.7v if I need to exchange it. The Tracer manual does not mention anything about this specific use-case. Just "Best", "Max" but no "Min". There's a video on Youtube showing a Midnite Solar MPPT charge controller charging a 24v battery with a 60 cell with much less efficiency. Would I be going a step backwards ordering a 24v inverter? Should I wire the batteries in parallel and keep everything 12v? Thank you.

 

[gnubie]

Check the Vmp. If you are using a MPPT controller you want Vmp to be at least 32V, IMO. I'd like to see at least 36V really. If you have panels connected in series you add the voltage to get the string Vmp.

With Voc of about 40V I'm guesstimating your Vmp is about 32V, check the specs on the back of the panel. A 24V battery will range up to 29.4V under absorption charging. 32V will only just let most MPPT controllers work. Some MPPT controllers also have a pseudo PWM mode and will go into this mode under conditions like this. As the sun warms the cells their Vmp will drop, potentially taking that 32V down below battery voltage and charging will cease or reduce significantly as the MPPT controller is now working above Vmp where the current quickly falls off.

*lots of edits to add more detail about Vmp vs battery vs MPPT vs temperature*

 

[Wiznaz]

Check the Vmp. If you are using a MPPT controller you want Vmp to be at least 32V, IMO. I'd like to see at least 36V really. If you have panels connected in series you add the voltage to get the string Vmp.

With Voc of about 40V I'm guesstimating your Vmp is about 32V, check the specs on the back of the panel. A 24V battery will range up to 29.4V under absorption charging. 32V will only just let most MPPT controllers work. Some MPPT controllers also have a pseudo PWM mode and will go into this mode under conditions like this. As the sun warms the cells their Vmp will drop, potentially taking that 32V down below battery voltage and charging will cease or reduce significantly as the MPPT controller is now working above Vmp where the current quickly falls off.

Thanks, Gnubie. Vmp is 33.05v. Right now it's not worth the hassle of hauling around a 2nd panel since I only need to use the solar 1 or 2 days per week. The van is very small. The 60 cell takes up the entire roof. Maybe I should exchange it for that Solaria 355W. That panel's Vmp is 39.5v

 

[gnubie]

That's really going to come down to convenience. A 12V inverter will run your aircon, you just need to beef up the cabling. The cabling you are running under the weatherstrip is the DC cabling to the battery accessed from outside of the van? If you have restrictions like that 24V might be your best option. You can't run the wires through holes etc?

 

[Wiznaz]

That's really going to come down to convenience. A 12V inverter will run your aircon, you just need to beef up the cabling. The cabling you are running under the weatherstrip is the DC cabling to the battery accessed from outside of the van? If you have restrictions like that 24V might be your best option. You can't run the wires through holes etc?

There's spacing under the van meant for a spare tire that I was going stow the batteries sealed in an enclosure. It's a space issue with a tiny van but it excels at everything else. I could drill a hole or 2 but I like everything to look clean so I will have to plan it out.