Am I leaving power on the table?

[thepartyhound]

I just ordered a Victron 75/15 (12V) smart MPPT for my small camper system. I have 200Ah of LiFePo⁴ batteries and a Multiplus 12/3k. My power use is very minimal at the moment and I typically can run off these batteries for 2-3 days between recharging. I had an extra solar panel I had planned to put on the camper. It is a 72 cell 315W panel that should just fit. Should I have ordered a bigger SCC to take full advantage of the (theoretical) 315W? It will be flat mounted, so not likely to produce the full output. Or is this going to be a pretty close matchup and the extra money for the next step up SCC would have just been wasted?

 

[sunshine_eggo]

75/15 outputs ~220W @ 14.5V

Flat panels on a roof - most of the year, you will probably not lose out on much, but in the summer, you will.

 

[skyking1]

It depends on where you usually camp. The site I go to every July, roof mounts are worthless. The RV is parked firmly in the shade of the trees. I have loose panels that I can get about 6 hours of good sun on if I move them frequently.

 

[sunshine_eggo]

Lightweight deployable panels FTW....

Cheap Dual Flex Panel Mobile/Tilting Stand

Here’s a super lightweight and cheap adjustable stand design that I use for 100w flex panel pairs. Each unit costs

diysolarforum.com

 

[skyking1]

I have a pair of 100W renogy that I slap together with some clamps, and do the dance to stay in the sun. 120' trees.

 

[thepartyhound]

Ok, thanks everyone. It sounds like that 75/15 should work fine. If I ever decide to add another panel I'm sure I'll be cussing myself out for not getting the 100/30 to start with, but I doubt I'll add another panel.

 

[sunshine_eggo]

I have a pair of 100W renogy that I slap together with some clamps, and do the dance to stay in the sun. 120' trees.

Bruh!

 

[mciholas]

It will be flat mounted

Expect to clean it periodically.

Mike C.

 

[thepartyhound]

Expect to clean it periodically.

Mike C.

Yeah, I'm sure you're right.

 

[2Big2B]

Bruh!

View attachment 215708

 

[sunshine_eggo]

I suggested the PVC frames and provided a link.

I believe you came back with some sort of foamboard or other backing.

 

[thepartyhound]

FWIW to anyone that stumbles upon this thread down the road, I have been seeing a pretty consistent 220W from this flat mounted, used, 315W panel. That is in the middle of the day with full sun.

 

[Dinobot248]

Jumping in kind of late but I prefer the 100/20 SCC. It provides some future proofing and adapts to multiple voltages.

It’s the smallest 48v version SCC. It can operate at 12v, 24v, and 48v … or max 260w, 520w, 1100w respectively (can’t remember the exact max watts).

 

[sunshine_eggo]

Jumping in kind of late but I prefer the 100/20 SCC. It provides some future proofing and adapts to multiple voltages.

It’s the smallest 48v version SCC. It can operate at 12v, 24v, and 48v … or max 260w, 520w, 1100w respectively (can’t remember the exact max watts).

The issue with the 100/20 is that it's very difficult to get sufficiently high Vmp for 48V charging and retain a low temperature voltage margin.

 

[KnobCountry]

Many of the panels that have a 19-22v range can be 3s and get 60-70v total which is well under the limit.

3s works great with the 100/20 but typically with panels 200 watts and under. Definitely for smaller systems or systems where separately oriented strings each use their own scc.

for example:

Specifications
-Dimension: 55.28*27.76*1.38 inches
-Weight: 24lbs
-Max Power Output(W): 200W
-Voltage MPP Vmp(V): 18.60V
-Current MPP Imp(A): 10.76A
-Voltage Open Circuit Voc(V): 21.8V
-Short Circuit Current Isc(A): 11.47A

 

[sunshine_eggo]

Many of the panels that have a 19-22v range can be 3s and get 60-70v total which is well under the limit.

Except those are 48-56Vmp, which is insufficient to reliably charge 48V batteries, especially when you factor in the drop in voltage associated with heating even in mild ambient conditions.

3s works great with the 100/20 but typically with panels 200 watts and under. Definitely for smaller systems or systems where separately oriented strings each use their own scc.

for example:

Specifications
-Dimension: 55.28*27.76*1.38 inches
-Weight: 24lbs
-Max Power Output(W): 200W
-Voltage MPP Vmp(V): 18.60V
-Current MPP Imp(A): 10.76A
-Voltage Open Circuit Voc(V): 21.8V
-Short Circuit Current Isc(A): 11.47A

18.6V * 3 = 55.8V

Panels sitting in the sun in 20°C ambient see cells working at 45°+C and experience a ~10% drop in Vmp, so now you're looking at closer to 51Vmp.

I think we can both agree that 51Vmp is not optimal for charging 48V batteries. Yes, you'll get some charging sice Voc will be higher, but you'll be operating the very undesirable region between Vmp and Voc.

4S would be great, but then your margin drops to 12V. As long as it doesn't get too cold, you're good.

 

[Dinobot248]

You guys make a good point about the relatively higher Vmp required to charge a 48v system when Voc is capped at 100v (and less when considering temperature compensation).

I think the 75 / 15 SCC is more than adequate for the OP’s 12v system for around $70.

I run a 48v system and appreciate the 100 / 20 SCC for $90 to allow me to use the SCC in different use cases such as temporary 12v system for camping to making use of a small batch leftover panels for my 48v primary system during winter time.

 

[sunshine_eggo]

You guys make a good point about the relatively higher Vmp required to charge a 48v system when Voc is capped at 100v (and less when considering temperature compensation).

I think the 75 / 15 SCC is more than adequate for the OP’s 12v system for around $70.

I run a 48v system and appreciate the 100 / 20 SCC for $90 to allow me to use the SCC in different use cases such as temporary 12v system for camping to making use of a small batch leftover panels for my 48v primary system during winter time.

Don't get me wrong... it's blue; therefore, I want to marry it! But some miss this limitation and are disappointed with performance by leaving too much headroom, BUT with 90Voc or less and temps above freezing, it should work great.

 

[robbob2112]

Don't get me wrong... it's blue; therefore, I want to marry it!

Be honest - you sleep with the blue and just hope it is only late.

 

[sunshine_eggo]

Be honest - you sleep with the blue and just hope it is only late.

quite the opposite... BLUE BABIES!