Need help rating charge controller based on solar array!!

[Dieselstein]

I’m considering buying two 245 watt 37.5v solar panels and using with the 24v LiFePo4 raw prismatic battery bank that Will designed. I’m having a bit of trouble, any insight?

 

[svetz]

Take a look at this and see if it doesn't explain things: Figuring out how many panels in series and parallel based on your MPPT

 

[Dieselstein]

Oof. This may be a little advanced for me currently, as I’m beginning to build my first system. My plan was series connect four 100w 12v panels to a Victron 100/20a charge controller. I found some lightly used panels with the 245w 37.5v rating (which would save me space on top of my van) but now I’m concerned that if I take this route, I may have to change components. I’m just not sure where to start.

 

[svetz]

Will's videos are the place to start! Understanding the basics will get you over the hump.

Beginner Video Playlist

 

[svetz]

Oof. This may be a little advanced for me currently, as I’m beginning to build my first system. My plan was series connect four 100w 12v panels to a Victron 100/20a charge controller. I found some lightly used panels with the 245w 37.5v rating (which would save me space on top of my van) but now I’m concerned that if I take this route, I may have to change components. I’m just not sure where to start.

If sounds like you're getting it!

A 245W 37.5V panel is probably around 6.5 amps (check the datasheet of your solar panels to be sure, but I'll use that for these sample calculations).
I'll also assume you have two panels, but adjust as needed...

If you put two in series you'd have 2x37.5 = 75V and 6.5 amps. Or in parallel 37.5V @ 2x 6.5 = 13 amps.

Update: Oops! Deleted the error that Will corrects below (Thanks Will!)

 

[Will Prowse]

That 13 amps is at the working voltage of the panel. For calculating the SCC size, you need to divide by nominal voltage of the battery bank because SCC's are rated at the output, not the input. The input will always be lower than the output, current wise.

So: solar array wattage / nominal voltage of the battery bank = current rating of SCC

So we can do this one really quick:
490 (watts of solar array) / 24 (voltage of battery bank) = 20.4166 (amp output rating of the charge controller)

20.4 amps is larger than the max rating of the victron 20A controller! But no need to worry! The victron has some headroom current wise and safety functions, so you can connect this SCC to these two panels. You can put the panels in series or parallel. But you should be good to go!

 

[Dieselstein]

Okay now that makes perfect sense! Would it be more appropriate to for the 30a controller since the headroom is cutting so close?

 

[Dieselstein]

Thank you for this! I’ve done quite a lot of binge watching every relavent video of Will’s! I suppose my impatience has gotten the best of me. Thank you both for your expediency!

 

[Craig]

Okay now that makes perfect sense! Would it be more appropriate to for the 30a controller since the headroom is cutting so close?

Yes and always figure for possible future expansion.