diy solar

diy solar

Anyone care to check my math before I pull the trigger?

308andyj

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Apr 11, 2021
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I am about to purchase 16 Lishen 272ah cells (3.2V each).

I want a 24v system so I plan to do a 2P8S battery bank. I will be getting a Growatt 24v SPF 3000TL all-in-one unit.

The battery bank will be 544 ah (13 kwh) once assembled.

I have 10 cheap panels which have been collecting dust for years, They're 12v 100w for a total of 500w at 24v.

My area gets right at 5 hours of "sun hours" daily average throughout the year. My 500w of panels will yield 2.5kwh @ 24v (104ah) daily if the 5 hours average holds true.

If I were to ever discharge the bank to 20% I would have 435ah to replenish to get back to 100% (Which on LiFePo4 is not good anyway) which would take just over 4 days of good sun.

I'm almost embarrassed to admit, I'm an electrician by trade, but some of this is all new to me. No one ever asked for my knowledge of theory to determine if I was fit to run conduit and pull wire, haha. I would appreciate anyone who can verify my math looks ball park correct, or if I am missing any gross details.

Thank you all!
 
Most opt for 2X 8S batteries in parallel each with their own BMS vs. 2P8S. This allows for easy redundancy if you have a cell go out. If you have a parallel cell go out, it will take down its cell-mate(s), and you have to reconfigure the whole bank to find the problem.

BMS?

10X 100W panels are still 1000W even at 24V. Power is constant. Voltage is doubled, current is halved.

You have 1000W of solar. Since you're using MPPT, you'll probably want to go 5S2P to bump your voltage up. Assuming your 5S Voc is less than 145V.
 
Most opt for 2X 8S batteries in parallel each with their own BMS vs. 2P8S. This allows for easy redundancy if you have a cell go out. If you have a parallel cell go out, it will take down its cell-mate(s), and you have to reconfigure the whole bank to find the problem.

BMS?

10X 100W panels are still 1000W even at 24V. Power is constant. Voltage is doubled, current is halved.

You have 1000W of solar. Since you're using MPPT, you'll probably want to go 5S2P to bump your voltage up. Assuming your 5S Voc is less than 145V.


Oof, thats an amatuer level mistake on my part, thanks for catching that!

Voc is 21.6 per panel so 5s2p would work.

I haven't decided on BMS yet, I see 2 other posters asking BMS questions with very similar setups as what I'm planning so I hope to glean some good info from their threads as people chime in.

In regards to the batter bank, you're suggesting batteries 1a,2a,3a,4a,5a,6a,7a,8a in series (272ah @ 12v) and batteries 1b,2b,3b,4b,5b,6b,7b,8b in series (a second string of 272ah @ 12v) then the 1a and 2a paralleled together to get the requisite 24v?

Thanks for your knowledge!
 
8cells in series = 24 volts , then another 8 in series =24 volts, two 24v in parallel still =24volts
 
Oof, thats an amatuer level mistake on my part, thanks for catching that!

Easy mistake. I kept flipping things in my head with my split phase setup... I have 2X 5000W inverters (eventually) in parallel running split phase... so do I have 5000W of 240V or 10000W of 120V... NEITHER!

10,000W of 240V of course, just the same current as one in single phase 120VAC!

Voc is 21.6 per panel so 5s2p would work.

I haven't decided on BMS yet, I see 2 other posters asking BMS questions with very similar setups as what I'm planning so I hope to glean some good info from their threads as people chime in.

In regards to the batter bank, you're suggesting batteries 1a,2a,3a,4a,5a,6a,7a,8a in series (272ah @ 12v) and batteries 1b,2b,3b,4b,5b,6b,7b,8b in series (a second string of 272ah @ 12v) then the 1a and 2a paralleled together to get the requisite 24v?

No.

Battery A: series cells 1-8 w/BMS
Battery B: series cells 1-8 w/BMS

Parallel A and B like you would any other 24V battery.
 
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