24v LiFePO4 battery, 36 cell panels. Series/Parallel?

[DavidS]

I want to check my thought process here.  Building a 24v LiFePO4 battery from 280Ah EVE cells in 8s config.  This will be used in an RV, so there are constraints on the physical width of panels mounted on the roof, so 36 cell panels are preferred as 72 cell panels are generally too wide @ 39".  I understand that I can series connect two 36 cell panels, assuming Voc = 24.35V and Impp = 9.85A getting me ~48v @ 9.85 amps for 2 panels.

Question: can I parallel connect 2 additional, identical panels, effectively creating two 72cell panels via series/parallel connection of four 36 cell panels?  The theory being I'd have ~48v at ~20A?

Is there any additional recommended fusing/breakers if that arrangement is valid? 

thanks!

[Dacian Todea]

DavidS,

Two 36 cell panels in series will be in no way different from a single 72 cell panel and yes the DSSR20 can handle 4x 36 cell panels if connected first as groups of two panels in series then those two groups in parallel.

60 cell panels will be ideal with 8s LiFePO4 but they will be the same with as 72 cell panels just shorter.

Two 60 cell panels will likely cost less and provide the same current to battery. 

[DavidS]

Dacian, thanks, that matches my understanding.  It certainly could be much less expensive to pay for four 300 or 400 watt, 60 cell panels, particularly if I can get good used cells.  The question is, can I make them fit?

Appreciate your time and response.  Take care and keep warm :).

[Dacian Todea]

I think with the most efficient cells 60 cell panels will be at most 350 to 360W. 

The difference is that 60 cell panels are just shorter by two rows than the 72 cell panels as 60 cells are usually constructed as 6x10 vs the 72 cell panels that are 6x12 cells.

So say a 300W panel made with 60 cells will be equivalent with a 360W 72 cell panel assuming the exact same efficiency cells are used but when charging a 8s LiFePO4 through the DSSR20 the 60 cell panel will perform exactly the same as the 72 cell panel so you just pay extra for 12 cells and it takes more space.

Thus two 60 cell panels will be equivalent with 4x 36 cell panels in terms of output in to the 8s battery but the 4x 36 cell panels will take quite a bit more area on the roof.   The 36 cells are made as 4x9 cells so they are just slightly shorter (about one 6" cell) shorted than the 60cell 6x10 panels.

So if you can fit 3x 60 cell panels instead of the 4x 36 cell panels then you get 50% more energy using almost the same area.