5kW 48V system sizing, circuit & safety check

timblack1

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Joined
May 19, 2021
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12
Hi,

I'm designing a 5kW 48V system to upgrade my camping trailer from having 12V batteries and a 120VAC to 12VDC converter to having 5kW of solar panels, a 48V MPP Solar Hybrid LVX 6048 all-in-one, a 17.9 kWh 48V battery, and a 48V-12V DC buck converter. My current design is attached. Did I get the sizing, circuits, wire gauges and fuse sizes right?

Thank you for any help you can offer!

Kind regards,

Tim
 

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TomC4306

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I'm trying to understand your battery diagram. It seems you have 20 each 3.2v cells and wish to use all of them. This layout inside a 'battery pack' is not going to work. My opinion is to string 16s, with a 16s bms and have 4 spare cells. Or build a separate 12v battery for whatever. Or buy 12 more cells, get a second 16s bms and then parallel those 2 'batteries' together. Or buy 12 more cells and parallel ALL 16 inside the battery with one 16s bms.
 

timblack1

New Member
Joined
May 19, 2021
Messages
12
I'm trying to understand your battery diagram. It seems you have 20 each 3.2v cells and wish to use all of them. This layout inside a 'battery pack' is not going to work. My opinion is to string 16s, with a 16s bms and have 4 spare cells. Or build a separate 12v battery for whatever. Or buy 12 more cells, get a second 16s bms and then parallel those 2 'batteries' together. Or buy 12 more cells and parallel ALL 16 inside the battery with one 16s bms.
Thank you for this feedback; I need it. I have not bought the cells yet. My goal is to have a battery with at least a 16.1 kWh capacity, to have 3 days' worth of capacity to handle a 5.38 kWh/day load. 20 3.2V Lishen 280 Ah cells would provide that capacity, but I've been unsure whether there is a configuration which could make them work with the 48V inverter. What is a better way to provide the 16.1+ kWh capacity (and hopefully end up with 48V)? Use individual cells with a different Ah capacity?

Would the battery layout I proposed fail to charge/discharge some cells to their full capacity?

[Edited to say 280 Ah instead of 240 Ah.]
 
Last edited:

TomC4306

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Mar 2, 2021
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P=I*E.
Power (watts)= amps*volts.
Algebra gets you any unknown from 2 known.
Watts gets you amps.
Watt hours gets you amp hours.

16100 wh/51.2 volts = 314 amp hours.
(3.2v x 16 cells =51.2 volts nominal)
240ah batteries 16 cells, serially attached
With a 100amp 16s bms.
12288 wh. With 5120 watts available at the inverter. But maxing the bms.

Make 2 of those 16 cell with 100 amp
bms batteries. Paralleled.
You'll have 200 amps max available at the battery. More than the inverter could ever draw.

Or... Buy slightly smaller capacity cells.
Cheaper. More available. Redundancy. One battery CAN handle the load if need be but in normal ops running 50%
 

TomC4306

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Mar 2, 2021
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It's not viable.
The 12s will discharge twice as fast as the 8p because they have half the capacity. You'll hit low voltage disconnect on one of those 12s cells and the bms will shut off the battery. Meanwhile, your 8p are sitting there half full.

Not to mention the opportunity for all 20 cells to get wildly out of balance.
Capacity CAN be increased but there are realities to how electrons move that govern the increment and design.
 

TomC4306

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Mar 2, 2021
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It would never balance. As soon as you started discharging.

Try it. Take pic.

Edit. Heck I don't think you could charge it as a unit.
 
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Jimmynik1

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Apr 3, 2021
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Wouldn't charge or discharge evenly. I'd get 12 more cells and have a 32 cell bank in a 16s2p.
 
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