It can be done for MUCH less than $28k.
I've even gone so far as laying up a few drawings for a modular battery system of my own design.
I've been designing my system to be housed in 2u rack chassis, similar to a computer server or audio amplifier. This one, in particular, is designed to function as 4 strings, each providing 210AH of storage capacity at 3.2v, which if wired in series would work out to a 12v 210AH pack in each 2u chassis. Wire 4 such chassis together in series, and you'd have a 51.2v (48v nominal) 210AH pack, in 8 rack spaces. Total cost of materials works out to probably something like $500 per 2u chassis. Give or take a bit, as I haven't priced in some smaller costs like bus bars, connectors, etc. yet.
We can go into much greater detail privately, if you're interested, but this same chassis could be configured several different ways, by grouping the cells differently, for example as 6 parallel strings of 20 cells, 12 strings of 10 cells, etc.
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