[rescue] DC Voltage distro

[Sandwich Maker]

" From: patrick at mail.zill.net (Patrick Giagnocavo)
" 
" On Wed, Sep 22, 2004 at 02:00:37PM -0400, Sandwich Maker wrote:
" > " From: patrick at mail.zill.net (Patrick Giagnocavo)
" > " 
" > " Aside:
" > " 
" > " The old telco batteries are 2V batts and thus you need at least 24 of
" > " them for 48VDC.  Or just 6 for 12VDC.
" > 
" > nit: 2v is a -cell-.  a -battery- of cells makes a 'battery'.
" 
" Maybe internally it is composed of a series of 2V cells wired in such
" a way as to be a battery?

telco tend to be single cells, and they made some really big ones -
2000a iirc, something like 2' dia x 3' tall.

balancing a parallel stack isn't easy since cell voltage changes with
charge, electrolyte density, temp etc.  in a series stack, all the
current goes through every cell.

10ka batt plants are not unknown.  but that's paralleling entire 48v
stacks, probably through some sort of power monitor. [0]

anecdote: when i worked at attbl, a craft person told me of the time
he dropped a crescent wrench onto the bus bars of a big metro co while
working on the overhead track.  it should've been clipped to a
lanyard, per safety regs...

the wrench melted in half.  the 48v bus didn't even glitch.

he was lucky.  he could've caused a -very-nasty- batt explosion, just
from the current surge.

he kept the melted handle end on his belt as a safety reminder for
many years after.

[0] batts were supposed to hold up for 4h.  the genset to take over,
in a medium co, was a 3600hp diesel.
________________________________________________________________________
Andrew Hay                                  the genius nature
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