Re: How much current safe for 30m extension?

bigcat_at_meeow.co.uk
Date: 02/13/05 

Date: 13 Feb 2005 05:45:44 -0800

Watson A.Name - "Watt Sun, the Dark Remover" wrote:
> <bigcat@meeow.co.uk> wrote in message
> news:1108237955.923021.112840@f14g2000cwb.googlegroups.com...

> > I'm in the U.S. and this gives me a flashback on what I was
thinking
> > about trying a few months ago. We have a PBX at work that's on 48V
> > batteries, but the batteries are 9 yrs old and need replacing.
They
> > cost a bundle
>
> > the first logical thought is repair them rather than replace. Lead
> > acids are often repairable.
>
> > Quick repair, doesnt always work:
> > rinse muck out of cells
> > fill with new acid
> > charge, monitor acid conc and adjust as required
>
> > This wont fix all cells by any means, but many it will. Those it
wont
> > in some cases may only need replacement plates and acid, which can
be
> > made much cheaper than buying new batts.
>
> No. These cells are 4V each, 12 in all, each is sealed.

If youre going to deal with this you'll need a few clues. You will have
2v cells in pairs, 4v lead acid cells dont exist.

> Well, except
> for the one cell that's split open because of internal pressure. :-(

> They must be replaced, not repaired. This is a phone system, where
it
> has to be online during emergencies. Batteries that are working
fine,
> but over 5 years old are considered unacceptable and must be
replaced.

thats fine if youve got the money, but since they've been there 9
years, and are found dead but still in service, I was guessing you
havent.

Is it April yet?

> > Also they can often be run happily at 24v, if not in all cases. If
> > yours could, it may be that your present cells would do that as is.

> This is *not* how you run a battery backup system! The idea is to
start
> out with fully charged batteries, so that when the power fails for a
> long period, the voltage of the batteries may drop to 40V or less as
the
> cells discharge, keeping the PBX online. If you start at anything
less,
> even 36V, you're not going to have any discharge time before the PBX
> system crashes. It would be essentially worthless.

You misunderstood completely, hopelessly, and idiotically. 48v is a
very old standard, and many 48v telecomms systems are now run on 24v.
If your equipment were 24v compatible, as some is, you would only need
half the number of cells.

And if you had no money to replace the battery, odds are you'll have at
least 50% of cells still serviceable.

Even if its not 24v compatible, you would get better backup performance
by removing any dead cells from the chain. Whether you remove them or
not you'll run below voltage.

> > What capacity at 48v is it on?
>
> The rectifier can put out a hundred amps at 54VDC, to charge the
> batteries while the PBX is also running. That must be 6 or so kW
input.
> So I would guess that it's 208V at 30A breakers. But with no
batteries,
> the load would be more like 3.6kW.

3.6kW @ 50v = 72A, so if we take an ultra-vague guess at C/10 charging,
that gives us 700ah capacity @48v. Ouch.

> > I thought it would be possible to run a power cable
> > underground to the big UPS we have in our computer room. Problem
is
> > that the PBX's rectifier takes 30A max at 120VAC, or about 3.6kW.
>
> > i've no ida how that fits into this
>
> The UPS in the computer room is 150kW, easily capable of handling
> another 6 or so kW.
>
> > And
> > the distance between is about 1300 feet or about 400m.
> >
> > I would guess that the UPS output should go into a transformer and
> come
> > out 480VAC, so the cable losses would be minimized. Then another
> transf
> > on the PBX end to bring it back to 120VAC. But should I expect to
> have
> > a max loss of 5% at max current, or what?
>
> > well how much current are you shifting??
> > And why cant you power your pbx from whatever power source it uses
> now?
> > You havent given us nearly enough information.
>
> The batteries and rectifier are a "UPS" to protect the PBX against
> failure of the commercial power. That's what it's being fed from.

Why dont you find someone else to discuss this with.

> > I think I came up with 4GA
> > cable, but at 480VAC, I'm guessing that it would have to be special
> > insulated underground cable.
>
> > standard uk T&E should do that, but shuold be insulation tested to
the
> > required v first. Costs around £12 a 100m reel, depending on
copper
> > size.
>
> Assuming that it could be pulled 1300 feet. Probably not without
> damage.

You thread it section by section. Regular access points also allow a
section can be replaced if it fails. The T&E also includes a spare
conductor for backup.

> Heavily insulated cable would be needed. Probably double
> jacketed.

T&E is double insulated, high v rated and cheap - thats why i suggested
it.

> > Someday I'll have to ask one of the electricians that work on our
HV
> > stuff. We have 4160VAC underground around campus but that's all
> > specialized switchgear, etc.
>
> > rf transmission co-ax is rated pretty high... and its concentric.
>
> RF transmission coax is typically copper clad steel wire center
> conductor, which would have much greater I^2R loss than pure copper
> cable at 50 or 60Hz. That would be unacceptable.

obviously it depends what v and i youre running it at, its very much
higher v rated than T&E.

> > T&E's probably far more practical.
>
> Thanks, but I've never heard of T&E here in the U.S. I don't know
what
> that acronym stands for.

twin and earth, its our standard house wiring cable, 2 cores double
insulated plus one single, comes in various copper sizes, is very cheap
and can be bought at thousands of outlets... here, anyway. I dont know
what export would cost.

You would need to get some basic concepts sorted out before youre in a
position to imlpement anything.

NT

Relevant Pages

  • Re: How much current safe for 30m extension?
    ... monitor acid conc and adjust as required ... >> This wont fix all cells by any means, ... Batteries that are working ... > RF transmission coax is typically copper clad steel wire center ...
    (sci.electronics.equipment)
  • Re: How much current safe for 30m extension?
    ... monitor acid conc and adjust as required ... >> This wont fix all cells by any means, ... Batteries that are working ... > RF transmission coax is typically copper clad steel wire center ...
    (sci.electronics.equipment)
  • Re: How much current safe for 30m extension?
    ... monitor acid conc and adjust as required ... >> This wont fix all cells by any means, ... Batteries that are working ... > RF transmission coax is typically copper clad steel wire center ...
    (sci.electronics.components)
  • Re: Why Do Batteries Suck So Much And When Is It Going To Stop?
    ... > different from a lead-acid cell from the Civil War era. ... at best three years to batteries that routinely have a five-year warranty. ... Maintenance free (actually recombinant cells) that capture the gases ... more difficult to recycle, and work continues on that front (because there ...
    (rec.woodworking)
  • Re: A dSLRs fastest shutter-speed is really no faster than its flash-sync speed.
    ... if F cells would be used instead of AA cells. ... I made a JOKE about batteries in hybrid cars, ... people who don't use their cameras much (my "accident camera" in the ... out, the NiMH proponents repeatedly misrepresent the facts, and I don't ...
    (rec.photo.digital)