Re: Batteries in Power Supply

Ive worked on telephone switch battery supplies. when replacing lead
acid batteries they must all be from the same mfg and lot# period in
order to charge and prevent problems between the 24 cells.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Carl Mikkelsen, Oasis"
<Carl.Mikkelsen@o...> wrote:

> Just some thoughts ...
>
> It was suggested that the capacitors could be eliminated if a

battery was

> floated across the power supply output. This started me thinking

about the

> battery problem.
>
> I would be concerned about the high ripple current through the

battery, and

> constant switching between charge and discharge cycles (120 times

per second).

>
> I would not build my own system to work this way, and would put

significant

> filter capacitance on the output.
>
> I would also used lead-acid batteries, because they are rated for
> "floating" on the mains.
> I would use a regulated output supply, set to the proper float

voltage for

> the batteries I was using.
> I would make the power supply be able to gracefully and safely

handle the

> over current condition, and allow power to come from the batteries

(current

> fold back regulation).
>
> I would oversize the batteries, making the batteries peak discharge

current

> be five times the expected peak servo load. [This may be too

conservative,

> and I could change my mind after seeing the battery prices. :) ]
>
> My father would have built this by taking a 12.6 V filament

transformer,

> running it through a selenium bridge rectifier, across some caps,

through a

> light bulb, across the battery, and off to the load.
>
> Since we like to use higher voltages as CNC supplies, I'd be

concerned

> about cell-to-cell variance when placing a large number of cells

(lead-acid

> or otherwise) in series. Each lead-acid cell has a voltage of

about 2

> volts, so you must stack 30 cells to reach 60 volts. That allows a

lot of

> cells for temperature variances, chemistry changes, and other nasty

things

> to build up and cause problems.
>
> Some one of you who is a battery engineer will probably point out

the

> fallacies in what I've said. Even if some of my concerns are

valid, there

> may be rational ways to engineer around them.
>
> It is an area where a little engineering up-front (temp sensors,

current

> monitoring, maybe a little closed-loop charging control) could save

a lot

> of grief later.
>
> I'm sure it is possible to make it work. The phone company had 48V
> lead-acid batteries floating across the line supply for decades,

and maybe

> still does. It's been quite a few years since I've been inside a

phone

> exchange building.
>
> -- Carl
>
>
> At 03:27 PM 1/21/2003 -0500, you wrote:
> >So would this mean that you could eliminate the filter capacitor

from the

> >power supply leaving only the transformer and rectifier?
> >
> >John Guenther
> >'Ye Olde Pen Maker'
> >Sterling, Virginia
> >
> > > -----Original Message-----
> > > From: Mariss Freimanis <mariss92705@y...>
> > > [mailto:mariss92705@y...]
> > > Sent: Tuesday, January 21, 2003 14:14 PM
> > > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > > Subject: [CAD_CAM_EDM_DRO] Re: Batteries in Power Supply
> > >
> > >
> > > Bernard,
> > >
> > > I posted on this topic about a year or so ago. The main

advantage

> > > mentioned was servomotors usually have a large ratio between

their

> > > stall current and continuous rated current (5:1 or more).
> > >
> > > This means the power supply has to be 5 or more times bigger

than

> > > needed just to supply very short duty cycle accel/decel current
> > > pulses to the motor.
> > >
> > > The idea was to use low AH batteries to supply this current;

they

> > > would recharge between accel/decel cycles. The battery would

connect

> > > to the power supply "+" terminal via a rectifier and it's

voltage

> > > would be a little less than the power supply's voltage. Under

load,

> > > the supply voltage would sag, forward biasing the rctifier from

the

> > > battery, which would now supply the pulse current.
> > >
> > > A motor stall current 5 times the max rated current implies a

4% duty

> > > cycle. Theoretically a 1AH battery could supply 20A for 3

minutes,

> > > way longer than you would need for accel/decel (1 or 2 seconds).
> > >
> > > Now the supply could be sized for the continous current only

plus

> > > what would be needed to restore the battery charge (5 * 4% =

20% or

> > > 1.2 times the max rated motor current).
> > >
> > > Mariss
> > >
> > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bernard R
> > > <bwjarandall@c...>" <bwjarandall@c...> wrote:
> > > > In view of the recent interest in power supplies has anyone

thought

> > > > about using batteries in place of capacitors in some of the

larger

> > > > power supplies?
> > > >
> > > > If you used 5 12Volt batteries and 1 6Volt that would give a
> > > nominal
> > > > 66 volt, with a fully charged cell rising to 2.4 volt that

would

> > > give
> > > > an additional 13.2Volt (33 cells @ .2V) for a total of 79.2

volt.

> > > >
> > > > In a recent post Les Watts reports routinely seeing 60 amp

peaks on

> > > > rapid starts, stops and sharp curves, I would think batteries

could

> > > > cope with these conditions better than conventional caps.
> > > >
> > > > I'm not sure if there would be any significant price

difference

> > > > between caps and batteries, a 100Volt 15,000uF cap is almost

$30

> > > and
> > > > for a reasonably stiff supply you probably need at least 3 in
> > > > parallel. Without doing the math, I suspect you need more

than 3

> > > caps
> > > > to cope with 60 Amp peaks. 12V 4.5 A/Hour batteries are about

$17

> > > and
> > > > 6V 4.2 A/H $9 for a total of $94. (12V 12 AH cost $$35)
> > > >
> > > > I'm not sure how effectively batteries kill spikes, I would

have

> > > > thought much better than caps, they should absorb overvoltage

from

> > > > motors quite effectively.
> > > >
> > > > I can see difficulties in disconnecting the batteries from the
> > > drives
> > > > at power off, but this should be offset by being able to use
> > > smaller
> > > > transformers.
> > > >
> > > > Just a thought.
> > > >
> > > > Bernard
> > >
> > >
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> >
> >OFF Topic POSTS: General Machining
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> >goto: aol://5863:126/rec.crafts.metalworking or go thru

Google.com to

> >reach it if you have trouble.
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> >
> >http://groups.yahoo.com/group/jobshophomeshop I consider this to

be a

> >sister site to the CCED group, as many of the same members are

there, for

> >OT subjects, that are not allowed on the CCED list.
> >
> >NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
> >THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO

EXCEPTIONS........

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> >List Mom
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>
>
> Carl Mikkelsen
> Oasis Semiconductor, Inc.
> 201 Jones Road
> Waltham, MA 02451
> (781) 647 8775