From: jgd@dixie.com (John De Armond)
Subject: Re: Why not 120 VDC ? Why invert?
Date: Sun, 24 Jan 93 04:24:52 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.

ems@michael.apple.com (E. Michael Smith) writes:

>Pondering this ...  Why not just use a 120 VDC battery pack and use
>120 VDC for a bunch of stuff?  Lightbulbs don't care if they are
>AC or DC, nor do electric stoves and ovens or other resistive heat
>sources.  Some tools have AC/DC motors.  120 VDC was a common motor
>type not too long ago, so I'd expect some motors to be available
>for things like house fans.

There are a couple of issues.  Filament-type lamps DO care about DC
according to my lighting handbook.  Seems the cathode end of the 
filament thins faster than the rest because of voltage potential between
the anode and cathode strips off vaporized tungsten atoms.  The
suggestion is of the lamp is run on DC to make some provision for
reversing the polarity fairly often.  

The next issue is that DC motors will either be noisy (universal) or
expensive (brushless DC).  A low end consumer appliance would likely
get a universal motor for both the AC and DC version.

Next is the issue of keeping that many batteries equalized.  One of
the systems I was responsible for at the Sequoyah nuclear plant was
the 250 volt, 200,000 amp-hour emergency instrument battery system.
Learned a lot about maintaining large battery strings.  The issue is
the cells develop sulfation and/or self-discharge at different 
rates.  In a long string like that, the odds will result in one or several
cells differing markedly from the rest.  One technique that helps mitigate
the effect is to periodically apply an equalizing charge.  This is a
fairly heavy overcharge, using the theory that the weak cells will be
charged while the strong cells are relatively harmlessly overcharged.
When you have a few thousand megawatts available, hard overcharging 
isn't hard :-).  A solar charger would have to be oversized to do that.

Even with equalizing charging system, cells were replaced fairly 
frequently.  We'd do a weekly "cadmium test" that involves dipping a 
cadmium rod connected to an instrument down in the acid and reading
the condition as a voltage between cadmium rod and (I think) the 
+ terminal of the cell.  Frequent cell replacement was the rule even
though we used the pure lead plate station type batteries.

We'd also once every outage fully discharge the battery and measure its 
capacity.  Mike, you'd have loved the dummy load involved :-)  Trailer
mounted with a connection for a fire hose for cooling.  Cable about 
the size of your wrist.

I think we can probably learn something from the industrial electromotive
field.  Most fork lifts and other electrically driven industrial devices
I've ever worked on use batteries somewhere close to 48 volts.  I'd feel 
fairly confident someone sat down and figured out the optimal balance between 
cell size, voltage, maintenance and charging requirements.

John
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