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From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: sci.electronics,sci.energy
Subject: Re: Battery Queries
Message-ID: <861@eplunix.UUCP>
Date: 15 Mar 90 17:54:38 GMT
References: <32769@brunix.UUCP>
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 36

In article <32769@brunix.UUCP>, kjb@cs.brown.edu (Ken Basye) writes:
> 
> +5V  - ~25 Amps = 125W
> +12V - ~3  Amps =  36W
> -12V - ~1  Amps =  12W
> 
> We may also be powering some other small devices and motors as well,
> so I'd prefer to have more available when it's needed.  We'd like to
> ba able to run all this for at least a couple of hours before
> recharging, and it would be OK if the charging took overnight (We're doing
> research, not making a product someone would want to take home).  

OK, these are your keys. Since you need a couple of hours use, and you
probably want your voltages to remain high, and your batteries to be
multiply rechargable, and to be idiot proof (for lab safety) gel cells
are probably the way to go. But, geez, 25 amps at 5V? For two hours?
That's 50 amp-hours times a fudge factor of three to prevent deep discharging
your cells. That's 150 amp-hours in a, say, 6V battery. *ouch* 

Yuasa makes a 10 A-hour 6 Volt sealed lead-acid. Using that as a standard,
they're 5 lbs. a piece, at 6" by 2" by 4". That's 75 lbs., 720 cubic inch.
Your other requirements pale in comparison to this.

Your 12 Volt supply can use only 18 A-hours, and your -12 can use 6 A-hours.
Yuasa makes a 12-Volt 24 A-hour one, 7" by 5" by 7", 17 lbs. And a 12-Volt
6 A-hour one that's 4" by 6" by 3", 5.28 lbs. 

You probably want to actually use batteries rather than DC-DC converters.
They're noisy and generate considerable heat, and they are not efficient.
Anybody heard of one with more than 50% efficiency? Save yourself the trouble:
use separate battery systems.

-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu