Xref: utzoo sci.electronics:10646 sci.energy:1703
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!think!eplunix!raoul
From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: sci.electronics,sci.energy
Subject: Re: Battery Queries
Message-ID: <862@eplunix.UUCP>
Date: 16 Mar 90 18:49:46 GMT
References: <32769@brunix.UUCP> <861@eplunix.UUCP> <7308@rice-chex.ai.mit.edu>
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 41

In article <7308@rice-chex.ai.mit.edu>, mikec@wheaties.ai.mit.edu (Mike E. Ciholas) writes:
> In article <861@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
> >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.
> >
> 
> I can get a DC-DC converter from Mechler (Ashland, MA) that takes anywhere
> from 8 to 80 volts, produces 5V at 25A, and does this with a minimum of
> 82% and as high as 94% efficiency.  It does not generate any audible noise
> and has only 30mv P-P ripple (electrical noise).  As it is high efficiency,
> I cannot see how it will generate lots of heat.

Hmmm, then I take it back. When I looked at DC-DC converters, I couldn't
find anything with the efficiencies or noise limits I needed. On the other
hand, it was for sensitive multi-channel medical equipment, so they also
had to be really small and almost utterly noise free. (1 mV was too much.)

> proper voltage.  You will need a battery voltage that is a minimum of
> 2 volts higher than the desired voltage for a linear regulator (and if
> you use a switching regulator, you are using a DC-DC converter without
> knowing it).  At 5 volts, this ammounts to only 71% efficient.  Second,

How much regulation is needed? For the 5 Volt digital supply, he may need
that much regulation and the resultant power losses. For the +/- 12 Volt
supply, I suspect precise regulation to be unnecessary. So the batteries
would still work there.

Your reliability and failure concerns are valid, but I trust batteries
more than I trust DC-DC converters. It takes a *lot* to fry a gel-cell,
and I've seen some DC-DC's destroyed in the lab by accidents. 

So what about using a lot of 12V batteries to power the 5V DC-DC and 
maybe a 12V DC-DC, or use one of the 12V units reversed for the -12V?
You'd still only need a 12V or 15V power supply to recharge them.

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