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From: mikec@wheaties.ai.mit.edu (Mike E. Ciholas)
Newsgroups: sci.electronics,sci.energy
Subject: Re: Battery Queries
Message-ID: <7308@rice-chex.ai.mit.edu>
Date: 16 Mar 90 00:33:10 GMT
References: <32769@brunix.UUCP> <861@eplunix.UUCP>
Reply-To: mikec@rice-chex.ai.mit.edu (Mike E. Ciholas)
Organization: MIT Artificial Intelligence Laboratory
Lines: 40

In article <861@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
>In article <32769@brunix.UUCP>, kjb@cs.brown.edu (Ken Basye) writes:
>> 
>> +5V  - ~25 Amps = 125W
>> +12V - ~3  Amps =  36W
>> -12V - ~1  Amps =  12W
>> 
>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.
>

Well, let's see...

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.

I do NOT reccomend using separate batteries for each voltage required.
First, some sort of voltage regulation will be required to maintain the
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,
as the batteries will discharge unevenly, the system may be subject to
complete absence of one of the voltages possibly resulting in system
failure or destruction.  Third, if you have multiple batteries, they will
never undergo uniform discharge/charge cycles with the result that one
battery may wear out before the others.  Fourth, charging multiple batteries
can be a pain.  With one battery you need only one charger.

Mike Ciholas

email:  mikec@ai.mit.edu
snail:  289 Highland Ave #108/Somerville, MA 02144
phone:  (617) 623 3563
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