🚀 go-pugleaf

I'm in need of answers to a few questions about batteries.  I don't
know if there's enough general interest in either of these groups to
justify using follow-ups; I'll leave that to the individual
responders.  If there is interest, I'll summarize all the e-mail
responses I get.

To introduce the problem, I'm working in mobile robotics, and looking
into various ways to power some VME boards with batteries.  The
manufacturers of these boards (reasonably enough) aren't designing
them with mobile robots in mind, so they aren't too concerned about
power.  They also generally use serial and video standards which
require +/- 12V in addition to +5V for logic.  Between a Moto 147SA-1,
3 vision processing boards from Itex, and a few other small devices,
the approximate power requirements as taken from the manufacturers
specs are:

+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).  
Our robots are built on bases that are 12" in diameter and are rated
to carry about 45 lbs, although we have put as much as 80 lbs on them,
at which point they have problems going over small bumps in the floor.
We may also be getting some larger bases with much larger payloads,
perhaps capable of carrying a standard-size deep-cycle marine battery.

Ok, now on with the questions:

1) Assuming that we use high-power, triple-output DC-DC converters to
supply the 3 required voltages, is using higher voltages (say 48V as
opposed to 12V) a big win?  If so, is putting 4 12V batteries in
series a reasonable way to do this?  Is there a better way to provide
3 different voltages?

2) Manufacturers of Gel-Cell lead-acid batteries (like Panasonic) rate
their batteries by amp-hours, but using a rate of discharge that
drains the battery in 20 hours.  Thus, a battery with a 10 Ah rating
can be expected to produce .5 amps for 20 hours.  What's a good rule
of thumb for converting such a rating to higher discharge rates?  My
experience has been that if you draw 10 amps from such a battery, it
won't last anywhere near an hour, and may not even be able to hold its
rated voltage.  Given that the Ah rating over 20 hours gives a
reasonable current, how far above that current can you go and still
hold rated voltage?  

3) I've found 12V batteries for wheelchairs that come in two flavors.
The sealed gel-cells are rated at 30Ah, while the wet batteries of
about the same size and weight and are rated at 23A for 54 minutes.
Does this mean that wet cells are better at providing higher current
over shorter times?

4) If you've read this far, perhaps you're interested or altruistic
enough to say something about how you might tackle this problem?

Thanks very much,
	Ken

Internet/CSnet  kjb@cs.brown.edu     U.S. MAIL  Ken Basye
UUCP            uunet!brunix!kjb                Box 1910
                                                Dept. of Computer Science
                                                Brown University
                                                Providence, RI  02912