At 08:43 AM 8/28/2008 -0700, Keith Ovregaard wrote:
>That would be the cats meow, but not cheap. I figure you could fit about 256
>Li-Ion 18650 Cylindrical 3.7V 2200mAh Cells under the seat. They cost about
>$7 each, so $1792!!! Not sure if they could be wired up to make 12V though.
>Tesla electric car uses 6,831 of these!

Just as well they're expensive IMO. They're very intolerant of
improper charging regimes, and express their displeasure by catching
fire or exploding.  They need active charge-monitoring
electronics.  It's my impression that such systems still need expert
engineering and are not at the DIY level yet for large capacities.  I
wouldn't care to sit on a bunch of them myself.  But here's a place
that sells them for $9/pr... <http://www.dealextreme.com/details.dx/sku.6979>

As to the 12v, four in series would be nominal 14.8v.  A couple of
big diodes with big heat sinks (or some fancier but more efficient
electronics) would bring that down to 13+ for lighting and other
circuits that cared. Or use three in series for 11.7v nominal,
probably better all around and certainly less wasteful.

However...256 cells would give 4x64, maximum rated discharge on the
cell is less than four amps, so your max rated current would be less
than 250 amps, so no good as starting battery (normal rated current
would be 30+ amps).  Nominal capacity, though, would be 140 amp-hours.

3x85 (255 total) would give 185 amp-hours nominal, 340 amps max draw,
45 amps normal draw.

Either way the pack would be dissipating 80+ watts internally at max
draw (plus whatever the connections dissipate), might need some
cooling. At normal draw only about 1.5 watts, no problem.

Could be a great house battery for people not running toasters,
coffee machines, monster stereos etc.  But I can see issues with
connections and repairability -- I suspect it would be better to use
existing battery packs and/or larger cells.

And I still wouldn't want to sit on them.

David

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"