On Mon, 05 Jan 2009 19:56:28 -0800, Adam Aglionby wrote:
a) I'm not sure how car batteries would fare for a moderate, constant load
(given that car use is generally a short, high-current draw and then a
charge cycle),

Probably less than ideal, deep cycle batteries are always a better
bet and not ludicrously more expensive. Traction batteries on other hand....

Yeah, I've not looked into cost yet. A quick google seems to indicate
somewhere in the $90-$140 range for car batteries, which is probably about
60-100 quid or so. That's probably about what I remember them being in
England, too.

b) The transmission losses involved might just be too great for a
low-voltage DC system to really be viable.

24V is popular for that very reason, half the copper of 12V for a given
load or double the load. Vehicle makers were very keen on changing
everything over at one point.

I think I remember when my dad's old Beetle had 6V electrics...
But yeah, I'd be as happy dealing with 24V as 12V I think.

PVC insulated cable tends to survive the outdoors fine, especially
buried out of the Sun , no UV, containment not really neccessary apart
from perhaps mechanical protection in some areas.

Well out here in my area of USVille there's no building code (no building
regs or planning permission) but there is an electrical code, and I
know that specifies conduit for outdoor wiring. Of course they assume
that's at domestic AC rates, not LV-DC, but I don't think they make the
distinction. I'll have the check though, as it's possible that the
rules only apply for things directly connected to the house fusebox -
maybe if it's downstream of a transformer or off-grid altogether (e.g.
solar) then I can do what I like.

(obviously car/truck batteries would need to be outdoors, but we've got
a spare shed tht's got some useful ventilation, so that'd do I think)

Wouldn`t neccessarily need to be outdoors, lot of public buildings have
cabinets full of the things somewhere for emergency lighting.

http://www.batteryfaq.org/

Hmm, I've seen sealed ones in telcos before, but haven't ever had a
change to see a battery-noly emergency setup (we had a diesel genset at
one site, but of course that just needed a single truck battery for
starting purposes, rather than a big bank).

That site still seems to say (I've only had a quick flick through so
far) that ventilation's vital when charging car batteries due to the gases
given off. Of course "how much is enough" is the vital question... maybe
it's only an issue if you have the battery in a tiny box and go stick your
head right in there :-)

cheers

Jules