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re: two dick nick
2 apr 2000
george ghio   wrote:

>"say 1 watt per battery, at 24 vdc, for a 24 v system."
 
>my god nick! 

if so, you might have more under your hat, george :-)

>20 hours to warm the batteries?

well then, let's use 5 watt resistors. still cheap and easy to wire and
control, at 24 vdc. we don't need to heat the batteries quickly, because
we are discharging them over 5 days, and we don't want to cramp available
house power, nor use expensive wiring and high currents that are harder
to switch. 

>do you ring up your eletricity supplier the day before to get more power?

no. but then, i seldom run out, as you might in scenario 1:

it's 0 c outdoors for a month, and your maximum dod is 80%, one time
during that month. our battery stores 200 ah at 6 v, ie 1200 wh, and
it weighs 50 lb. it takes 20 wh to heat 50 pounds of lead 25 c, which
"releases" 23% more capacity, ie 276 wh, for a net gain of 256 wh...
consider these two scenarios, if you please:

  1. keep the battery 25 c for the whole month.

     at 25 c, the self-discharge rate is about 12%/week, according to
     fig. 5.9 on page 93 of dr. komp's book, so by the end of the
     month, it's down to 0.88^4 = 60% of capacity, ie it only contains
     0.6x1200 = 720 wh. so it goes dead before the end of the month,
     and it's lost some lifetime as well, compared to scenario

  2. keep the battery 0 c initially, and warm it to 25 c at the end.

     at 0 c, the self-discharge rate is about 2% per week, so by the
     end of the month, it's only down to 0.98^4 = 92% of its capacity,
     and removing another 20 wh to heat it to 25 c makes this 1087 wh,
     ie 90.5% of the initial charge...

in scenario 2, the battery stores 1087/720 = 51% more energy, no?

why throw away money for panels and batteries and battery lifetime
and heated floorspace and live in the dark by keeping the batteries
warm all the time?

>between warming the batteries for optimum charging and warming them
>again for optimum output when do they get cold?

how much do low temperatures affect charging? (were you going to look
that up, george?). do recall (after a briefing about the last several
hundred years of basic technological progress) that batteries don't have
much thermal mass, so they can quickly be warmed and naturally cool
in a box surrounded by cold air, to wit:

let's put 8 50 pound batteries inside a 2'x4'x1' tall 4" styrofoam box
put together with deck screws and some 1/4" plywood between 2 2" foam
sheets, with white latex paint on the outside. its thermal conductance
is 32ft^2/r20 = 1.6 btu/h-f. keeping it 25 c on a 0 f day requires
(25-0)1.8(1.6) = 72 btu/h or 21 watts, on average.

the batteries might have 16 btu/f of thermal capacitance. they might sit
on a 1/2" self-molded cement bed with 8 5 watt resistors or some nichrome
wire in the cement, and some reinforcing mesh underneath, on top of foam.
this adds about 8 btu/f to the capacitance, making the rc time constant
24btu/f/(1.6btu/h-f) = 15 hours. this could be shortened if needed.

if we were clever, we might reprogram the gen start output of a trace 4024
inverter to turn on a tiny fan or actuate a 2 watt honeywell 6161b1000
damper motor to move outdoor air through the box when the batteries were
sufficiently charged and warmer than 0 c, and warmer than outdoor air. 

a roofpond on top of the box might be a nice option, on the shady side
of a house, with a 2" styrofoam perimeter berm on top of the box and a
layer of uv polyethylene film over that, with some cap strips to hold
the poly on top of the berm. put a hole in the film center so rainwater
flows into the covered 2" pond, but it can't evaporate, so it loses heat
by night-sky radiation in summertime and freezes in wintertime to help
limit the lower battery temp without heating.

or put the heater and batteries inside a zomeworks cool cell. these are
used worldwide by savvy solar power consultants to keep batteries cool
and make them last a lot longer. 

nick




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