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re: battery bank in house
3 jun 2005
sylvan butler wrote:
>...i insist on understanding everything from the ground up
how irritating :-)
>>>>>>> asqcm = 100qv, where qv=.006 * n * iamps
electrodes used in electrolysis have a magic measured "electrode potential,"
(call it ve--i'm not sure it can be calculated) that explains why 100% of
the electrical energy doesn't go into producing h2 and o2. the wasted power
(as much as 70% in electrolysis) is ive in watts, and the wasted energy is
power times time in watt-hours. some of the wasted energy ends up as heat,
but it seems that some also goes into h2 and o2 when charging batteries. it
takes a certain amount of energy to produce a mole (22.4 liters) of h2, about
0.1 kwh/ft^3, iirc. if we subtract the heat power, maybe the charging energy
equals the energy stored plus the energy used to make h2.
if you dig more into this, you might find that about 10% of the charging
energy ends up as h2, since batteries have about 90% "coulombic efficiency"
(round trip energy) if fully charged, altho this can be close to 100% if
they are only 80% charged. csiro thinks this can also extend battery life,
altho p.eng. simon wiles at surrette doubts this, since leaving a battery
partially-charged results in sulfation, and this requires equalization
at higher temps to remove, and higher temps reduce battery life.
but this stuff all seems pretty murky. surrette's tech bulletins suggest doing
equalization with a high voltage and low current. how can that happen? they
also suggest equalizing at the c/20 rate, and say stop equalization and let
the battery cool for a while if it reaches 125 f.
bulletin 610 (battery ventilation) assumes a battery is fully discharged
and then fully charged as quickly as possible. closely paraphrasing:
as the cell voltage reaches 2.35 the majority of gassing begins.
at this point about half the electrical energy causes water to break
into hydrogen and oxygen gas. if we know how much extra energy is
required to fully charge the battery then we know how much energy is
available to create hydrogen gas. this is generally accepted as 20%...
a 100 ah battery will require an electrical input of 120 ah. not all
the extra energy creates hydrogen. some is dissipated as heat, but
for simplicity it is assumed it all does.
through calculations [heh] we can show that 1 ah of overchage will
produce 0.42 l of h2 per cell... for every volume of h2 a 1/2 volume
of o2 is produced. this must be considered because to remove the h2
we also have to remove the o2.
for example, charging 3 cells with 120 ah makes
20 ahx0.42lx3 = 25.2 l of h2, or 37.8 l of h2+o2.
...it can be shown that the fastest a battery can be charged without
permanent damage is 5 hours. gassing occurs in the last 20%, ie in
1 hour. assuming a constant rate above, we produce 37.8 l or 1.33 ft^3
per hour of gas. removing it as fast as it is produced requires moving
1.33/60 = 0.022 cfm.
hydrogen is explosive in concentrations above 4%. ventilation systems...
are of two types. the first for a tight battery box and the other for
a large room. the tight box may be vented using an explosion proof blower
(www.zephyrvent.com) that sucks air through the box... with a 25% capacity
safety factor.
if the room is large the gas will mix with the air, so more air must be
evacuated. methods and calculations used are beyond the scope of this
bulletin. (how many cfm of ventilation would keep 0.022 cfm of gas below
a 4% concentration in an 8' cube? :-)
...by controlling the cell voltage gassing can be greatly reduced, with
atmospheric venting via a static vent or "chimney," usually sufficient
for small systems. hydrocaps reduce the amount of h2 released by 2/3.
so how does this turn into cm^2, as in:
>>>>>>> asqcm = 100qv, where qv=.006 * n * iamps?
perhaps this contains some assumptions about the bouyancy of the gas
vs air and the height of the chimney, as in
cfm = 16.6asqrt(hdt), an empirical formula for an air thermal chimney.
what's the equivalent dt for h2+o2?
nick
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