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Jarrod Jablonski <JJ@Ha*.ne*> writes:
> Captured o-rings are dangerous due to combustion from battery off
> gassing and are not recommended.
This is an issue when charging, but not really when discharging. Also,
the lead-gel YUASA batteries used (don't have it here in front of me,
but I think it is YUASA), are able to absorb the gas produced to a
certain degree.
The basic chemical reaction is as follows:
[anode] PbO_2 + 4H^+ + SO_4^{2-} = PbSO_4 + 2H2O
[cathode] Pb + SO_4^{2-} = PbSO_4 + 2e^-
[overall] PbO_2 + 2H_2SO_4 + Pb = 2PbSO_4 + 2H2O
Direction of equilibrium reactions:
---> Discharge
<--- Charge
During the final stage of the charging cycle there are competing
chemical reactions taking place. Water is decomposed to produce
hydrogen and oxygen gases. The charging efficiency drops dramatically
at this stage. The presence of bubbles causes a stirring action of the
acid, thus sulphuric acid is then mixed within the electrolyte.
During the final stage of charging or under overcharge condition, the
charging energy is consumed for electrolytic decomposition of
water. Oxygen is released as gas from the positive plate into the
sealed cell compartment. The oxygen then rapidly diffuses through the
separator or cracks in the gel. The oxygen reacts with the moist
negative active material to form lead oxide. The oxidation of the
negative causes a slight depolarization and inhibits the release of
Hydrogen gas. The lead oxide reacts to form lead sulphate that on
further charge changes to form lead and sulphuric acid. This results
in no loss of oxygen or hydrogen and therefore eliminates the need to
add H_2O.
Now, for the discharge part, there is no production of gaseous
hydrogen, and any (unlikely) hydrogen leaking out was most probably
produced by overcharge.
Bottom line is: Do not overcharge - a decent charger will prevent
it. Let the battery sit for a while after charging before you close
the lid. Vent a canister that has not been used for a while.
Gas in the canister that has leaked from the sealed battery during
dive is highly improbable, and I see no special reason to have a
o-ring system that can act as a relief for an explosion in the
canister that will not happen. I do not think this represent a danger
for a battery that has been well vented during charging and set to
rest before use.
Please correct me if I missed something out here (is has been a while
since I graduated chem. eng.). Do you have any examples of hydrogen
gas trapped in the sealed battery that has caused explosion in a
canister - i.e. if the battery was vented during charging.
Saying this, I have not yet had a leak in my apollo 14, and I'm
looking forward to going diving the next four days with my repaired
Goodman handle.
Olaf
--
Olaf Trygve Berglihn <olafb@pv*.or*>
--
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