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I suppose the issue is do you place your bets on nobody ever overcharging
their battery or do you figure that sooner or later someone will and plan
accordingly?
This is pretty much a no-brainer if you ask me. I think there are too many
rocket scientists in this sport to do it any good.
Jim
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> From: Olaf Trygve Berglihn <olafb@pv*.or*>
> Date: 23 May 2001 15:04:38 +0200
> To: Jarrod Jablonski <JJ@Ha*.ne*>
> Cc: techdiver@aquanaut.com
> Subject: Re: EE and steel screws in aluminum (was Re: halcyon vs DiveRite)
>
> 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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