Kirjoittanut: elwari
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2. FUNCTION OF VRLA-GEL-BATTERIES
2.1 Chemical Reactions
The following formula shows the chemical reactions as a so-called overall
reaction for discharge and charge:
Pb + PbO 2 + 2H 2 SO 4
Discharge
2PbSO 4 + 2H 2 O
Charge
During discharge (reaction from left to right side), the lead of the negative
electrode (active material) and the lead dioxide of the positive electrode
are transformed into lead sulphate. The sulphuric acid is transformed into
sulphate (lead sulphate) and water. The formation of water shows that the
acid concentration is decreased. On the other hand, the acid is diluted by
the formed water.
During charge (reactions from right to left), these processes take place in
the reverse direction.
2.2 Gassing, Recombination
A special feature of VRLA-batteries consists in the recombination of oxygen
during charge. The cycle starts at the positive electrode:
H 2 O
1?2 O 2 + 2H + + 2 e -
Water is decomposed and gaseous oxygen is formed. The hydrogen ions
remain dissolved in the electrolyte and are not released as gas. The
electrons move away via the positive electrode
What is happening now with the oxygen?
How does it make its way to the negative electrode?
In lead-acid batteries of the vented design with „free“ electrolyte, it is
practically impossible for the oxygen to move to the negative electrode.
Immediately after having „left“ the positive electrode, it bubbles up and
escapes through the vent plug.
In VRLA batteries, a densely porous medium is offered to the oxygen to
facilitate its movement (AGM-batteries: pores in the glass mat; Gel-
batteries: cracks in the gel).
At the negative electrode, the following reactions take place:
(1) Pb + 1?2 O 2
(2) PbO + 2H + + SO 4 2- PbO
PbSO 4 + H 2 O
(3) PbSO 4 + 2e - Pb + SO 4 2-
The oxygen oxidizes lead to lead oxide (formula (1)). This lead oxide is
further transformed into lead sulphate with participation of hydrogen
dissolved in the electrolyte and sulphate ions (formula (2)). This represents
practically a discharge process. Further products of this reaction are water
and heat, in addition to the ohmic losses during charging. Thus, the oxygen
is recombined and water is re-converted. The circle is closed.
The recombination efficiency in Gel-batteries is
? 95 %, typically 98%.
2. FUNCTION OF VRLA-GEL-BATTERIES
2.1 Chemical Reactions
The following formula shows the chemical reactions as a so-called overall
reaction for discharge and charge:
Pb + PbO 2 + 2H 2 SO 4
Discharge
2PbSO 4 + 2H 2 O
Charge
During discharge (reaction from left to right side), the lead of the negative
electrode (active material) and the lead dioxide of the positive electrode
are transformed into lead sulphate. The sulphuric acid is transformed into
sulphate (lead sulphate) and water. The formation of water shows that the
acid concentration is decreased. On the other hand, the acid is diluted by
the formed water.
During charge (reactions from right to left), these processes take place in
the reverse direction.
2.2 Gassing, Recombination
A special feature of VRLA-batteries consists in the recombination of oxygen
during charge. The cycle starts at the positive electrode:
H 2 O
1?2 O 2 + 2H + + 2 e -
Water is decomposed and gaseous oxygen is formed. The hydrogen ions
remain dissolved in the electrolyte and are not released as gas. The
electrons move away via the positive electrode
What is happening now with the oxygen?
How does it make its way to the negative electrode?
In lead-acid batteries of the vented design with „free“ electrolyte, it is
practically impossible for the oxygen to move to the negative electrode.
Immediately after having „left“ the positive electrode, it bubbles up and
escapes through the vent plug.
In VRLA batteries, a densely porous medium is offered to the oxygen to
facilitate its movement (AGM-batteries: pores in the glass mat; Gel-
batteries: cracks in the gel).
At the negative electrode, the following reactions take place:
(1) Pb + 1?2 O 2
(2) PbO + 2H + + SO 4 2- PbO
PbSO 4 + H 2 O
(3) PbSO 4 + 2e - Pb + SO 4 2-
The oxygen oxidizes lead to lead oxide (formula (1)). This lead oxide is
further transformed into lead sulphate with participation of hydrogen
dissolved in the electrolyte and sulphate ions (formula (2)). This represents
practically a discharge process. Further products of this reaction are water
and heat, in addition to the ohmic losses during charging. Thus, the oxygen
is recombined and water is re-converted. The circle is closed.
The recombination efficiency in Gel-batteries is
? 95 %, typically 98%.