Environmental Engineering Reference
In-Depth Information
1
0.9
0.8
C/20
C/8
0.7
Discharge limit
1C
0.6
3C
2C
2
5
10
20
30
60
2
3
5
10
20
minutes
hours
Discharge time at 20 °C (68 °F)
Figure 10.6 Discharge behaviour of lead-acid battery for various discharge rates
The discharge behaviour described in Figure 10.6 will shift left (shorter time
intervals) as the battery is cooled. For example, at 0
C the 3
C
rate will result in a
discharged battery in approximately 5 min on this same scale.
Lead-acid batteries are amongst the oldest known rechargeable electro-
chemical couples. During discharge both electrodes are converted to lead sulphate
and during charge the electrodes are restored. However, on charge, oxygen is lib-
erated at the positive electrode and hydrogen at the negative electrode. This side
reaction causes disassociation of water by electrolysis, resulting in water loss that
must be periodically replenished, and hence the battery requires maintenance.
During the early 1970s, maintenance free batteries were introduced that reduced
water loss through oxygen recombination with freshly formed elemental lead on the
negative electrode. In the presence of the sulphuric acid electrolyte, this oxygen
combines with lead to form lead sulphate, causing depolarization of the negative
electrode and thereby effectively suppressing hydrogen formation. Oxygen released
through electrolysis is able to accomplish this because it has access via voids
between the electrodes to react with the lead where the electrolyte is immobilized.
Immobilization of electrolyte in the inter-electrode spaces was accomplished in two
ways: (1) by use of an absorbent glass mat of a highly porous microfibre con-
struction that is only partially saturated with electrolyte and (2) with gelled elec-
trolyte. Adding fumed silica to the electrolyte causes it to congeal into a gel. When
the battery is recharged, some water is lost, the gel dries and on subsequent
recharging cracks and fissures propagate in the gel, thereby acting as channels for
oxygen to find its way to the negative electrode and recombine. Use of a pressure
relief valve helps to further regulate the flow of oxygen from positive to negative
electrode. The large, prismatic, maintenance free or valve regulated lead-acid
(VRLA) batteries normally have 1-2 psi gauge (psig) pressure thresholds on
the relief valve. Smaller, spiral wound VRLA batteries can have pressures as high
