68 Chapter 7 Structure of UltraBattery TM + Separator + PbO 2 PbO 2 Carbon electrode Pb Lead–acid cell Asymmetric supercapacitor + Pb i i i T i 2 UltraBattery TM Carbon electrode PbO 2 Figure 1 Schematic representation of UltraBattery configuration and operation. Soluble lead acid cell diagram, showing component materials. Courtesy of CSIRO. bottom of the battery, this process is what limits the cycle life of traditional lead acid batter- ies. For this reason most lead acid batteries are designed with some additional space at the bottom of the cells for the flaked material to collect in order to prevent accidental short circuiting between the anodes.
The other challenge with the traditional lead acid is the gassing that occurs as the acid reacts with the lead. This gassing is what drives the need for frequent testing of the electrolyte and the need to service and refill the battery. Another type of lead acid battery that has been introduced to the market more recently is the absorbed glass mat (AGM) lead acid battery. The main difference between the tradi- tional lead acid and the AGM is that in the AGM battery, a sponge type fiberglass mat separator is used to hold the electrolyte instead of submerging the lead plates in the sulfuric liquid electrolyte. This reduces the potential for leakage and spilling of the acid electro- lytes.
The AGM is therefore able to be “sealed” as the gasses are reabsorbed into the fiberglass mat. And as they are sealed, there is no need to service the batteries. The AGM battery falls into the category of being a valve-regulated lead acid (VRLA) battery. All batteries in this category are sealed and are able to recombine the gasses back into the electrolytes. This category of lead acid batteries are also frequently called sealed or mainte- nance-free for the same reason.
While they have the disadvantage of using hazardous materials such as lead and sulfuric acid, this is partially overcome as lead acid batteries are one of the most highly recycled products in the market today with a recycle rate in the United States of between 95 and 98%. This, of course, does not mitigate the environmental risk in regions of the world